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Co-operative brood rearing, seen here in honeybees, is a condition of eusociality.

Eusociality (Greek εὖ eu 'good' and social) is the highest level of organization of sociality. It is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping generations within a colony of adults, and a division of labor into reproductive and non-reproductive groups. The division of labor creates specialized behavioral groups within an animal society, sometimes called castes. Eusociality is distinguished from all other social systems because individuals of at least one caste usually lose the ability to perform behaviors characteristic of individuals in another caste. Eusocial colonies can be viewed as superorganisms.

Eusociality has evolved among the insects, crustaceans, trematoda and mammals. It is most widespread in the Hymenoptera (ants, bees, and wasps) and in Isoptera (termites). A colony has caste differences: queens and reproductive males take the roles of the sole reproducers, while soldiers and workers work together to create and maintain a living situation favorable for the brood. Queens produce multiple queen pheromones to create and maintain the eusocial state in their colonies; they may also eat eggs laid by other females or exert dominance by fighting. There are two eusocial rodents: the naked mole-rat and the Damaraland mole-rat.[1] Some shrimps, such as Synalpheus regalis, are eusocial.[2] E. O. Wilson and others have claimed that humans have evolved a weak form of eusociality. It has been suggested that the colonial and epiphytic staghorn fern, too, may make use of a primitively eusocial division of labor.

History

[edit]
Suzanne Batra introduced the term "eusocial"[3] after studying nesting in Halictid bees including Halictus latisignatus,[4] pictured.

The term "eusocial" was introduced in 1966 by Suzanne Batra, who used it to describe nesting behavior in Halictid bees, on a scale of subsocial/solitary, colonial/communal, semisocial, and eusocial, where a colony is started by a single individual.[3][4] Batra observed the cooperative behavior of the bees, males and females alike, as they took responsibility for at least one duty (e.g., burrowing, cell construction, oviposition) within the colony. The cooperativeness was essential as the activity of one labor division greatly influenced the activity of another. Eusocial colonies can be viewed as superorganisms, with individual castes being analogous to different tissue or cell types in a multicellular organism; castes fulfill a specific role that contributes to the functioning and survival of the whole colony, while being incapable of independent survival outside the colony.[5]

In 1969, Charles D. Michener[6] further expanded Batra's classification with his comparative study of social behavior in bees. He observed multiple species of bees (Apoidea) in order to investigate the different levels of animal sociality, many of which are different stages that a colony may pass through. Eusociality, which is the highest level of animal sociality a species can attain, specifically had three characteristics that distinguished it from the other levels:[3]

  1. Egg-layers and worker-like individuals among adult females (reproductive division of labor, with or without sterile castes)
  2. The overlap of generations (mother and adult offspring)
  3. Cooperative care of the brood
Weaver ants, here collaborating to pull nest leaves together, can be considered eusocial, as they have a permanent division of labor.

E. O. Wilson extended the concept to include other social insects, such as ants, wasps, and termites. Originally, it was defined to include organisms (only invertebrates) that fulfilled the same three criteria defined by Michener.[3][7][8][9]

Eusociality was then discovered in a group of chordates, the mole-rats.[10] Some researchers have argued that another possibly important criterion for eusociality is "the point of no return".[3] This is characterized by having individuals fixed into one behavioral group, usually before reproductive maturity. This prevents them from transitioning between behavioral groups, and creates a society with individuals truly dependent on each other for survival and reproductive success. For many insects, this irreversibility has changed the anatomy of the worker caste, which is sterile and provides support for the reproductive caste.[3][9] Other researchers have suggested that cooperative breeding and eusociality are not discrete phenomena, but rather form a continuum of fundamentally similar social systems whose main differences lie in the distribution of lifetime reproductive success among group members.[11] Vertebrate and invertebrate cooperative breeders can be arrayed along a common axis, that represents a standardized measure of reproductive variance. In this view, loaded terms like "primitive" and "advanced" eusociality should be dropped. An advantage of this approach is that it unites all occurrences of alloparental helping of kin under a single theoretical umbrella (e.g., Hamilton's rule).[12]

Diversity

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Most eusocial societies exist in arthropods, while a few are found in mammals.[13] Some ferns may exhibit a form of eusocial behavior.[14][15]

In insects

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See also: Sexual selection in social insects and Identity in social insects

Eusociality has evolved multiple times in different insect orders, including hymenopterans,[16] termites,[17] thrips,[18] aphids,[18] wasps,[19] and beetles.[20]

In hymenopterans

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Swarming Iridomyrmex purpureus ants. The young queens are black, winged, and much larger than the wingless workers.

The order Hymenoptera contains the largest group of eusocial insects, including ants, bees, and wasps—divided into castes: reproductive queens, drones, more or less sterile workers, and sometimes also soldiers that perform specialized tasks.[21] In the well-studied social wasp Polistes versicolor,[22] dominant females perform tasks such as building new cells and ovipositing, while subordinate females tend to perform tasks like feeding the larvae and foraging. The task differentiation between castes can be seen in the fact that subordinates complete 81.4% of the total foraging activity, while dominants only complete 18.6% of the total foraging.[23] Eusocial species with a sterile caste are sometimes called hypersocial.[24]

While only a moderate percentage of species in bees (families Apidae and Halictidae) and wasps (Crabronidae and Vespidae) are eusocial, nearly all species of ants (Formicidae) are eusocial.[25] Some major lineages of wasps are mostly or entirely eusocial, including the subfamilies Polistinae and Vespinae. The corbiculate bees (subfamily Apinae of family Apidae) contain four tribes of varying degrees of sociality: the highly eusocial Apini (honey bees) and Meliponini (stingless bees), primitively eusocial Bombini (bumble bees), and the mostly solitary or weakly social Euglossini (orchid bees).[26] Eusociality in these families is sometimes managed by a set of pheromones that alter the behavior of specific castes in the colony. These pheromones may act across different species, as observed in Apis andreniformis (black dwarf honey bee), where worker bees responded to queen pheromone from the related Apis florea (red dwarf honey bee).[27] Pheromones are sometimes used in these castes to assist with foraging. Workers of the Australian stingless bee Tetragonula carbonaria, for instance, mark food sources with a pheromone, helping their nest mates to find the food.[28]

Beside corbiculate bees, eusociality is documented within Apidae in xylocopine bees, where only simple colonies containing one or two "worker" females have been documented in the tribes Xylocopini and Ceratinini, though some members of Allodapini have larger eusocial colonies.[29][30][31] Similarly, in the Colletidae, there is only one species reported to exhibit any form of social behavior; occasional nests of the species Amphylaeus morosus contain a female and a "guard" (a sister or daughter of the female that founded the nest), creating very small social colonies, where both females are capable of reproduction though only the foundress female appears to lay eggs.[32] In Halictidae (sweat bees), by contrast, eusociality is well-documented in hundreds of species, primarily in the genera Halictus and Lasioglossum. In Lasioglossum aeneiventre, a halictid bee from Central America, nests may be headed by more than one female; such nests have more cells, and the number of active cells per female is correlated with the number of females in the nest, implying that having more females leads to more efficient building and provisioning of cells.[33] In several species with only one queen, such as Lasioglossum malachurum in Europe, or Halictus rubicundus in North America, the degree of eusociality depends on the clime in which the species is found - they are solitary in colder climates and social in warmer climates.[34][35][36]

Myrmecocystus honeypot ants, showing the repletes or plerergates, their abdomens swollen to store honey (top), with ordinary workers (bottom)

Reproductive specialization in Hymenoptera generally involves the production of sterile members of the species, which carry out specialized tasks to care for the reproductive members. Individuals may have behavior and morphology modified for group defense, including self-sacrificing behavior. For example, members of the sterile caste of the honeypot ants such as Myrmecocystus fill their abdomens with liquid food until they become immobile and hang from the ceilings of the underground nests, acting as food storage for the rest of the colony.[37] Not all social hymenopterans have distinct morphological differences between castes. For example, in the Neotropical social wasp Synoeca surinama, caste ranks are determined by social displays in the developing brood.[38] Castes are sometimes further specialized in their behavior based on age, as in Scaptotrigona postica workers. Between approximately 0–40 days old, the workers perform tasks within the nest such as provisioning cell broods, colony cleaning, and nectar reception and dehydration. Once older than 40 days, S. postica workers move outside the nest for colony defense and foraging.[39]

In termites

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Termites live in large nests, with queen, king, soldier (red heads), and worker (pale heads) castes.

