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Herd of American bison at Genesee Park

Sociality is the degree to which individuals in an animal population tend to associate in social groups (gregariousness) and form cooperative societies.

Sociality is a survival response to evolutionary pressures.[1] For example, when a mother wasp stays near her larvae in the nest, parasites are less likely to eat the larvae.[2] Biologists suspect that pressures from parasites and other predators selected this behavior in wasps of the family Vespidae.

This wasp behaviour evidences the most fundamental characteristic of animal sociality: parental investment. Parental investment is any expenditure of resources (time, energy, social capital) to benefit one's offspring. Parental investment detracts from a parent's capacity to invest in future reproduction and aid to kin (including other offspring). An animal that cares for its young but shows no other sociality traits is said to be subsocial.

An animal that exhibits a high degree of sociality is called a social animal. The highest degree of sociality recognized by sociobiologists is eusociality. A eusocial taxon is one that exhibits overlapping adult generations, reproductive division of labor, cooperative care of young, and—in the most refined cases—a biological caste system.

One characteristic of social animals is the relatively high degree of cognitive ability. Social mammal predators such as spotted hyena and lion have been found to be better than non-social predators such as leopard and tiger at solving problems that require the use of innovation.[3]

Presociality

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Solitary animals such as the jaguar do not associate except for courtship and mating.[4] If an animal taxon shows a degree of sociality beyond courtship and mating, but lacks any of the characteristics of eusociality, it is said to be presocial.[5] Although presocial species are much more common than eusocial species, eusocial species have disproportionately large populations.[6]

The entomologist Charles D. Michener published a classification system for presociality in 1969, building on the earlier work of Suzanne Batra (who coined the words eusocial and quasisocial in 1966).[7][8] Michener used these terms in his study of bees, but also saw a need for additional classifications: subsocial, communal, and semisocial. In his use of these words, he did not generalize beyond insects. E. O. Wilson later refined Batra's definition of quasisocial.[9][10]

Subsociality

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Subsociality is common in the animal kingdom. In subsocial taxa, parents care for their young for some length of time. Even if the period of care is very short, the animal is still described as subsocial. If adult animals associate with other adults, they are not called subsocial, but are ranked in some other classification according to their social behaviours. If occasionally associating or nesting with other adults is a taxon's most social behaviour, then members of those populations are said to be solitary but social. See Wilson (1971)[9] for definitions and further sub-classes of varieties of subsociality. Choe & Crespi (1997)[11] and Costa (2006)[12] give readable overviews.

Subsociality is widely distributed among the winged insects, and has evolved independently many times. Insect groups that contain at least some subsocial species are shown in bold italics on a phylogenetic tree of the Neoptera (note that many non-subsocial groups are omitted):[13]

Neoptera
Idioprothoraca

Embioptera (webspinners)[14][15]

Rhipineoptera
Dictyoptera

Blattodea (cockroaches, inc. eusocial termites)[16]

Mantodea (mantises)

Orthoptera (grasshoppers, crickets)[17]

Dermaptera (earwigs)[18][19][20]

Eumetabola
Parametabola

Zoraptera (angel insects)[21]

Paraneoptera
Condylognatha

Thysanoptera (thrips)[22]

Hemiptera (bugs)

Membracidae (treehoppers, thorn bugs)[23][24]

Pentatomidae (shield bugs)[25]

Reduviidae (predatory bugs)[26][27][28]

Tingidae (lace bugs)[29][30]

many families[31][32]

Psocoptera (bark lice)[33]

Endopterygota
Coleoptera[60]

Tenebrionidae (leaf/flower beetles)[48][49]

Erotylidae (pleasing fungus beetles)[50]

Chrysomelidae (leaf beetles)[51][52][53][54][55][56][57][58][59]

Neuropteroidea

Raphidioptera (snakeflies)

Neuroptera (lacewings, alderflies, and allies)

Antliophora (true flies, scorpionflies, fleas)

Trichoptera (caddisflies)

Lepidoptera (butterflies and moths)[61]

Hymenoptera (sawflies, wasps, ants, bees)[62] (apart from eusocial species)

Solitary but social

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A wide-eyed mouse lemur gnaws at a snack it holds in its hands.
The mouse lemur is a nocturnal, solitary-but-social lemur native to Madagascar.

