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Cathemerality
Cathemerality
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The lion is a cathemeral felid.

Cathemerality, sometimes called metaturnality, is an organismal activity pattern of irregular intervals during the day or night in which food is acquired, socializing with other organisms occurs, and any other activities necessary for livelihood are undertaken.[1] This activity differs from the generally monophasic pattern (sleeping once per day) of nocturnal and diurnal species as it is polyphasic (sleeping 4-6 times per day) and is approximately evenly distributed throughout the 24-hour cycle.[2]

Many animals do not fit the traditional definitions of being strictly nocturnal, diurnal, or crepuscular, often driven by factors that include the availability of food, predation pressure, and variable ambient temperature.[3] Although cathemerality is not as widely observed in individual species as diurnality or nocturnality, this activity pattern is seen across the mammal taxa, such as in lions, coyotes, and lemurs.[4]

Cathemeral behaviour can also vary on a seasonal basis over an annual period by exhibiting periods of predominantly nocturnal behaviour and exhibiting periods of predominantly diurnal behaviour. For example, seasonal cathemerality has been described for the mongoose lemur (Eulemur mongoz) as activity that shifts from being predominantly diurnal to being predominantly nocturnal over a yearly cycle, but the common brown lemurs (Eulemur fulvus) have been observed as seasonally shifting from diurnal activity to cathemerality.[5]

As research on cathemerality continues, many factors that have been identified as influencing whether or why an animal behaves cathemerally. Such factors include resource variation, food quality, photoperiodism, nocturnal luminosity, temperature, predator avoidance, and energetic constraints.[6][7][8]

Etymology

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In the original manuscript for his article "Patterns of activity in the Mayotte lemur, Lemur fulvus mayottensis," Ian Tattersall introduced the term cathemerality to describe a pattern of observed activity that was neither diurnal nor nocturnal.[9] Although the term cathemeral was proposed, it was initially deemed as unnecessary new jargon and thus the term diel was used in the published version instead. In 1987, Tattersall gave a formal definition of cathemeral, turning to its Ancient Greek roots.

The word is a compound of two Greek terms: κᾰτᾰ́ (katá) 'through' and ἡμέρᾱ (hēmérā) 'day'. The term cathemeral, then, means 'through the day', where "day" refers to the full day from midnight to midnight. Tattersall credits his father, Arthur Tattersall, and Robert Ireland, two classicists, for considering this lexical problem and proposing its solution.[10]

Influences on cathemerality

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A common brown lemur, a cathemeral primate

Environmental factors

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Photoperiodism has been determined to heavily influence the distribution of activity throughout the day.[7] When there are increased daylight hours and increased sunset delay, afternoon activity begins earlier which, in turn, increases the amount of diurnal activity. In contrast, when there is a decrease in daylight hours and decreased sunset delay, afternoon activity begins later which, in turn, increases the amount of nocturnal activity.[7]

Nocturnal luminosity influences the annual pattern of activity rhythms and affects both diurnal and nocturnal behaviour.[7] Nocturnal luminosity has been found to positively correlate with the amount of nocturnal activity and negatively correlate with diurnal activity. In other words, an animal's activity distribution may be somewhat dependent on the presence of the lunar disc and the fraction of illuminated moon in relation to sunset and sunrise times.[7]

Thermoregulation

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Thermoregulation has been said to be an adaptive response that enables cathemeral animals to minimize thermoregulatory stress and costs associated with maintaining temperature homeostasis.[8] A comparison between diurnal activity and ambient temperatures showed that cathemeral individuals demonstrate the least activity during the hottest part of the day and exhibit an increase in nocturnal activity, therefore exhibit a decrease in diurnal activity during the hot wet seasons.[11] For example, to reduce heat stress, eastern grey kangaroos (Macropus giganteus) will spend hot daylight hours in the shade and as a result, they increase their nocturnal activity.[12]

Golden-lined spinefoots display cathemeral behaviour to avoid predators

Predator avoidance

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It has been hypothesized that cathemeral animals alter their activity patterns over a 24-hour period as a mechanism to avoid predation.[8] For example, lemurs are subject to a large amount of predation, such as from diurnal raptors or fossa, when active during the day. In an attempt to minimize the risk of predation, lemurs limit their amount of diurnal activity.[8] Predation masking effects have also been exhibited by the golden-lined spinefoot (Siganus lineatus), a tropical reef fish, which shifts its activity based on specific predation pressures in specific environments.[13] Other hypotheses suggest that cathemerality provides less predictable activity patterns to predators, making the cathemeral individual "temporally cryptic." This provides an advantage to cathemeral animals as their predators are unable to anticipate the activity pattern of their prey due to their irregular, flexible schedule.[5]

Energetic constraints

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It has been reported that body size is correlated with cathemeral behaviour.[14] Cathemerality allows animals to forage for food at any point during the 24-hour day to maximize energy and nutrient intake. This implies that they have the opportunity to eat up to twice as much as nocturnal or diurnal species. Since larger animals possess larger energy requirements, they tend to spend more time foraging than smaller animals, and behave cathemerally as a result.[14] In contrast, small body size causes increased metabolic rate which may also result in cathemeral activity as an attempt to improve foraging efficacy, as seen in shrews and voles.[15]

Evolution of cathemerality

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The evolutionary disequilibrium hypothesis suggests that cathemerality is a result of the diversification of more specialized activity patterns, and somewhat viewed as a transition state between nocturnality and diurnality.[2] For example, the ancestral primate is said to be nocturnal, but may have displayed flexibility in activity patterns which facilitated the evolution of cathemerality. This hypothesis is supported by the comparative approach of vision morphology in primates.[16] Nocturnal primates possess visual systems that were primarily dependent on sensitivity to light as opposed to quality of acuity. Nocturnal primates also exhibit larger cornea size, high rod-to-cone ratio, and greater photoreceptors relative to ganglion cells when compared to diurnal primate vision systems.[16] Cathemeral primates exhibit a vision system that is intermediate of diurnal and nocturnal primates, which suggests an evolutionary accommodation that provided nocturnal species with the opportunity of diurnal activity when advantageous.[17]

See also

[edit]
Look up cathemeral in Wiktionary, the free dictionary.