Termites (order Blattodea, infraorder Isoptera) make up another large portion of highly advanced eusocial animals. The colony is differentiated into various castes: the queen and king are the sole reproducing individuals; workers forage and maintain food and resources;[40] and soldiers defend the colony against ant attacks. The latter two castes, which are sterile and perform highly specialized, complex social behaviors, are derived from different stages of pluripotent larvae produced by the reproductive caste.[17] Some soldiers have jaws so enlarged (specialized for defense and attack) that they are unable to feed themselves and must be fed by workers.[41]

In beetles

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Austroplatypus incompertus is a species of ambrosia beetle native to Australia, and is the first beetle (order Coleoptera) to be recognized as eusocial.[42][20] This species forms colonies in which a single female is fertilized, and is protected by many unfertilized females, which serve as workers excavating tunnels in trees. This species has cooperative brood care, in which individuals care for juveniles that are not their own.[20]

In gall-inducing insects

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Kladothrips rugosa, a gall-forming thrips (larva on left, adult on right) with galls (centre) on Acacia leaves. Its soldier caste defends the colony in its gall fortress.

Some gall-inducing insects, including the gall-forming aphid, Pemphigus spyrothecae (order Hemiptera), and thrips such as Kladothrips (order Thysanoptera), are described as eusocial.[18][43] These species have very high relatedness among individuals due to their asexual reproduction (sterile soldier castes being clones produced by parthenogenesis), but the gall-inhabiting behavior gives these species a defensible resource. They produce soldier castes for fortress defense and protection of the colony against predators, kleptoparasites, and competitors. In these groups, eusociality is produced by both high relatedness and by living in a restricted, shared area.[44][45]

In crustaceans

[edit]

Eusociality has evolved in three different lineages in the colonial crustacean genus Synalphaeus. S. regalis, S. microneptunus, S. filidigitus, S. elizabethae, S. chacei, S. riosi, S. duffyi, and S. cayoneptunus are the eight recorded species of parasitic shrimp that rely on fortress defense and live in groups of closely related individuals in tropical reefs and sponges.[46] They live eusocially with a single breeding female, and a large number of male defenders armed with enlarged snapping claws. There is a single shared living space for the colony members, and the non-breeding members act to defend it.[47]

The fortress defense hypothesis additionally points out that because sponges provide both food and shelter, there is an aggregation of relatives (because the shrimp do not have to disperse to find food), and much competition for those nesting sites. Being the target of attack promotes a good defense system (soldier caste); soldiers promote the fitness of the whole nest by ensuring safety and reproduction of the queen.[48]

Eusociality offers a competitive advantage in shrimp populations. Eusocial species are more abundant, occupy more of the habitat, and use more of the available resources than non-eusocial species.[49][50][51]

In trematodes

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The trematodes are a class of parasitic flatworm, also known as flukes. One species, Haplorchis pumilio, has evolved eusociality involving a colony creating a class of sterile soldiers.[52] One fluke invades a host and establishes a colony of dozens to thousands of clones that work together to take it over. Since rival trematode species can invade and replace the colony, it is protected by a specialized caste of sterile soldier trematodes.[53] Soldiers are smaller, more mobile, and develop along a different pathway than sexually mature reproductives. One difference is that a soldier's mouthparts (pharynx) is five times as big as those of the reproductives. They make up nearly a quarter of the volume of the soldier. These soldiers do not have a germinal mass, never metamorphose to be reproductive, and are, therefore, obligately sterile.[53] Soldiers are readily distinguished from the immature and mature reproductive worms. Soldiers are more aggressive than reproductives, attacking heterospecific trematodes that infect their host in vitro. H. pumilio soldiers do not attack conspecifics from other colonies. The soldiers are not evenly distributed throughout the host body. They are found in the highest numbers in the basal visceral mass, where competing trematodes tend to multiply during the early phase of infection. This strategic positioning allows them to effectively defend against invaders, similar to how soldier distribution patterns are seen in other animals with defensive castes. They "appear to be an obligately sterile physical caste, akin to that of the most advanced social insects".[53]

In nonhuman mammals

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Model of naked mole-rat burrow with soldiers, workers, and queen, a social structure similar to the castes of the eusocial insects

Among mammals, two species in the rodent group Phiomorpha are eusocial, the naked mole-rat (Heterocephalus glaber) and the Damaraland mole-rat (Fukomys damarensis), both of which are highly inbred.[54][55] These mole-rats live in harsh, limiting environments, where dispersal is difficult and dangerous and cooperation is required to find food and defend against predators.[56] Most colony members are workers, and they cooperatively care for offspring of a single reproductive female (the queen) to whom they are closely related. These mole-rats are eusocial under any definition of the term. Interestingly, the discovery of male and female dispersers has revealed that there is a mechanism of inter-colony outbreeding in naked mole-rats.[57] Outbreeding reduces intra-colony genetic relatedness, but reduces inbreeding depression. Dispersers are morphologically, physiologically and behaviorally distinct from colony members and actively seek to leave their burrow when an escape opportunity presents itself.[58][59][60] These individuals are equipped with generous fat reserves for their journey.[60] Though they possess high levels of luteinizing hormone, dispersers are only interested in mating with individuals from foreign colonies rather than their own colony's queen. They also show little interest in working cooperatively with workers in their natal colony.[60] Hence, disperser morphs are well-prepared to promote the establishment of new, initially outbred colonies, before cycles of inbreeding resume.

Some mammals in the Carnivora and Primates have eusocial tendencies, especially meerkats (Suricata suricatta) and dwarf mongooses (Helogale parvula). These show cooperative breeding and marked reproductive skews. In the dwarf mongoose, the breeding pair receives food priority and protection from subordinates and rarely has to defend against predators.[61]

In humans

[edit]
Further information: Group selection

Scientists have debated whether humans are prosocial or eusocial.[62] Edward O. Wilson called humans eusocial apes, arguing for similarities to ants, and observing that early hominins cooperated to rear their children while other members of the same group hunted and foraged.[63] Wilson and others argued that through cooperation and teamwork, ants and humans form superorganisms.[64][65][66] Wilson's claims were vigorously rejected by critics of group selection theory, which grounded Wilson's argument,[63][67][68] and because human reproductive labor is not divided between castes.[67]

Though controversial,[69] it has been suggested that male homosexuality[70] and female menopause[71] could have evolved through kin selection.[72][73] This would mean that humans sometimes exhibit a type of alloparental behavior known as "helpers at the nest", with juveniles and sexually mature adolescents helping their parents raise subsequent broods, as in some birds,[74] some non-eusocial bees, and meerkats.[75] These species are not eusocial: they do not have castes, and helpers reproduce on their own if given the opportunity.[76][65][77]

In plants

[edit]
The staghorn fern Platycerium bifurcatum may display a simple form of eusociality.

One plant, the epiphytic staghorn fern, Platycerium bifurcatum (Polypodiaceae), may exhibit a primitive form of eusocial behavior amongst clones. The evidence for this is that individuals live in colonies, where they are structured in different ways, with fronds of differing size and shape, to collect and store water and nutrients for the colony to use. At the top of a colony, there are both pleated fan-shaped "nest" fronds that collect and hold water, and gutter-shaped "strap" fronds that channel water: no solitary Platycerium species has both types. At the bottom of a colony, there are "nest" fronds that clasp the trunk of the tree supporting the fern, and drooping photosynthetic fronds. These are argued to be adapted to support the colony structurally, i.e. that the individuals in the colony are to some degree specialized for tasks, a division of labor.[14][15][78]

Evolution

[edit]
Main article: Evolution of eusociality

Phylogenetic distribution

[edit]
Further information: Sociality

Eusociality is a rare but widespread phenomenon in species in at least seven orders in the animal kingdom, as shown in the phylogenetic tree (non-eusocial groups not shown). All species of termites are eusocial, and it is believed that they were the first eusocial animals to evolve, sometime in the Late Jurassic period (~150 million years ago).[79] The other orders shown contain both eusocial and non-eusocial species, including many lineages where eusociality is inferred to be the ancestral state. Thus the number of independent evolutions of eusociality (clades) is not known. The major eusocial groups are shown in boldface in the phylogenetic tree.

Eukaryotes

Paradox

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Prior to the gene-centered view of evolution, eusociality was seen as paradoxical: if adaptive evolution unfolds by differential reproduction of individual organisms, the evolution of individuals incapable of passing on their genes presents a challenge. In On the Origin of Species, Darwin referred to the existence of sterile castes as the "one special difficulty, which at first appeared to me insuperable, and actually fatal to my theory".[80] Darwin anticipated that a possible resolution to the paradox might lie in the close family relationship, which W.D. Hamilton quantified a century later with his 1964 inclusive fitness theory. After the gene-centered view of evolution was developed in the mid-1970s, non-reproductive individuals were seen as an extended phenotype of the genes, which are the primary beneficiaries of natural selection.[81]

Inclusive fitness and haplodiploidy

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Argument that haplodiploidy favors eusociality

[edit]
Further information: Inclusive fitness and Haplodiploidy
Honey bees are haplodiploid, with haploid males and diploid females. It has been suggested that this organisation favours eusociality, but haplodiploidy is neither necessary nor sufficient for eusociality to emerge.