Solitary-but-social animals forage separately, but some individuals sleep in the same location or share nests. The home ranges of females usually overlap, whereas those of males do not. Males usually do not associate with other males, and male offspring are usually evicted upon maturity. However, this is opposite among cassowaries, for example. Among primates, this form of social organization is most common among the nocturnal strepsirrhine species and tarsiers. Solitary-but-social species include mouse lemurs, lorises, and orangutans.[63]

Some individual cetaceans adopt a solitary but social behavior, that is, they live apart from their own species but interact with humans. This behavior has been observed in species including bottlenose dolphin, common dolphin, striped dolphin, beluga, Risso's dolphin, and orca. Notable individuals include Pelorus Jack (1888–1912), Tião (1994–1995), and Fungie (1983–2020). At least 32 solitary-sociable dolphins were recorded between 2008 and 2019.[64]

Parasociality

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Not to be confused with parasocial interaction.

Sociobiologists place communal, quasisocial, and semisocial animals into a meta-class: the parasocial. The two commonalities of parasocial taxa are the exhibition of parental investment, and socialization in a single, cooperative dwelling.[5]

Communal, quasisocial, and semisocial groups differ in a few ways. In a communal group, adults cohabit in a single nest site, but they each care for their own young. Quasisocial animals cohabit, but they also share the responsibilities of brood care. (This has been observed in some Hymenoptera and spider taxa,[65] as well as in some other invertebrates.)[5] A semisocial population has the features of communal and quasisocial populations, but they also have a biological caste system that delegates labor according to whether or not an individual is able to reproduce.

Beyond parasociality is eusociality. Eusocial insect societies have all the characteristics of a semisocial one, except overlapping generations of adults cohabit and share in the care of young. This means that more than one adult generation is alive at the same time, and that the older generations also care for the newest offspring.

Eusociality

[edit]
Main articles: Eusociality and Evolution of eusociality
Bees almost completely cover a honeycomb suspended from a tree branch.
Giant honey bees cover the honeycomb of their nest.

Eusocial societies have overlapping adult generations, cooperative care of young, and division of reproductive labor. When organisms in a species are born with physical characteristics specific to a caste which never changes throughout their lives, this exemplifies the highest acknowledged degree of sociality. Eusociality has evolved in several orders of insects. Common examples of eusociality are from Hymenoptera (ants, bees, sawflies, and wasps) and Blattodea (infraorder Isoptera, termites), but some Coleoptera (such as the beetle Austroplatypus incompertus), Hemiptera (bugs such as Pemphigus spyrothecae), and Thysanoptera (thrips) are described as eusocial. Eusocial species that lack this criterion of morphological caste differentiation are said to be primitively eusocial.[5]

Two potential examples of primitively eusocial mammals are the naked mole-rat and the Damaraland mole-rat (Heterocephalus glaber and Fukomys damarensis, respectively).[66] Both species are diploid and highly inbred, and they aid in raising their siblings and relatives, all of whom are born from a single reproductive queen; they usually live in harsh or limiting environments. A study conducted by O'Riain and Faulkes in 2008 suggests that, due to regular inbreeding avoidance, mole rats sometimes outbreed and establish new colonies when resources are sufficient.[67]

Eusociality has arisen among some crustaceans that live in groups in a restricted area. Synalpheus regalis are snapping shrimp that rely on fortress defense. They live in groups of closely related individuals, amidst tropical reefs and sponges.[68] Each group has one breeding female; she is protected by a large number of male defenders who are armed with enlarged snapping claws. As with other eusocial societies, there is a single shared living space for the colony members, and the non-breeding members act to defend it.[69]

Human eusociality

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E. O. Wilson and Bert Hölldobler controversially[70] claimed in 2005 that humans exhibit sufficient sociality to be counted as a eusocial species.[71]

See also

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References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Sociality

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Sociality refers to the extent to which individuals of a species form enduring groups with conspecifics, engaging in interactions that vary from temporary aggregations to complex societies characterized by division of labor and mutual dependence. In biological terms, it encompasses behaviors influenced by the presence of others, including group , defense, and , often quantified by metrics such as group size, stability, and interaction frequency. Evolutionarily, sociality emerges when the net fitness gains—such as reduced predation risk, improved resource access, and shared —surpass the drawbacks, including intra-group competition, parasite transmission, and energetic demands of coordination. This adaptive strategy has arisen independently across phyla, from eusocial insects exhibiting to mammalian herds displaying kin-based alliances, with empirical data linking greater sociality to prolonged lifespans and cognitive sophistication. In humans, sociality manifests in hierarchical communities sustained by reciprocity, cultural transmission, and large-scale , enabling technological and societal advancements while imposing costs like conflict and pressures.