According to inclusive fitness theory, organisms can gain fitness by increasing the reproductive output of other individuals that share their genes, especially their close relatives. Natural selection favors individuals to help their relatives when the cost of helping is less than the benefit gained by their relative multiplied by the fraction of genes that they share, i.e. when Cost < relatedness * Benefit. W. D. Hamilton suggested in 1964 that eusociality could evolve more easily among haplodiploid species such as Hymenoptera, because of their unusual relatedness structure.[82][83][84]

In haplodiploid species, females develop from fertilized eggs and males develop from unfertilized eggs. Because a male is haploid, his daughters share 100% of his genes and 50% of their mother's. Therefore, they share 75% of their genes with each other. This mechanism of sex determination gives rise to what W. D. Hamilton first termed "supersisters", more closely related to their sisters than they would be to their own offspring.[84] Even though workers often do not reproduce, they can pass on more of their genes by helping to raise their sisters than by having their own offspring (each of which would only have 50% of their genes). This unusual situation, where females may have greater fitness when they help rear sisters rather than producing offspring, is often invoked to explain the multiple independent evolutions of eusociality (at least nine separate times) within the Hymenoptera.[85]

Argument that haplodiploidy does not favor eusociality

[edit]

Against the supposed benefits of haplodiploidy for eusociality, Robert Trivers notes that while females share 75% of genes with their sisters in haplodiploid populations, they only share 25% of their genes with their brothers.[86] Accordingly, the average relatedness of an individual to their sibling is 50%. Therefore, helping behavior is only advantageous if it is biased to helping sisters, which would drive the population to a 1:3 sex ratio of males to females. At this ratio, males, as the rarer sex, increase in reproductive value, reducing the benefit of female-biased investment.[87]

Further, not all eusocial species are haplodiploid: termites, some snapping shrimps, and mole rats are not. Conversely, non-eusocial bees are also haplodiploid, and among eusocial species many queens mate with multiple males, resulting in a hive of half-sisters that share only 25% of their genes. The association between haplodiploidy and eusociality is below statistical significance.[88] Haplodiploidy is thus neither necessary nor sufficient for eusociality to emerge.[89] Relatedness does still play a part, as monogamy (queens mating singly) is the ancestral state for all eusocial species so far investigated.[90] If kin selection is an important force driving the evolution of eusociality, monogamy should be the ancestral state, because it maximizes the relatedness of colony members.[90]

Evolutionary ecology

[edit]

Increased parasitism and predation rates are the primary ecological drivers of social organization. Group living affords colony members defense against enemies, specifically predators, parasites, and competitors, and allows them to gain advantage from superior foraging methods.[9] The importance of ecology in the evolution of eusociality is supported by evidence such as experimentally induced reproductive division of labor, for example when normally solitary queens are forced together.[91] Conversely, female Damaraland mole-rats undergo hormonal changes that promote dispersal after periods of high rainfall.[92]

Climate too appears to be a selective agent driving social complexity; across bee lineages and Hymenoptera in general, higher forms of sociality are more likely to occur in tropical than temperate environments.[93] Similarly, social transitions within halictid bees, where eusociality has been gained and lost multiple times, are correlated with periods of climatic warming. Social behavior in facultative social bees is often reliably predicted by ecological conditions, and switches in behavioral type have been experimentally induced by translocating offspring of solitary or social populations to warm and cool climates. In H. rubicundus, females produce a single brood in cooler regions and two or more broods in warmer regions, so the former populations are solitary while the latter are social.[94] In another species of sweat bees, L. calceatum, social phenotype has been predicted by altitude and micro-habitat composition, with social nests found in warmer, sunnier sites, and solitary nests found in adjacent, cooler, shaded locations. Facultatively social bee species, however, which comprise the majority of social bee diversity, have their lowest diversity in the tropics, being largely limited to temperate regions.[95]

Multilevel selection

[edit]
Further information: Group selection

Once pre-adaptations such as group formation, nest building, high cost of dispersal, and morphological variation are present, between-group competition has been suggested as a driver of the transition to advanced eusociality. M. A. Nowak, C. E. Tarnita, and E. O. Wilson proposed in 2010 that since eusociality produces an extremely altruistic society, eusocial groups should out-reproduce their less cooperative competitors, eventually eliminating all non-eusocial groups from a species.[96] Multilevel selection has been heavily criticized for its conflict with the kin selection theory.[97]

Reversal to solitarity

[edit]

A reversal to solitarity is an evolutionary phenomenon in which descendants of a eusocial group evolve solitary behavior once again. Bees have been model organisms for the study of reversal to solitarity, because of the diversity of their social systems. Each of the four origins of eusociality in bees was followed by at least one reversal to solitarity, giving a total of at least nine reversals.[6][7] In a few species, solitary and eusocial colonies appear simultaneously in the same population, and different populations of the same species may be fully solitary or eusocial.[94] This suggests that eusociality is costly to maintain, and can only persist when ecological variables favor it. Disadvantages of eusociality include the cost of investing in non-reproductive offspring, and an increased risk of disease.[98]

All reversals to solitarity have occurred among primitively eusocial groups; none have followed the emergence of advanced eusociality. The "point of no return" hypothesis posits that the morphological differentiation of reproductive and non-reproductive castes prevents highly eusocial species such as the honeybee from reverting to the solitary state.[27]

Physiology and development

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Pheromones

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Pheromones play an important role in the physiological mechanisms of eusociality. Enzymes involved in the production and perception of pheromones were important for the emergence of eusociality within both termites and hymenopterans.[99] The best-studied queen pheromone system in social insects is that of the honey bee Apis mellifera. Queen mandibular glands produce a mixture of five compounds, three aliphatic and two aromatic, which control workers.[100] Mandibular gland extracts inhibit workers from constructing queen cells, which can delay the hormonally based behavioral development of workers and suppress their ovarian development.[101][100] Both behavioral effects mediated by the nervous system often leading to recognition of queens (releaser) and physiological effects on the reproductive and endocrine system (primer) are attributed to the same pheromones. These pheromones volatilize or are deactivated within thirty minutes, allowing workers to respond rapidly to the loss of their queen.[101]

The levels of two of the aliphatic compounds increase rapidly in virgin queens within the first week after emergence from the pupa, consistent with their roles as sex attractants during the mating flight.[100] Once a queen is mated and begins laying eggs, she starts producing the full blend of compounds.[100] In several ant species, reproductive activity is associated with pheromone production by queens.[100] Mated egg-laying queens are attractive to workers, whereas young winged virgin queens elicit little or no response.[100]

Physiology of eusociality in fire ants: three queen pheromones help to create and maintain the eusocial state of the colony. Loss of a primer pheromone triggers the development of replacement queens (dashed lines).[100][101]

Among ants, the queen pheromone system of the fire ant Solenopsis invicta includes both releaser and primer pheromones. A queen recognition (releaser) pheromone is stored in the poison sac along with three other compounds. These compounds elicit a behavioral response from workers. Several primer effects have also been demonstrated. Pheromones initiate reproductive development in new winged females, called female sexuals.[100] These chemicals inhibit workers from rearing male and female sexuals, suppress egg production in other queens of multiple queen colonies, and cause workers to execute excess queens.[100][101] These pheromones maintain the eusocial phenotype, with one queen supported by sterile workers and sexually active males (drones). In queenless colonies, the lack of queen pheromones causes winged females to quickly shed their wings, develop ovaries and lay eggs. These virgin replacement queens assume the role of the queen and start to produce queen pheromones.[100] Similarly, queen weaver ants Oecophylla longinoda have exocrine glands that produce pheromones which prevent workers from laying reproductive eggs.[101]

Similar mechanisms exist in the eusocial wasp Vespula vulgaris. For a queen to dominate all the workers, usually numbering more than 3000 in a colony, she signals her dominance with pheromones. The workers regularly lick the queen while feeding her, and the air-borne pheromone from the queen's body alerts those workers of her dominance.[102]

The mode of action of inhibitory pheromones which prevent the development of eggs in workers has been demonstrated in the bumble bee Bombus terrestris.[101] The pheromones suppress activity of the endocrine gland, the corpus allatum, stopping it from secreting juvenile hormone.[103] With low juvenile hormone, eggs do not mature. Similar inhibitory effects of lowering juvenile hormone were seen in halictine bees and polistine wasps, but not in honey bees.[101]

Other mechanisms

[edit]

A variety of other mechanisms give queens of different species of social insects a measure of reproductive control over their nest mates. In many Polistes wasps, monogamy is established soon after colony formation by physical dominance interactions among foundresses of the colony including biting, chasing, and food soliciting. Such interactions create a dominance hierarchy headed by larger, older individuals with the greatest ovarian development. The rank of subordinates is correlated with the degree of ovarian development.[101] Workers do not oviposit when queens are present, for a variety of reasons: colonies tend to be small enough that queens can effectively dominate workers; queens practice selective oophagy; the flow of nutrients favors queen over workers; and queens rapidly lay eggs in new or vacated cells.[101]