Definition and Scope

Core Definition and Characteristics

Sociality denotes the propensity of conspecifics—individuals of the same —to aggregate into groups and engage in recurrent interactions that deviate from solitary behavioral patterns, often yielding mutual influences on , , and acquisition. This phenomenon encompasses a from transient associations to persistent societies, where the presence of others modulates physiological, cognitive, and behavioral processes, such as heightened vigilance or coordinated . Empirical observations across taxa reveal sociality as a heritable trait shaped by genetic predispositions, with group formation typically emerging when benefits like predator avoidance outweigh risks of or transmission. Key characteristics include the scale and stability of group size, ranging from pairs or small kin clusters to large colonies exceeding thousands, as quantified by metrics like the sociality index that integrates association duration and interaction frequency. Interactions within social groups frequently involve communication signals—vocal, chemical, or visual—to coordinate activities, resolve conflicts, or transmit information, with studies on and cetaceans documenting how such signals enhance collective decision-making. Division of roles may appear even in basal forms, where dominant individuals secure mating access while subordinates gain indirect fitness benefits through , though this varies by and is not universal. Sociality's core features also encompass trade-offs in energy allocation, where group members incur costs like increased parasite exposure—evidenced by higher rates in dense avian flocks—but achieve gains in or information sharing, as modeled in game-theoretic analyses of dilemmas. Unlike asocial strategies, social systems demand cognitive investments for recognition of allies and foes, correlating with enlarged regions in social mammals, per comparative neuroanatomical from over 200 species. These traits underscore sociality as an adaptive response to environmental pressures rather than an inherent moral framework, with empirical validation from longitudinal field studies tracking in response to changes.

Classification Systems

Classification systems for sociality in animals typically array behaviors from to complex cooperative societies, with the most formalized schemes originating in studies of insects, particularly . These frameworks, such as those developed by Charles D. Michener, identify progressive stages based on nest-sharing, brood care cooperation, reproductive roles, and generational overlap. Michener's 1969 classification for bees delineates six key stages of social organization:
StageDescription
SolitaryIndividuals forage, nest, and reproduce independently, with no cooperative brood care or division of labor.
SubsocialAdults provide extended care to their own offspring for a limited period before dispersal, as seen in some cockroaches.
CommunalMultiple females share a nest and resources but rear broods separately without cooperation.
QuasisocialMothers and offspring cooperate in brood care within a shared nest, with all individuals potentially reproducing.
SemisocialBuilds on quasisociality with a primitive worker caste, where some forgo personal reproduction to assist colony tasks.
EusocialFeatures semisocial traits plus overlapping generations of adults, enabling lifelong sterile workers to support reproductives.
Eusociality, the apex of these systems, requires three defining traits: cooperative brood care (offspring or siblings raised collectively), reproductive division of labor (typically one or few queens and sterile workers), and multigenerational overlap within the group. This level occurs in over 15,000 species, predominantly ants, bees, wasps, and termites, but also select vertebrates like naked mole-rats. Parallel sequences, such as parasocial (emphasizing nest-sharing and brood without initial generational overlap) and subsocial (focusing on prolonged leading to castes), apply across social insects. In vertebrates, classifications diverge, often integrating systems—monogamous pairs, polygynous harems, or promiscuous groups—with spatial behaviors like territoriality, rather than rigid insect-like stages, though eusocial elements appear in species with helper-at-the-nest systems. outlined 10 qualities of advanced sociality, including group stability, communication, and role specialization, applicable beyond insects. Contemporary views treat sociality as a multidimensional continuum influenced by ecological pressures, challenging binary or stepwise categorizations by incorporating dynamic interactions like and seasonal variability.

Evolutionary Origins

Phylogenetic Patterns

Advanced forms of sociality, such as characterized by reproductive division of labor, cooperative brood care, and overlapping generations, have arisen independently at least 11 times in arthropods but remain phylogenetically clustered within specific insect orders. is most prevalent in the (ants, bees, and wasps), encompassing over 15,000 described , and the (), with approximately 2,800 , where it dominates these clades. Sporadic occurrences appear in other arthropod groups, including , , some beetles, and snapping , but these represent fewer than 1% of eusocial overall. Phylogenetic analyses indicate that these transitions often stem from subsocial precursors involving maternal care, with haplodiploid sex determination facilitating in , though not universally required across taxa. In vertebrates, is exceedingly rare, documented only in two genera of African mole-rats (Heterocephalus and Fukomys within Bathyergidae), where colonies feature a single breeding female and non-reproductive workers. Broader social behaviors, such as group-living and , show more variable phylogenetic distribution; for instance, comparative studies of over 1,000 mammalian reveal solitary living as the ancestral state, with independent transitions to pair-living or group-living occurring in lineages adapted to specific ecological pressures like predation or resource distribution. In , ancestral social organization likely involved flexible pair-living with fluid group associations, evolving into multimale-multifemale groups in many anthropoid lineages, as inferred from Bayesian phylogenetic generalized linear mixed models across 216 . Avian sociality similarly exhibits multiple origins of in over 300 across diverse orders, often linked to harsh environments delaying independent breeding. Phylogenetic comparative methods highlight correlated evolution between sociality and life-history traits across taxa; group-living mammals, for example, exhibit longer lifespans (up to 2-3 times that of solitary counterparts) and extended generation times, suggesting selection for delayed in social contexts. Sociality also influences , with eusocial lineages showing elevated rates of expansions and positive selection in genes related to immunity and sensory , as observed in comparative genomic analyses of and . These patterns underscore that while basic gregariousness may evolve readily under predation or pressures, advanced sociality requires rare synergistic preconditions, resulting in its patchy distribution rather than uniform phylogenetic spread.

Key Drivers and Transitions

Ecological factors, including predation and resource distribution, drive the initial formation of groups by providing benefits such as improved predator detection and foraging efficiency in patchy environments. High predation pressure selects for grouping behaviors that enhance collective vigilance, reducing individual through shared alarm calls and tactics observed in various taxa. Similarly, clumped resources favor aggregation to exploit food patches more effectively, as solitary individuals face higher from dispersers. Genetic mechanisms, particularly , underpin transitions to advanced sociality by favoring behaviors that increase . Under Hamilton's rule, evolves when the product of genetic relatedness and benefit to recipients exceeds the altruist's cost (rB > C). In haplodiploid , female siblings share 75% relatedness due to , exceeding parent-offspring relatedness and promoting worker sterility to rear sisters over personal reproduction. Ancestral further maximizes colony relatedness, facilitating eusociality's origin as evidenced in comparative studies across bees, wasps, and ants. Major evolutionary transitions to sociality involve two stages: cooperative group formation followed by integration into a higher-level entity with division of labor. serves as a precursor, evolving into subsocial systems where offspring assist parents, then permanent groups with overlapping generations. In eusocial , this culminates in castes with reproductive division, where and workers specialize, transforming colonies into Darwinian individuals capable of collective . Such transitions are rare, occurring independently about 15 times, predominantly in under specific ecological and genetic conditions.

Spectrum of Social Behaviors

Presocial Strategies

Presocial strategies encompass behavioral patterns in animals that involve limited cooperative interactions, such as or temporary group associations, but without the reproductive division of labor, overlapping generations of adults, or cooperative brood care by non-reproductives characteristic of . These strategies often represent transitional stages in social evolution, providing selective advantages like enhanced protection or resource sharing while avoiding the costs of permanent group commitment. Subsociality, a key presocial form, features direct in post-hatching care, typically by one guarding, provisioning, or defending young until . In , this is exemplified by earwigs (Forficula spp.), where females remain with eggs to prevent fungal overgrowth and , then regurgitate food to first-instar nymphs, increasing juvenile survival by up to 50% compared to unguarded broods. Giant water bugs ( spp.) demonstrate paternal subsociality, with males carrying egg masses dorsally for 2-3 weeks, periodically surfacing to oxygenate them and preventing or predation. Assassin bugs () exhibit maternal guarding of early nymphs against parasitoids, a that boosts nymphal eclosion rates in field observations. Such tactics evolve in response to high juvenile mortality, favoring parents that delay dispersal to maximize without forgoing personal reproduction. Parasociality involves multiple reproductives cooperating in nest-building or foraging but retaining individual reproductive potential, often leading to dominance contests over egg-laying. This is observed in some halictid bees, where co-foundresses share burrow excavation and pollen provisioning, yet foundresses dominate reproduction through physical aggression, with subordinates laying fewer viable eggs. In burrowing like Anurogryllus muticus, females aggregate loosely for oviposition , benefiting from vigilance against predators, though groups dissolve post-hatching without sustained . These strategies mitigate solitary risks like nest usurpation but incur conflicts, as all group members compete for limited resources, constraining group stability compared to eusocial systems. In vertebrates, presocial equivalents include familial aggregations for juvenile protection, as in some crocodilians where mothers guard hatchlings from conspecifics for weeks post-emergence, or in burying beetles (Nicrophorus spp.) where parents prepare carrion provisions and defend broods, reducing larval starvation in competitive environments. Empirical studies link these behaviors to ecological pressures like predation intensity, with presocial groups achieving higher reproductive success than solitaries in unstable habitats.