In primitively eusocial bees (where castes are morphologically similar and colonies are small and short-lived), queens frequently nudge their nest mates and then burrow back down into the nest. This draws workers into the lower part of the nest where they may respond to stimuli for cell construction and maintenance.[101] Being nudged by the queen may help to inhibit ovarian development; in addition, the queen eats any eggs laid by workers.[101] Furthermore, temporally discrete production of workers and gynes (actual or potential queens) can cause size dimorphisms between different castes, as size is strongly influenced by the season during which the individual is reared. In many wasps, worker caste is determined by a temporal pattern in which workers precede non-workers of the same generation.[104] In some cases, for example in bumblebees, queen control weakens late in the season, and the ovaries of workers develop.[101] The queen attempts to maintain her dominance by aggressive behavior and by eating worker-laid eggs; her aggression is often directed towards the worker with the greatest ovarian development.[101]

In highly eusocial wasps (where castes are morphologically dissimilar), both the quantity and quality of food are important for caste differentiation.[101] Recent studies in wasps suggest that differential larval nourishment may be the environmental trigger for larval divergence into workers or gynes.[104] All honey bee larvae are initially fed with royal jelly, which is secreted by workers, but normally they are switched over to a diet of pollen and honey as they mature; if their diet is exclusively royal jelly, they grow larger than normal and differentiate into queens. This jelly contains a specific protein, royalactin, which increases body size, promotes ovary development, and shortens the developmental time period.[105] The differential expression in Polistes of larval genes and proteins (also differentially expressed during queen versus caste development in honey bees) indicates that regulatory mechanisms may operate very early in development.[104]

[edit]
See also: Hive mind and Group mind (science fiction)

Stephen Baxter's 2003 science fiction novel Coalescent imagines a human eusocial organization founded in ancient Rome, in which most individuals are subject to reproductive repression.[106] Harold Fromm, reviewing Groping for Groups by E. O. Wilson and others in The Hudson Review, asks whether Wilson's stated "wish" for humans to bring about "a permanent paradise for human beings" would mean "to be group-selected in factories in the style of Huxley's [1932 novel] Brave New World.[107]

The 1973 novel Hellstrom's Hive by Frank Herbert revolves around a secret society made up entirely of a race of bioengineered insect-like humanoids that is modeled after the behavior of social insect species.[108]

See also

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References

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Eusociality

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Eusociality is the highest level of social organization in the animal kingdom, characterized by three defining features: cooperative brood care in which colony members care for offspring produced by others, overlapping adult generations within the colony, and a division of reproductive labor into castes that include non-reproductive individuals specializing in tasks such as foraging, defense, and nest maintenance. This social structure enables colonies to function as integrated units, often likened to superorganisms, where individual fitness is subordinated to collective success. Eusociality occurs in a diverse array of taxa, most prominently among insects in the order Hymenoptera (ants, bees, and wasps) and Isoptera (termites); the eusocial species in these groups together comprise over 15,000 species (e.g., more than 15,700 described ant species as of 2022) and dominate their respective ecological niches. Beyond these, it has been documented in other arthropods such as aphids, thrips, ambrosia beetles, and snapping shrimp (Synalpheus regalis), as well as in two mammalian species: the naked mole-rat (Heterocephalus glaber) and the Damaraland mole-rat (Fukomys damarensis). These eusocial societies exhibit remarkable complexity, with colonies ranging from hundreds to millions of individuals, and castes differentiated by morphology, behavior, and physiology—such as queens focused on reproduction and workers dedicated to non-reproductive roles. The represents a major transition in , having arisen independently at least 12 times in arthropods (plus at least twice in mammals) across metazoans, though it remains rare, occurring in fewer than 1% of described animal species. Key hypotheses for its origins include , proposed by , which posits that evolves when benefits outweigh direct reproduction, particularly amplified in haplodiploid where sisters share 75% of genes on average, favoring worker sterility to rear siblings. Other factors, such as ecological pressures like nest defense and stability, and genomic influences including sex determination systems, contribute to its repeated emergence, though debates persist on the primacy of versus ecological drivers.

Definition and Characteristics

Core Criteria

Eusociality is characterized by three core criteria originally proposed by Charles D. Michener in 1969 and popularized by in 1971: cooperative brood care, in which all or most group members participate in the rearing of young, including offspring produced by other individuals; overlapping generations within the colony, where adults from different generations coexist and interact; and reproductive division of labor, whereby a small proportion of individuals specialize in while the majority forgo it to support the colony. These traits distinguish eusocial species from less complex social systems by emphasizing collective investment in over individual efforts. Eusociality can manifest as strict (also termed or advanced) or facultative (also termed primitive), depending on the degree of permanence. In strict eusociality, such as in , morphological and behavioral castes are irreversibly differentiated early in development, locking individuals into lifelong roles like non-reproductive workers or queens. Conversely, facultative eusociality, observed in some sweat bees (e.g., in the Halictus), involves reversible roles where individuals can switch between reproductive and non-reproductive behaviors based on environmental conditions or colony needs, without fixed morphological castes. This reproductive division of labor sets eusociality apart from subsociality, where occurs but generations do not overlap extensively, or communal breeding, where multiple individuals reproduce without specialized non-reproductive helpers. A hallmark of eusociality is the irreversible differentiation of castes, particularly in advanced forms, where non-reproductive individuals lose the physiological capacity for independent reproduction. In highly eusocial colonies, the proportion of reproductive individuals is typically below 1%, underscoring the extreme skew in reproductive output that sustains colony-level fitness.

Castes and Division of Labor

In eusocial colonies, individuals are organized into distinct castes that specialize in specific roles, primarily reproductive and non-reproductive forms. Reproductive castes typically include , which focus on egg-laying, and in , kings that contribute to alongside queens. Non-reproductive castes consist of workers, which handle tasks such as and , and soldiers in some , which serve as defensive specialists with enlarged mandibles or chemical weaponry adapted for colony protection. These castes arise through developmental pathways influenced by environmental and genetic factors, ensuring a structured that supports colony function. Division of labor within eusocial societies is achieved through mechanisms like age-based polyethism, where young workers typically perform indoor tasks such as nursing brood or maintaining the nest, while older individuals transition to riskier outdoor activities like . Task allocation can also depend on morphological traits, such as body size, with larger individuals often assigned to demanding roles like defense or resource transport, and smaller ones to precision tasks like brood care. This temporal and size-based partitioning allows colonies to adapt dynamically to changing needs without requiring constant reconfiguration of roles. Caste dimorphism is evident in morphological differences that reinforce specialization, such as the contrast between alates—winged reproductives equipped for dispersal and founding—and apterous workers in , which lack wings and are optimized for sustained labor within the nest. These physical distinctions, including variations in body size, development, and sensory structures, align members with their primary functions, minimizing overlap in capabilities. Specialization through castes and division of labor enhances fitness by improving overall efficiency in and task execution, allowing colonies to process food, defend territories, and rear offspring more effectively than if individuals multitasked. This structured organization reduces redundancy and energy waste, enabling larger sizes and greater productivity, as specialized workers outperform generalists in their assigned roles. By linking brood care to these divisions, colonies achieve sustained growth and resilience to environmental pressures.

Historical Development

Early Observations

Early observations of eusocial behaviors in date back to ancient times, with providing one of the first detailed accounts in his during the 4th century BCE. He described honeybee hives as organized communities featuring a single "king" bee (now recognized as the queen), numerous workers engaged in foraging and comb-building, and larger drones, emphasizing their cooperative labor and hierarchical structure without reproducing young themselves. noted the bees' collective defense of the hive and division of tasks, such as nursing larvae and gathering , portraying the society as a model of natural order. In the 18th and 19th centuries, European naturalists expanded these descriptions through direct fieldwork on and colonies. , in (1859), highlighted the complexity of and honeybee societies, particularly the existence of sterile worker castes that forgo reproduction to support the fertile queen and males. Darwin viewed these neuter as a significant challenge to , questioning how traits beneficial only to the colony could evolve since sterile individuals could not pass them on directly. He observed overlapping generations within hives, where workers cared for the queen's offspring, underscoring the in these groups. Jean-Henri Fabre's meticulous observations in the late 19th century, detailed in his multi-volume Souvenirs Entomologiques (1879–1907), focused on the instinctive behaviors of social insects like bees and ants without invoking evolutionary theory. Fabre documented nest construction, where workers collaboratively excavate chambers and provision cells with food, and foraging expeditions that demonstrated coordinated group efforts to locate and transport resources. His accounts emphasized the precision of these activities, such as the architectural symmetry in bee combs and the defensive strategies of ant colonies against intruders, revealing the depth of social integration in everyday routines. Carlo Emery's studies around the turn of the further illuminated interactions within societies, particularly through his examinations of . In works like his 1909 paper on the origins of dulotic and parasitic s, Emery described how certain species infiltrate host colonies, exploiting the division of labor by coercing workers to rear their brood. These early empirical insights into observable behaviors—such as communal brood care and —laid the groundwork for understanding eusocial organization prior to theoretical frameworks.

Formulation of the Concept

The foundational framework for understanding eusociality as a distinct level of in was laid by William Morton Wheeler in his 1928 book The Social : Their Origin and Evolution, which synthesized early observations of communal behaviors in , , wasps, and into a cohesive evolutionary narrative emphasizing polymorphism, division of labor, and colony integration as key social traits. Wheeler's work, drawing on descriptive accounts of bee hives and other colonies from the 19th and early 20th centuries, proposed pathways like the "subsocial" route—where evolves into group cohesion—providing the conceptual groundwork that later researchers built upon to categorize advanced . This framework contributed to the development of the concept of eusociality, with the term first coined by Suzanne W. T. Batra in 1966 to describe nesting behavior in halictine bees. It was formalized by Edward O. Wilson in his seminal 1971 book The Insect Societies, where he defined it as the highest stage of social evolution characterized by three core criteria: cooperative brood care (including care of offspring by non-parents), overlapping generations within a , and a reproductive division of labor with castes that include sterile individuals altruistically aiding reproduction. Wilson's synthesis integrated , , and comparative studies of hymenopterans and , distinguishing eusociality from lower social grades like subsociality or communal breeding, and emphasized its rarity and adaptive significance in insect societies. In the decades following Wilson's formulation, refinements emerged to broaden the concept's applicability, particularly through the work of Joan E. Strassmann and David C. Queller in the , who expanded the criteria to encompass facultative eusociality in primitively social wasps—where workers can reproduce under certain conditions—and advocated for its extension beyond to other taxa exhibiting similar traits, such as high relatedness and reproductive skew in non-hymenopteran groups. These expansions highlighted that eusociality need not be obligate or morphologically rigid, allowing for transitional forms observed in field studies of wasps like species. A key debate in the 1990s centered on the inclusivity of eusociality for non-insect arthropods, exemplified by whether sponge-dwelling snapping shrimp (Synalpheus spp.) qualified, given their marine habitat and lack of traditional insect-like castes; this was resolved through empirical studies demonstrating that species like Synalpheus regalis met Wilson's criteria, with a single breeding female, non-reproductive helpers cooperatively defending sponge colonies, and overlapping generations, thus confirming eusociality's occurrence outside terrestrial insects.

Taxonomic Diversity

In Hymenopterans

Eusociality is particularly prevalent in the order Hymenoptera, encompassing ants, bees, and wasps, where it is exhibited by over 15,700 described ant species as of 2022, along with numerous eusocial bees and wasps, representing a significant portion of the order's approximately 160,000 described species. Eusociality has arisen through multiple independent evolutionary origins from solitary ancestors. For instance, in bees, eusociality evolved from solitary forebears around 100 million years ago during the Late Cretaceous. Characteristic features of eusocial include female-biased sex ratios, often approaching 3:1 in favor of females at the population level, which aligns with their haplodiploid genetic system where females develop from fertilized eggs and males from unfertilized ones. in these societies demonstrate exceptional , with some queens surviving up to 30 years, far exceeding the lifespan of workers. Worker sterility is typically enforced through policing behaviors, where workers detect and remove eggs laid by other workers, thereby suppressing and maintaining the queen's monopoly on . Variations in eusociality exist across Hymenopteran taxa, ranging from primitive to advanced forms. In primitive eusociality, as seen in halictid bees (family ), castes are flexible and reversible, with individuals capable of transitioning between reproductive and non-reproductive roles based on age, dominance, or environmental cues, allowing all females to potentially reproduce under certain conditions. In contrast, advanced eusociality in honeybees (Apis mellifera) features morphologically distinct, permanently sterile workers that forgo entirely, dedicating their lives to , , and nest maintenance. Colony sizes in eusocial vary dramatically, reflecting ecological adaptations and social complexity. Primitive eusocial wasps, such as those in the genus , typically form small colonies of dozens of individuals, often starting with a single foundress and growing to 20–100 workers. At the other extreme, army ants (subfamily Dorylinae) maintain massive colonies numbering in the millions, enabling coordinated swarm raids that overwhelm prey in tropical forests.

In Termites

Termites (order Blattodea, clade Isoptera) represent one of the major independent origins of eusociality among insects, with over 3,100 described species as of 2024, all exhibiting this social organization. Unlike many other eusocial lineages, termites evolved from cockroach-like ancestors approximately 150 million years ago during the Jurassic period, making their sociality a deep-rooted trait that predates that of hymenopterans. Eusocial termite colonies are characterized by overlapping generations, cooperative brood care, and a reproductive division of labor, with a primary breeding pair consisting of a king and a long-lived queen that together found and sustain the colony. A distinctive feature of eusociality is the diploid inheritance system, which contrasts with the of hymenopterans, yet supports complex differentiation. The colony includes true worker s—sterile immatures dedicated to , nest maintenance, and brood care—though in some lower , these workers retain developmental plasticity and can become neotenic reproductives (immature secondary reproductives) if the primary pair dies. Soldiers form a specialized defensive , comprising 1-5% of the colony population, equipped either with enlarged mandibles for snapping attacks against intruders or nasute heads that deploy sticky chemical secretions for ranged defense. Termite nests, often constructed from soil, saliva, and feces, exhibit remarkable architectural complexity, particularly in mound-building species, where towering structures up to 8 meters high facilitate thermoregulation through passive ventilation systems that maintain internal temperatures around 30°C regardless of external fluctuations. These mounds also provide structural defense, forming a hardened outer barrier against predators and environmental hazards. In advanced termites like those in the genus Macrotermes, cooperative foraging involves workers collectively harvesting grass and wood outside the nest, which is then used to cultivate symbiotic fungi (Termitomyces spp.) in subterranean comb gardens, enabling efficient decomposition of lignocellulose for colony nutrition.

In Other Insects and Arthropods

Eusociality is exceedingly rare among insects outside the orders and Isoptera, with fewer than 100 known exhibiting the full suite of traits including cooperative brood care, overlapping generations, and reproductive division of labor. This scarcity highlights the exceptional evolutionary constraints and ecological niches required for such complex to emerge independently multiple times. Eusociality has also evolved in (order Thysanoptera), with approximately 7 exhibiting the trait, primarily in gall-forming such as those in the genera Oncothrips and Kladothrips. These colonies feature sterile castes that defend against intruders using modified forelimbs, facilitated by parthenogenetic in enclosed galls. In Coleoptera, the only obligately eusocial is the Australian ambrosia weevil , where colonies inhabit fungal-cultivated galleries in wood, featuring a single breeding female, non-reproductive helpers that forage and maintain the fungus garden, and overlapping generations. Unlike hymenopterans or termites, A. incompertus lacks distinct morphological castes, relying instead on behavioral defense by workers, with eusociality likely facilitated by the stable, defended wood-boring and diploid inheritance system. Among (Hemiptera: ), eusociality has evolved in several gall-inducing , particularly in the genus , where clonal reproduction through enables the production of sterile defender castes. In like Pemphigus spyrothecae, a foundress aphid induces a root on poplar , producing genetically identical offspring that differentiate into normal morphs for reproduction and sterile soldiers with enlarged forelegs for grasping and piercing intruders. These soldiers defend the gall against predators such as syrphid fly larvae, often sacrificing themselves in aggressive attacks, while the colony grows through multiple generations before winged dispersers emerge to found new galls. Approximately six to eleven aphid display true eusociality, with soldier castes comprising up to 20-50% of the colony in high-predation environments, underscoring the role of predation pressure and clonal genetics in promoting . In arthropods beyond insects, eusociality occurs in the family , specifically sponge-dwelling snapping of the genus Synalpheus. Eusocial colonies, first documented in the , consist of a dominant (typically a large female and male) and numerous non-breeding subordinates that perform guarding, cleaning, and brood care within the protected cavities of tropical marine s. Species such as Synalpheus regalis exhibit extreme reproductive skew, with subordinates—often siblings—refraining from reproduction and using their enlarged snapping claws for territorial defense against intruders, including conspecifics from other colonies. This marine eusociality, which has arisen at least four times independently within Synalpheus, is maintained by high sponge habitat patchiness, intense competition, and kin-structured colonies, paralleling systems but adapted to an aquatic context without .