Advanced Sociality

Advanced sociality encompasses intermediate to highly integrated forms of , including quasisocial, semisocial, and eusocial organizations, which feature , use, and varying degrees of reproductive specialization among group members. These behaviors contrast with presocial strategies by involving multiple adults in nest maintenance and brood rearing, often leading to more efficient colony-level adaptations. Quasisocial species involve adults of the same generation sharing a nest and cooperatively caring for brood, with individuals capable of recognizing and preferentially tending their own offspring while all retaining reproductive potential. This level is observed in certain bees, such as some Euglossine species, where females collaborate on nest defense and provisioning but do not exhibit caste differentiation. Semisocial groups extend this by incorporating a temporary reproductive division of labor, where dominant individuals monopolize reproduction within the cohort, suppressing subordinates who assist in foraging and guarding; examples include primitively eusocial halictid bees and some polistine wasps, where such hierarchies can revert if the dominant perishes. Eusociality represents the apex of advanced sociality, defined by three core traits: cooperative brood care (non-parental adults rearing young), overlapping adult generations within the , and a reproductive division of labor with morphologically or behaviorally distinct castes, typically sterile workers supporting fertile queens. This organization has evolved independently at least 11 times in insects, primarily in the order (, bees, wasps) and Isoptera (termites), as well as in some , , and ambrosia beetles; vertebrate examples are rarer, limited to the Heterocephalus glaber () and Fukomys damarensis (), where queens dominate reproduction in underground . Eusocial function as superorganisms, with workers specializing in tasks like , defense, and , enabling exponential growth in size—e.g., exceeding 2 million individuals—and heightened resilience to environmental pressures.

Adaptive Trade-offs

Benefits of Group Living

Group living reduces individual predation risk through mechanisms such as the dilution effect, where the probability of attack decreases as group size increases, and enhanced collective vigilance, allowing earlier detection of threats. In plains zebras, larger groups exhibit lower predation rates attributable to both dilution and reduced detection by predators. Empirical studies in fluctuating environments confirm that group size correlates with higher survival by mitigating predation rather than solely improving resource access. These anti-predator advantages are evident across taxa, including mammals and birds, where social aggregation dilutes encounter rates with predators. Foraging efficiency often improves in groups due to about food locations and exploitation of patches, outweighing intragroup in many . In feral horses, feeding rates increased with group size, and solitary individuals experienced higher weight loss compared to those in groups. Fish schools demonstrate that social interactions integrate individual and cues, achieving near-optimal and equitable resource distribution. However, efficiency peaks at intermediate group sizes in some , beyond which diminishes returns. Reproductive success benefits from sociality via mate access, shared , and reduced risk in larger groups. Long-term avian studies show groups less prone to , prompting reproductive concessions among competitors to maintain cohesion. Highly social mammals and birds exhibit delayed reproductive and higher lifetime output, linked to protective and gains. In breeders, subordinates contribute to defense and provisioning, elevating overall fledging rates. Additional physiological benefits include in cold climates, where huddling conserves heat, as observed in and , though these are secondary to ecological drivers. Overall, these advantages drive the of sociality where predation pressure and patchiness favor grouping over solitary living.