In Non-Arthropods

Eusociality is exceptionally rare among vertebrates, having evolved independently in only two genera of within the family Bathyergidae approximately 25 million years ago during the early . These include the (Heterocephalus glaber) and the (Fukomys damarensis, formerly Cryptomys damarensis), which exhibit the core traits of eusociality: cooperative brood care, reproductive division of labor with a single breeding queen and non-reproductive workers, and overlapping generations within colonies. In colonies, typically comprising 70–300 individuals, a single queen monopolizes , often through aggressive suppression of subordinates, while workers of both sexes perform , burrow maintenance, and pup care without reproducing. Breeding often involves incestuous mating between the queen and a related , maintaining high genetic relatedness that supports . Similarly, colonies, usually smaller with 10–25 members, feature a dominant and non-breeding helpers that contribute to colony tasks, with likewise skewed toward a single female and occasional among close kin. This mammalian eusociality contrasts with forms by occurring in diploid organisms adapted to arid, subterranean environments where digging and defense enhance survival. Beyond vertebrates, eusocial-like organization appears in trematode flatworms (phylum Platyhelminthes), particularly in larval stages within hosts. In species such as Himasthla elongata, clonal parthenogenetic larvae (parthenitae) form colonies exhibiting division of labor, with specialized non-reproductive "soldier" castes that aggressively defend against predators and rivals, while reproductive castes focus on producing transmission stages (cercariae) for infecting the next host. These castes differ morphologically—soldiers are larger with robust mouths for —and behaviorally, with soldiers comprising about 10–20% of the colony to optimize transmission success in the confined environment. This represents one of the few non-arthropod examples of eusocial traits, driven by intense within hosts. Eusociality in plants remains highly debated and unconfirmed, with no widely accepted cases meeting all core criteria. Some researchers have proposed primitive eusociality in colonial epiphytic ferns like Platycerium bifurcatum (staghorn fern), where sterile vegetative fronds (workers) perform resource acquisition and support, while fertile fronds (reproductives) specialize in spore production, suggesting division of labor in overlapping generations. However, this interpretation is contested, as it lacks irreversible caste differentiation and cooperative brood care equivalent to animal systems. Similarly, insect-induced galls on plants, such as those formed by fig wasps (Agaonidae) in fig syconia, have been analogized to eusocial structures due to the enclosed community dynamics, but these reflect insect sociality extended into plant tissue rather than true plant eusociality. In Acacia plants, symbiotic relationships with eusocial ants (e.g., Pseudomyrmex species) provide an extended phenotype where the plant manipulates ant behavior for defense via nectar and domatia, but this mutualism does not confer eusociality to the plant itself. Overall, plant "eusociality" claims emphasize clonal growth and specialization but fall short of the full definition.

Evolutionary Origins

Phylogenetic Distribution

Eusociality has arisen independently at least 15 times across metazoans, primarily in arthropods, with estimates varying due to ongoing phylogenetic refinements. In insects, this includes at least 10 origins: nine within the order Hymenoptera (encompassing ants, bees, and wasps, with recent genomic studies suggesting up to four in bees alone, two in vespid wasps, and one in ants) and one in termites (order Blattodea, formerly Isoptera). Additional independent evolutions have occurred at least three times in the crustacean genus Synalpheus (snapping shrimps), as well as in other arthropods including aphids (Hemiptera), thrips (Thysanoptera), and ambrosia beetles (Coleoptera). In mammals, eusociality evolved once in the rodent family Bathyergidae, represented by the naked mole-rat (Heterocephalus glaber) and Damaraland mole-rat (Fukomys damarensis). These multiple origins highlight eusociality as a convergent evolutionary strategy, appearing in disparate lineages despite varying ecological contexts. The fossil record offers direct evidence of early eusociality through specimens preserved in from the Lower , dating to approximately 99 million years ago. These fossils include soldier with specialized morphologies for colony defense and worker engaged in or combat, indicating advanced division of labor and cooperative behaviors characteristic of eusocial societies. Such findings demonstrate that eusociality in and predates the -Paleogene (K-Pg) by tens of millions of years, with no earlier unequivocal fossil evidence identified. Genomic phylogenies further refine the timing of eusociality's emergence in , revealing that it arose in some lineages after the K-Pg boundary (66 million years ago), coinciding with post-extinction ecological opportunities. For instance, ancestral state reconstructions in bees suggest transitions to eusociality occurred during the , following the diversification of flowering . These molecular estimates align with the data, portraying eusociality as an adaptive response to shifting environments across , with recent 2025 analyses indicating even higher numbers of origins within bee families like (at least 11). Eusociality's phylogenetic distribution is heavily skewed toward holometabolous insects—those undergoing complete , such as and —where larval and adult stages enable specialized castes and division of labor. This pattern is rare outside arthropods, absent in most s, with the notable exception of the aforementioned eusocial , which represent a singular vertebrate instance. Overall, the trait's sporadic occurrence underscores its evolutionary lability, confined to taxa with predisposing life-history traits like and nest fidelity.

The Paradox of Eusociality

Eusociality presents a profound evolutionary because it involves the and persistence of non-reproductive castes, such as workers, that forgo personal entirely, thereby reducing their direct fitness to zero while aiding the of others in the . This apparent conflict with the principles of , where traits enhancing individual are favored, has long puzzled biologists, as sterile individuals seemingly contribute nothing to their own genetic lineage through direct descent. Charles Darwin first articulated this challenge in 1859, describing the sterile "neuter" insects in and colonies as "one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory." He noted that could not act on these individuals if they produced no offspring, yet their widespread occurrence across social insect lineages demanded an explanation compatible with his framework. Empirical studies reveal that while high genetic relatedness within colonies—often resulting from or haplodiploid sex determination—facilitates , it alone does not fully account for the extreme sterility observed in eusocial species, as unresolved kin conflicts over reproduction persist and would favor cheaters who reproduce selfishly. Mathematical models demonstrate that sterility requires not only elevated relatedness but also mechanisms to suppress individual reproductive attempts, highlighting the incompleteness of relatedness as an explanatory factor without additional ecological or genetic stabilizers. In contrast to simpler forms of sociality, such as subsocial or primitively eusocial systems, where individuals frequently revert to solitary reproduction under changing conditions, transitions away from advanced eusociality with fixed sterile castes are exceedingly rare, underscoring the paradox's depth and the colony-level commitments that lock in this trait. For instance, in halictine bees exhibiting primitive eusociality, reversals to solitarity have occurred at least 12 times, whereas no such reversals are documented in advanced eusocial lineages like ants or honeybees.

Theories of Evolution

Inclusive Fitness and Kin Selection

Inclusive fitness represents an organism's total genetic contribution to the next generation, encompassing not only direct fitness through personal but also indirect fitness gained by aiding the reproductive success of genetic relatives, weighted by the coefficient of relatedness rr. This concept, introduced by , extends classical Darwinian fitness by accounting for the propagation of genes via , thereby providing a mechanism for the evolution of where individuals sacrifice their own to benefit relatives. Hamilton's rule, rB>CrB > C, quantifies the condition under which a social behavior evolves, where rr is the genetic relatedness between and recipient, BB is the fitness benefit to the recipient, and CC is the fitness to the . The rule derives from a genetical model of social behavior, where the change in frequency of a gene promoting altruism is positive if the inclusive fitness effect—rB−CrB - C—exceeds zero; this follows from partitioning the total fitness effect into direct and indirect components, using the Price equation to show that selection favors alleles increasing the weighted sum of effects on all individuals' fitness. In eusocial colonies, the rule applies to workers forgoing reproduction to help raise sisters: a worker's CC of sterility is offset if the benefit BB to the queen's production of additional sisters, multiplied by the worker's relatedness rr to those sisters, yields greater indirect fitness gains than the worker could achieve by reproducing independently. Experimental evidence from honey bees supports , as behaviors align with predictions by preferentially removing worker-laid eggs to favor queen production. Such findings demonstrate that worker , policed to align with , directly boosts colony-level fitness by prioritizing queen-reared . The framework of and extends to diploid eusocial taxa like , where high paternity skew maintains elevated average relatedness among colony members to satisfy Hamilton's rule despite symmetric . Termite colonies typically arise from lifelong monogamous pairs, resulting in full-sibling relatedness of r=0.5r = 0.5, equivalent to a worker's relatedness to its own ; however, extreme paternity skew in species with occasional multiple ensures most share a single father, preventing relatedness asymmetry and sustaining indirect fitness benefits that favor worker sterility when benefits BB from cooperative brood care exceed twice the reproductive cost CC. This mechanism underscores 's generality across genetic systems, resolving the of eusociality in diploids through assured high relatedness.

Haplodiploidy Hypothesis

The sex determination system, characteristic of the order (including , bees, and wasps), produces males from unfertilized haploid eggs and females from fertilized diploid eggs. Under this system, full sisters share 75% of their genes on average due to identical paternal contributions, whereas sisters share only 25% relatedness with brothers, creating an asymmetry in genetic relatedness among siblings. This asymmetry forms the basis of the haplodiploidy hypothesis, proposed by , which posits that the elevated relatedness between sisters favors the of worker sterility in females, as they gain greater by raising sisters (reproductives) rather than their own offspring. Hamilton argued that this predisposition explains the higher incidence of eusociality in haplodiploid insects compared to diploids. Critics, however, note that eusociality has independently evolved in diploid organisms like termites, where no such relatedness asymmetry exists, undermining the hypothesis as a necessary condition for eusociality. Additionally, multiple mating by queens (polyandry), common in many eusocial hymenopterans, reduces average sister-sister relatedness toward 50% or lower by introducing half-sisters, thereby diminishing the predicted advantage for altruism. Empirical support for the comes from observations of sex investment ratios in unmanipulated colonies. Trivers and Hare found that ant colonies allocate reproductive resources in a 3:1 female-to-male bias, aligning with the prediction that workers, more related to sisters, favor female production to maximize . This bias has been corroborated in various hymenopteran species, though its consistency varies with mating frequency and colony structure.

Multilevel Selection

Multilevel selection theory posits that operates simultaneously at multiple hierarchical levels, including both individuals and groups, to explain the of complex social behaviors like eusociality. In eusocial species, individual-level selection often favors selfish reproduction, but group-level (-level) selection promotes the success of colonies composed largely of altruists, as these groups outcompete less cooperative ones by enhancing overall productivity, defense, and survival. This framework views the colony as a where selection favors traits that benefit the collective unit, even at the expense of individual fitness components. A significant advancement in this approach came from Edward O. Wilson and colleagues in 2010, who shifted emphasis toward trait-group models, arguing that eusociality arises primarily through acting on colony-level traits rather than solely through gene-centered . In their model, eusociality evolves from solitary or subsocial ancestors via multilevel selection on pre-existing cooperative behaviors in ecologically favorable conditions, with the colony functioning as the primary . This perspective highlights how inter-colony competition drives the fixation of altruistic traits within groups, resolving aspects of the paradox of eusociality by prioritizing group dynamics over individual relatedness alone. Supporting evidence from computational models demonstrates that eusociality evolves more rapidly under multilevel selection in viscous populations, where limited dispersal maintains high local relatedness and reduces between-group mixing. For instance, simulations of structured populations show that sterile altruistic emerge and persist when migration rates are low, as colony-level benefits from outweigh individual-level costs, particularly in haplo-diploid and diploid systems. These models indicate that population viscosity amplifies group selection's effect, allowing eusocial traits to spread faster than under panmictic conditions. Multilevel selection integrates synergistically with kin selection to explain mechanisms like worker policing and conflict resolution in eusocial colonies. While kin selection accounts for the genetic incentives for altruism among relatives, multilevel selection elucidates how colony-level enforcement—such as policing egg-laying by subordinates—resolves reproductive conflicts to maximize group fitness, creating a feedback loop that stabilizes eusociality. This combined approach underscores how individual and group selections interact to maintain harmony in highly integrated societies like those of ants and bees.

Mechanisms and Regulation

Ecological Influences

Ecological pressures play a pivotal role in the origin and maintenance of eusociality by favoring group living over solitary reproduction in challenging environments. One key factor is the concept of assured fitness returns, where individuals delay dispersal from the natal nest in harsh conditions, opting to help rear siblings rather than risk independent reproduction with uncertain success. In such scenarios, high mortality rates for solitary foundresses—due to predation, resource scarcity, or environmental stressors—make solo nesting improbable, whereas contributing to an established colony guarantees at least partial inclusive fitness benefits through shared brood survival. This mechanism is particularly evident in primitively eusocial wasps and bees, where offspring remain philopatric, enhancing colony persistence amid unpredictable habitats. Group living in eusocial species also provides robust defense against predators and parasites, reducing individual risk through collective vigilance and specialized castes. In and , colony members form barriers, alarm systems, and soldier castes that deter intruders, such as rival or vertebrate predators, far more effectively than solitary individuals could. For instance, soldiers use chemical signals and physical aggression to protect nests from kleptoparasites and competitors, minimizing mortality in dense, high-risk habitats. This defensive advantage sustains large colonies, as the dilution of risk across group members outweighs solitary evasion strategies, contributing to the evolutionary stability of eusociality in these lineages. The spatial distribution of resources further influences eusocial colony formation, with clumped food sources promoting aggregation and cooperative exploitation. In , access to concentrated wood resources necessitates durable nests defended by workers, fostering division of labor and overlap of generations. Similarly, thrive in environments where prey or seeds are patchily distributed, allowing colonies to monopolize and store these resources through teams, which solitary could not efficiently secure. Such ecological structuring selects for eusocial traits by linking resource defense to group size, as larger collectives better exploit and protect ephemeral, high-value patches. Recent studies highlight how climate variability modulates facultative eusociality in bees, where environmental fluctuations drive shifts between solitary and social phenotypes. In species like Exoneura robusta, lower temperatures trigger increased social behaviors, such as co-nesting and alloparenting, enhancing survival during cooler, resource-scarce periods. Facultatively social sweat bees (Halictus rubicundus) exhibit latitudinal variation in sociality, with cooler, more variable climates favoring group formation to buffer against floral dearth and temperature extremes. Climate change exacerbates these dynamics, potentially reorganizing social expression in bees by altering phenological cues and resource predictability, thus influencing the prevalence of eusocial traits across populations.

Physiological and Developmental Controls

In eusocial insects, queen pheromones serve as proximate signals to maintain reproductive division of labor by suppressing worker reproduction. In honeybees (Apis mellifera), the queen mandibular (QMP), a blend of volatile fatty acids primarily consisting of 9-ODA, 9-HDA, HOB, and methyl p-hydroxybenzoate, is secreted from the 's mandibular glands and distributed throughout the via worker . QMP inhibits ovarian development in workers, preventing vitellogenin synthesis and , thereby enforcing sterility in the worker . This suppressive effect is dose-dependent and reversible upon queen removal, as demonstrated in experiments where synthetic QMP application to queenless workers reduced egg-laying rates by over 90%. Juvenile hormone (JH) regulates caste-specific physiology and behavior in eusocial , decoupling reproductive and somatic functions. In honeybee workers, elevated JH titers during the middle-to-late adult stage promote the transition from in-hive tasks to , enhancing locomotor activity and collection efficiency without stimulating reproduction. Conversely, low JH levels in workers correlate with reproductive activation in queenless conditions, where JH application suppresses ovary development and laying worker emergence. In queens, sustained high JH titers throughout adulthood support continuous and egg production, contrasting with the prereproductive role of JH in solitary ancestors. These patterns highlight JH's evolutionary repurposing for behavioral maturation in workers while retaining gonadotropic functions in reproductives. Nutritional cues during larval stages drive determination in through differential feeding by workers, influencing developmental trajectories without fixed genetic predestination. In like Reticulitermes spp., larvae destined for queen (reproductive) development receive protein-rich diets from worker trophallaxis, elevating JH synthesis and promoting (winged reproductive) morphogenesis with extended growth periods and larger ovaries. In contrast, underfed larvae default to worker or castes, characterized by abbreviated development and sterile morphology. This plasticity is evident in colony-founding experiments, underscoring nutrition's role in modulating JH-mediated . Recent research from the 2020s has revealed that stress-related hormones, such as biogenic amines including and corazonin, modulate flexibility in facultatively eusocial species, allowing adaptive shifts between solitary and cooperative phenotypes. Similarly, corazonin signaling influences identity transitions in like Harpegnathos saltator, where stress-induced upregulation promotes (worker-reproductive) reversion, increasing colony resilience. These findings indicate that integrate ecological pressures with physiological plasticity, enabling reversible expression in species with optional eusociality.

Molecular and Genomic Basis

The molecular and genomic basis of eusociality involves intricate gene regulatory networks that underpin differentiation and social behaviors in insects. In honeybees (Apis mellifera), transcriptomic studies have identified vitellogenin (vg) as a key molecular signature for queen development, with its expression in the brain distinguishing queens from workers during larval stages.00797-0) Insulin signaling pathways, including the insulin receptor substrate (IRS) and target of rapamycin (TOR), integrate nutritional cues to regulate fate, promoting queen determination under high-nutrient conditions while suppressing it in workers.31569-2) These pathways interact with epigenetic modifications, such as m6A methylation, to fine-tune during larval development, ensuring stable -specific phenotypes.31569-2) Recent single-cell analyses further reveal that vitellogenin and insulin-related transcripts form part of broader networks influencing neuronal differentiation in eusocial brains. Noncoding RNAs (ncRNAs) play crucial epigenetic roles in maintaining caste structures across , modulating without altering DNA sequences. In species like and , long noncoding RNAs (lncRNAs) and microRNAs (miRNAs) regulate developmental transitions by targeting transcription factors involved in caste polyphenism. Piwi-interacting RNAs (piRNAs) specifically contribute to caste maintenance by silencing transposable elements—selfish genetic components that could disrupt social harmony—through epigenetic mechanisms like in germ cells and somatic tissues. A 2024 review highlights how these piRNAs prevent genomic instability in eusocial colonies, where high relatedness amplifies the need for suppressing selfish elements to preserve cooperative behaviors. Phylogenomic analyses of eusocial genomes reveal signatures of adaptive , including expansions in families associated with sensory and immune functions essential for life. In , , and , olfactory receptor have undergone significant duplications, enabling enhanced chemical communication for task allocation and nestmate recognition. Similarly, immunity-related families, such as those encoding and Toll-like receptors, show expansions in social , reflecting the selective pressures of dense living and exposure. These patterns, identified through comparative phylogenomics across Hymenopteran and Isopteran lineages, indicate that eusociality drives convergent genomic innovations for social . Genomic evidence also illuminates mechanisms resolving and conflict in eusocial societies, particularly in where policing behaviors suppress worker to maintain stability. Studies identify candidate policing genes, such as those involved in egg recognition and , with caste-specific expression patterns that enforce reproductive and mitigate intragenomic conflicts over resource allocation. In Reticulitermes flavipes, genomic scans reveal signatures of selection on these loci, supporting their role in and the of worker sterility. Such findings underscore how genomic adaptations promote harmony in diploid eusocial systems like , complementing in haplodiploid .