Costs and Risks of Sociality

Social living imposes several inherent costs on individuals, primarily arising from heightened interactions that amplify , exposure, and visibility to threats, often offsetting the advantages of group formation. Empirical studies across taxa demonstrate that while group size can dilute predation risk in some contexts, it frequently elevates overall detectability by predators, as larger aggregations produce more noise, scent, or visual cues. For instance, in mammalian groups, the net adaptive value of sociality hinges on whether benefits like vigilance surpass these risks, but solitary strategies predominate in many lineages where such costs prove prohibitive. A primary risk stems from accelerated transmission, as proximity and frequent contacts facilitate the spread of pathogens among group members. Research synthesizing analyses in reveals that group size directly correlates with infection rates, with denser networks exacerbating outbreaks of parasites and viruses; for example, in and , empirical data show transmission probabilities scaling with contact frequency, leading to higher morbidity and mortality in social versus solitary populations. This vulnerability extends to zoonotic , where social clustering amplifies spillover risks, as documented in longitudinal field studies of mammals. Behavioral adjustments, such as temporary network plasticity to avoid infected individuals, can mitigate but not eliminate this cost, underscoring sociality's role in amplifying epidemiological burdens. Intra-group represents another substantial drawback, manifesting as over , mates, and that incurs energetic expenditures, injuries, and suppressed . In group-living mammals and birds, often results in subordinate individuals experiencing reduced intake or access, with studies on Ethiopian wolves illustrating how temporal resource predictability modulates these costs, favoring smaller groups to minimize conflict. Reproductive skew, including or dominance hierarchies, further erodes individual fitness; for example, in , alpha males' monopolization of breeding leads to elevated violence and lower for others, as quantified in long-term observational data. These dynamics highlight how social cohesion can paradoxically foster internal strife, with costs nonlinearly increasing in larger or more complex societies. Additional risks include elevated parasite loads and physiological stress from chronic social monitoring or submission, which can shorten lifespan or impair immune function. In eusocial insects like bees, worker sterility and impose direct reproductive costs, while in vertebrates, group foraging may heighten per capita energy demands without proportional gains, as evidenced by metabolic scaling models. Overall, these trade-offs explain the evolutionary persistence of presocial or solitary lifestyles in over 90% of animal , where isolation avoids such liabilities despite forgoing cooperative gains.

Sociality Across Taxa

Invertebrates

Eusociality represents the pinnacle of social organization in , defined by cooperative brood care, overlapping generations within colonies, and a division of reproductive and non-reproductive labor. This phenomenon occurs almost exclusively among arthropods, where it has evolved independently multiple times, enabling colonies to achieve efficiencies unattainable by solitary individuals. While only about 2% of exhibit , these account for a disproportionate share of , underscoring the adaptive success of in resource exploitation and defense. In the order , encompassing , bees, and wasps, is facilitated by haplodiploid sex determination, which promotes by rendering female workers more related to sisters than to their own offspring. , with over 10,000 eusocial , form colonies ranging from hundreds to millions of individuals, featuring specialized castes for , , and soldiering; for instance, raids involve coordinated mass attacks on prey. Honeybees (Apis mellifera) maintain colonies of up to 80,000 workers, with queens specialized for reproduction and workers performing age-based tasks from to . (order , formerly Isoptera), numbering around 3,100 —all eusocial—differ by being diploid and relying on symbiotic gut microbes for digestion, supporting massive mound colonies that regulate internal climates via ventilation. Beyond core eusocial , subsocial or primitively social behaviors appear in other arthropods, such as and , where some develop sterile castes to defend colonies against intruders. Marine snapping shrimps of the genus Synalpheus exhibit in sponge-dwelling colonies, with non-reproductive helpers defending territories via synchronized snapping claws; this trait has arisen at least four times independently, correlating with larger genomes rich in transposable elements. Social spiders, comprising about 25 permanently social across seven families, cooperate in web-building, prey capture, and brood care without rigid castes, often forming colonies of thousands that tackle prey larger than solitary spiders could manage. These examples highlight how in enhances survival through , though it demands mechanisms like chemical recognition to mitigate intra-colony conflict.

Vertebrates


Vertebrates exhibit a broad spectrum of s, from transient aggregations to enduring societies, underpinned by a conserved neural social behavior network comprising regions such as the , , and midbrain , which regulate , , and affiliation across , birds, and mammals. This network's homology suggests an ancient origin, with variations arising from ecological pressures like predation and resource distribution. Sociality in vertebrates often confers benefits such as enhanced predator detection and foraging efficiency, though costs like increased and disease transmission impose selective constraints. In , particularly teleosts, schooling—polarized, synchronized group movement—is prevalent, observed in over 4,000 , enabling dilution of predation risk and hydrodynamic advantages that reduce energy expenditure by up to 56% at high speeds compared to solitary . Approximately one-quarter of shoal throughout life, with many others doing so during vulnerable juvenile or reproductive phases, driven by sensory cues including detection of water movements. Examples include schools, where collective vigilance amplifies against predators. Amphibians display limited sociality, predominantly presocial with solitary adults aggregating transiently for breeding choruses in anurans, where males compete acoustically for mates, modulated by vasotocin to influence calling and . is rare but occurs in some species, such as poison dart frogs transporting tadpoles, though lacking the cooperative structures seen in higher vertebrates. Reptiles are generally solitary, with social interactions confined to , territorial defense, or brief parental guarding, as in crocodilians where females protect nests and juveniles for months post-hatching. However, some squamates form groups, particularly viviparous where live-bearing correlates with evolutionary transitions to sociality, including kin-based family units in certain . Snakes occasionally exhibit affiliative bonds, preferring familiar conspecifics, challenging prior views of reptilian . Birds frequently form flocks for and migration, with species like starlings demonstrating murmurations that confound predators through rapid, coordinated maneuvers. prevails in over 3% of species, such as woodpeckers storing nuts communally and scrub-jays aiding breeders in care, often favoring kin to maximize . Territorial and song are regulated by vasotocin in the social behavior network, varying with group size and density. Mammals achieve the most complex vertebrate sociality, with herd-living ungulates like aggregating for anti-predator vigilance and resource defense, packs of wolves cooperating in hunts that succeed in 10-15% of pursuits versus solitary failures. Eusocial-like structures emerge in naked mole-rats, featuring castes, reproductive division, and among highly related individuals, though not fully equivalent to insect eusociality due to diploid and occasional breeding by subordinates. Pair-bonding in species like prairie voles involves vasopressin-mediated affiliation, paralleling network functions in other vertebrates.

Human Sociality

Biological Foundations

Human sociality is rooted in evolutionary adaptations that favored for survival advantages, such as improved efficiency, predator defense, and cooperative child-rearing in ancestral environments spanning the Pleistocene epoch. and genetic evidence indicates that early hominins formed multi-family bands of 50–150 individuals, enabling resource sharing and division of labor that exceeded solitary or small-pair strategies observed in other . This ultra-social structure coevolved with cognitive capacities for and shared , distinguishing humans from other great apes by promoting scalable cooperation beyond immediate kin. At the neurobiological level, social cognition relies on a distributed "social brain" network encompassing the medial prefrontal cortex for mentalizing others' intentions, the for , and the for detecting social threats and emotional cues. studies, including fMRI, reveal heightened activation in these regions during tasks involving , fairness judgments, and group coordination, with the integrating conflict monitoring in social exchanges. Disruptions in these circuits, as seen in conditions like autism spectrum disorder, impair reciprocal interactions, underscoring their causal role in typical social functioning. Hormonal mechanisms further underpin affiliation and bonding, with oxytocin released from the during physical contact and gaze reciprocity to enhance trust and pair-bond formation. Intranasal oxytocin administration in experiments increases generosity in economic games and reduces responses to fearful faces, facilitating prosocial approach behaviors. complements this by modulating aggression and mate-guarding, particularly in males, while dynamics balance affiliation with competitive stress in hierarchical groups. These neuroendocrine systems, conserved from mammalian ancestors yet amplified in humans through extended , enforce adaptive reciprocity in large-scale societies.

Debates on Eusocial Classification

Eusociality is defined by three primary criteria: overlapping generations within a , cooperative brood care involving individuals other than parents, and a reproductive division of labor where some group members forgo personal to assist others. These traits are observed in select such as hymenopterans (, bees, wasps) and , as well as rare vertebrates like naked mole rats, where morphological and behavioral castes enforce sterility in non-reproductive individuals. Proponents of classifying humans as eusocial, notably biologist E.O. Wilson, argue that human societies display analogous features, including multigenerational family units, alloparenting (care of offspring by non-parents such as grandparents and aunts), and societal divisions of labor that enhance group productivity over individual reproduction. In his 2012 book The Social Conquest of Earth, Wilson posits humans as "eusocial apes," attributing dominance among primates to these traits, which parallel insect superorganisms and favor group-level selection over strict kin selection. A 2010 paper co-authored by Wilson, Martin Nowak, and Corina Tarnita further suggests eusociality evolves through group formation and assortment, loosely applying this to humans as dominant land vertebrates. Critics challenge this extension, emphasizing that humans lack obligatory reproductive castes or morphological adaptations enforcing sterility, essential for canonical . Evolutionary biologists like Joan Strassmann and David Queller argue Wilson's redefinition dilutes the term, as remains facultative—most individuals reproduce, and helpers (e.g., in bands) retain reproductive potential, unlike workers. They contend via or high relatedness better explains , and applying to humans overlooks individual fitness maximization, with historical reproductive skew (e.g., effective female breeding populations 17 times higher than males in some analyses) insufficient for eusocial status. This debate ties to broader disputes on eusocial origins, where Wilson's group-centric models faced mathematical critiques for underemphasizing relatedness thresholds (typically r > 0.5 for stability). Some researchers propose a continuum, grading species by reproductive skew rather than binary castes, potentially placing humans midway between solitary and fully eusocial taxa due to cultural enforcement of division of labor. However, empirical data from show lifetime non-reproduction rates below 1-2% in most populations, far short of the near-total sterility in eusocial , undermining strict classification. Recent models affirm eusociality's rarity requires mechanisms like maternal or nest defense, absent in .