Transitions and Variations

From Solitary Ancestors

The evolution of eusociality from solitary ancestors is inferred through comparative analyses of extant species and limited fossil evidence, revealing a stepwise progression rather than abrupt shifts. In many lineages, particularly within the , the transition begins with subsocial care, where solitary females provide extended maternal provisioning to offspring, creating opportunities for delayed dispersal and interaction among siblings. This phase prefigures communal nesting, in which multiple females share nests without pronounced reproductive dominance, as seen in parasocial groups. These stages culminate in primitive eusociality, characterized by initial division of labor and reproductive skew, often retaining flexibility as evidenced by halictid bees (family ), where allows switches between solitary and social phenotypes in response to environmental cues, highlighting the reversibility of early social stages. Fossil records provide snapshots of these transitions, with amber inclusions from (ca. 99 million years ago) preserving early eusocial and exhibiting morphological castes indicative of role specialization, such as wingless workers, suggesting that solitary nesting behaviors in stem-group ancestors facilitated the emergence of cooperative brood care by the era. Comparative phylogenomics further supports this pathway, showing that eusociality in bees and wasps arose multiple times from solitary forebears, with genomic signatures of relaxed selection on solitary-specific genes in social derivatives. At the origins of role specialization, recent experimental work demonstrates how early-season helping behaviors in primitively eusocial wasps yield increasing returns to cooperation; in , helpers contribute disproportionately to colony founding, enhancing productivity beyond solitary efforts and stabilizing the shift toward reproductive division of labor.00088-9) Solitary ancestors often possessed genetic predispositions, such as nesting and maternal care behaviors, that pre-adapted them to sociality by promoting philopatry and resource defense in overlapping generations. For instance, genomic comparisons in bees reveal that solitary species already express genes for nest construction and offspring guarding, which were co-opted in eusocial lineages to support cooperative foraging and defense, underscoring how ancestral solitary traits lowered the threshold for group formation. Initial barriers to eusociality, including reproductive conflicts among co-foundresses over nest inheritance, were likely overcome through the evolution of mechanisms that favor helping full siblings, reducing selfish behaviors and promoting colony cohesion from the outset. In wasps, olfactory cues learned early in life enable precise of relatives, mitigating potential or resource theft in nascent groups and facilitating the stable emergence of helping roles.

Reversals to Solitarity

Reversals to solitarity represent a rare but documented evolutionary phenomenon in eusocial lineages, where descendants of social groups transition back to solitary reproduction and foraging behaviors, often in primitively eusocial taxa. In bees, such reversals are particularly well-studied due to the lability of social organization in groups like the Halictidae (sweat bees and allies). Phylogenetic analyses of the genus Lasioglossum (subgenus Evylaeus) reveal at least 12 independent losses of eusociality, with species reverting to solitary nesting after initial transitions to cooperative brood care. Similarly, a comprehensive genomic study of sweat bees documents multiple independent origins of eusociality, with at least four in Augochlorini, followed by multiple losses, including over a dozen in related halictine clades, highlighting the repeated discarding of social traits such as worker sterility and cooperative foraging in favor of individual reproduction. In , reversals are less common but manifest through the of social parasitism, where eusocial ancestors lose key social structures like the worker . For instance, in the genus Formica, permanent inquiline parasites such as certain Formica have evolved from temporary parasitic ancestors by completely forgoing worker production, relying instead on host colonies to rear sexual ; this represents a degeneration to a solitary-like reproductive strategy within a parasitic context. Facultative eusociality in some further illustrates conditional reversals at the level, where individuals shift to solitary in environments with abundant resources, reducing the fitness advantages of . Mechanisms driving these reversals often involve ecological shifts that diminish the benefits of eusociality, such as improved resource availability or reduced predation pressure, which favor solitary reproduction over cooperative investment. In primitively eusocial bees, this can lead to the suppression of worker caste development, resulting in neotenic reproductives—females that retain reproductive capability without specializing as non-reproductive helpers. Evolutionary models demonstrate that even modest advantages to solitary strategies, such as higher individual fecundity in low-risk environments, can facilitate rapid invasions by solitary lineages, promoting the breakdown of reproductive division of labor. The evolutionary lability of eusociality is evident in its asymmetric transition rates, with phylogenetic reconstructions and simulations indicating that losses of sociality occur more frequently than gains in primitively eusocial haplodiploid insects like halictid bees, though reversals are common in early, primitive stages. Recent 2024 genomic analyses of Lasioglossum species reveal convergent changes in enhancer activity associated with independent losses of sociality, further illustrating the genetic flexibility underlying these transitions. These patterns challenge earlier views of eusociality as a "point of no return," emphasizing its dependence on ecological contexts and the potential for social traits to be shed when they no longer confer survival advantages.

Eusociality in Humans and Culture

Analogies in Human Societies

In his 2012 book The Social Conquest of Earth, biologist proposed that humans display eusocial-like traits, characterized by division of labor, cooperative group living, and altruistic behaviors that enhance collective survival, drawing parallels to the advanced sociality seen in insects such as and . Wilson argued that these features emerged through multilevel selection pressures acting on both individuals and groups, positioning human societies as a form of incipient eusociality adapted to our species' unique . Critics, however, contend that human social structures do not meet the core biological criteria for eusociality, such as fixed, morphologically distinct castes and the monopolization of reproduction by a limited number of individuals across overlapping generations; instead, is broadly distributed, and roles are fluid and culturally influenced rather than genetically rigid. Furthermore, —through learned behaviors, norms, and institutions—dominates human social dynamics, overriding the instinctual, kin-based mechanisms typical of eusocial and rendering direct analogies problematic. Supporting evidence for partial parallels includes observations from societies, where cooperative child-rearing and non-reproductive roles foster group cohesion; for instance, among the Hadza of , grandmothers play a crucial role in provisioning weaned children with calorie-rich foods, enabling mothers to resume reproduction sooner and increasing overall grandchild survival rates by up to 50%. This "" illustrates and division of labor in human bands, akin to worker assistance in eusocial colonies, though without the irreversible sterility seen in insects. Post-2020 scientific debates have increasingly emphasized that social complexity has arisen through with eusociality, via distinct genetic and cultural pathways rather than shared genomic mechanisms. As of , theoretical work posits that humans are undergoing a cultural-driven evolutionary transition toward greater integration, resembling superorganism-like structures, but this remains distinct from biological eusociality due to the primacy of learned over genetic transmission. In ' short story "" (1905), giant in the are depicted as exhibiting advanced , organizing coordinated attacks on s and suggesting an emergent societal structure akin to a conquering empire. This portrayal draws on early 20th-century fascination with tropical , framing the as a model of efficient, hierarchical that challenges human dominance. The 1998 animated films and both anthropomorphize ant societies to explore themes of caste systems and internal conflicts within eusocial structures. In , the worker protagonist Z rebels against a rigid, militaristic , highlighting tensions between individual desires and collective duties enforced by and royal . Similarly, presents the as a yet oppressed community under external threats from grasshoppers, with characters embodying diverse roles like inventors and foragers to emphasize communal problem-solving and interdependence. Documentaries in the have increasingly focused on honeybee societies, underscoring threats to eusocial order from environmental stressors. For instance, The Pollinators (2020) examines commercial operations and the impacts of habitat loss and rising temperatures that disrupt patterns and increase colony collapse risks. These films blend scientific footage with discussions of environmental perils, portraying bee eusociality as both resilient and vulnerable to anthropogenic changes. Eusociality often serves as a metaphor for extreme collectivism in science fiction, exemplified by the Borg in Star Trek: The Next Generation (1989–1994) and subsequent series. The Borg Collective operates as a hive-mind cybernetic society where individuality is assimilated into a unified whole, echoing eusocial insect dynamics like pheromone-driven conformity and reproductive division but amplified to critique totalitarian uniformity. This representation evolved from early insectoid concepts to humanoid forms, reinforcing the Borg as a cautionary symbol of lost autonomy in pursuit of collective perfection.

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