Molecular and Genetic Basis

Kin Selection and Inclusive Fitness

extends the concept of individual fitness to include an organism's effects on the of genetic relatives, weighted by the coefficient of relatedness r, which measures the probability that a in the actor is identical by descent to a in the recipient. This framework, formalized by in 1964, posits that acts on promoting behaviors that maximize , encompassing both personal reproduction (direct fitness) and indirect benefits to kin. describes the evolutionary process whereby such spread because altruistic acts toward relatives enhance the propagation of shared , even at a personal cost. Hamilton's rule, rB > C, quantifies the condition for altruism to evolve: the benefit B to the recipient's fitness, multiplied by relatedness r, must exceed the actor's fitness cost C. In genetic terms, r averages 0.5 for full siblings or offspring under diploid inheritance but reaches 0.75 for sisters in haplodiploid systems like Hymenoptera (bees, ants, wasps), where females develop from fertilized eggs sharing all paternal genes. This asymmetry favors worker sterility in females, as aiding sisters yields higher indirect fitness than personal reproduction, explaining the evolution of eusociality in over 90% of hymenopteran species with facultative or obligate castes. Experimental evidence includes manipulations of colony relatedness in wasps and ants, where reduced r (e.g., via multiple queens or introduced unrelated individuals) decreases altruism and increases queen production by workers, confirming kin-biased investment. At the molecular level, kin selection operates through genomic mechanisms enabling recognition of relatives, such as cuticular hydrocarbons in serving as kinship cues for differential treatment. Genes underlying these traits, like those in the desat family in or odorant receptors in , correlate with social behaviors that align with predictions. In vertebrates, genomic analyses reveal 's role in traits like in birds and mammals, where high r (e.g., 0.5–0.75 in nuclear families) sustains delayed dispersal and . Critics, including Nowak, Tarnita, and Wilson (2010), argue is mathematically equivalent to standard models and unnecessary as a distinct , potentially overlooking non-kin in complex societies. However, defenders counter that equivalence does not negate its value for partitioning fitness effects and predicting outcomes in kin-structured populations, with empirical tests (e.g., microbial and experiments) validating Hamilton's rule over alternatives. The theory remains central to understanding the genetic underpinnings of sociality, though debates persist on its scope beyond additive effects.

Genomic Influences

Heritability estimates for social behaviors across demonstrate a substantial genomic contribution, with narrow-sense (h²) ranging from 0.04 to 0.35 for traits like human-directed contact-seeking in dogs and social network centrality in wild great tits. These values, derived from quantitative genetic analyses including twin studies and pedigree data, indicate that additive genetic variance underlies variation in sociability, independent of environmental factors. Selection experiments in model organisms further confirm this, as artificial selection for high versus low sociability in deer mice yields divergent lineages with differential in 174 loci, including those affecting and . In vertebrates, systems provide concrete genomic mechanisms. Polymorphisms in the (OXTR) and arginine vasopressin receptor 1A (AVPR1A) modulate social affiliation and pair bonding; for example, microsatellite repeat length in the AVPR1A promoter region differs between monogamous prairie voles (Microtus ochrogaster), which exhibit high expression in reward pathways, and less social montane voles (Microtus montanus). Genome-wide association studies (GWAS) in dogs identify loci near genes for synaptic function and signaling associated with human-directed sociability, such as those influencing trainability and . In , mutations in specific disrupt group cohesion, linking defective to pathways in neural development. Human social traits, including aspects of extraversion and , show polygenic inheritance, with GWAS meta-analyses implicating variants in OXTR and related loci that explain small but significant portions of variance in . These effects interact with sex and early environment but stem from sequence variation; for instance, OXTR rs53576 polymorphism correlates with prosociality in relational contexts across cohorts. In livestock like pigs, GWAS for socially affected traits reveal loci influencing and affiliation, highlighting conserved genomic architecture. For eusocial insects, genomic influences manifest in caste-specific gene regulation rather than simple allelic variation. Comparative genomics of ants and bees uncover shared upregulated genes in reproductive castes, including those for vitellogenin and signaling, enabling without genotypic change. Hymenopteran genomes feature mechanisms like that silence worker reproductive genes, facilitating division of labor; this epigenetic overlay on fixed genomic templates supports colony-level . Overall, sociality's genomic basis is polygenic and context-dependent, with pathways conserved across taxa while permitting evolutionary divergence.

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