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Cathemerality

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Cathemerality is an activity pattern in animals characterized by irregular and sporadic intervals of behavior distributed approximately evenly across the 24-hour daily cycle, encompassing both daylight and darkness without adherence to strict diurnal, nocturnal, or crepuscular rhythms. The term derives from the Greek roots kata (throughout) and hemera (day), emphasizing activity that occurs at any time. This flexible schedule contrasts with more rigid temporal partitioning and is observed in behaviors such as feeding, traveling, and social interactions. The concept of cathemerality was first formally defined in 1987 by anthropologist Ian Tattersall, based on field observations of the brown lemur (Eulemur fulvus) in Madagascar during the 1970s, where activity levels showed no significant bias toward light or dark phases. Initially coined to describe this pattern in strepsirrhine primates, the term gained broader application after Tattersall's 2006 review, which highlighted its occurrence in genera like Eulemur and Hapalemur, as well as sporadically in platyrrhine primates such as Aotus and Alouatta. Over time, recognition expanded beyond primates, revealing cathemerality as a widespread temporal strategy. Cathemerality documented in diverse taxa, including arthropods, , birds, and mammals, with the latter for approximately 10.5% of the 5,033 known terrestrial mammal . In mammals, it appears frequently in orders like Carnivora and Artiodactyla; key examples include megaherbivores such as and rhinoceroses, which forage opportunistically around the clock, and certain shrews driven by short cycles. Among , it remains most prevalent in Malagasy lemurs, potentially as an adaptation to reduce predation or exploit variable resources in island ecosystems. This pattern serves as a key temporal niche, enabling animals to partition time flexibly in response to ecological pressures.

Definition and Characteristics

Definition

Cathemerality is an activity characterized by irregular, patchy bouts of distributed throughout the 24-hour cycle, with significant in activities such as occurring across both and phases. This is often polyphasic, with multiple periods of activity and interspersed throughout the cycle, contrasting with the monophasic sleep cycles of strictly diurnal or nocturnal . Unlike diurnal , which are primarily active during daylight hours, nocturnal confined to nighttime, or crepuscular organisms peaking and , cathemerality represents a flexible, non-committal temporal niche that avoids rigid alignment with environmental cycles. Core attributes of cathemerality include bimodal or multimodal activity peaks that lack fixed circadian entrainment, allowing opportunistic responses to varying conditions rather than prescriptive timing. This results in a more even distribution of activity across the diel cycle, sometimes described as "metaturnality" in alternative terminology for the same irregular pattern. Cathemerality is a widespread chronotype observed across diverse taxa, highlighting its ecological versatility.

Key Characteristics

Cathemerality is characterized by a polyphasic activity , consisting of multiple bouts of activity interspersed with periods throughout the 24-hour cycle, rather than a single consolidated period of . These bouts vary in , with the timing and distribution flexible in response to immediate environmental conditions such as availability or . This contrasts briefly with monophasic , where activity is concentrated in one continuous daytime block. Physiologically, cathemeral animals exhibit entrainment primarily to exogenous environmental cues like lunar cycles or daily light-dark transitions, which mask or override an underlying endogenous circadian rhythm that approximates a 24-hour period. This allows for rapid phase shifts in activity, enabling individuals to adjust their behavioral timing within hours or days without disrupting overall rhythmicity. Unlike strictly nocturnal or diurnal species with rigid internal clocks, cathemerals show reduced reliance on fixed endogenous timing, facilitating adaptive responses to variable conditions. Activity patterns are quantified using methods such as , which employs accelerometers to record movement and distinguish active from phases; focal behavioral observations, often conducted in 30-minute intervals to capture bout irregularity; and radio-tracking or GPS collars to monitor spatial movement and temporal overlap between diurnal and nocturnal phases. These techniques reveal the irregular bout and phase overlap, with particularly effective for long-term, non-invasive in wild populations. Cathemeral behavior demonstrates high intra-individual flexibility, where the same animal may shift activity timing across days—for instance, being more nocturnal one day and diurnal the next—while populations often maintain consistent overall patterns of 24-hour distribution. This individual-level variability underscores the adaptive nature of cathemerality, allowing opportunistic responses to daily fluctuations, whereas population-level consistency ensures species-wide temporal niche occupation.

Etymology and History

Etymology

The term "cathemerality" was coined in 1978 by paleoanthropologist Ian Tattersall during a field study of lemur behavior in Madagascar. Tattersall introduced the word to describe an unusual activity pattern observed in the Mayotte lemur (Eulemur fulvus), characterized by irregular bouts of activity distributed evenly across both day and night. Etymologically, "cathemeral" is a compound derived from the Ancient Greek roots kata (κατά), meaning "through" or "throughout," and hemera (ἡμέρα), meaning "day." This construction conveys the idea of activity occurring "throughout the day," encompassing periods of both daylight and darkness without strict adherence to diurnal or nocturnal rhythms. Although proposed in 1978, the term received its first formal definition in a 1987 publication, where Tattersall explicitly outlined its application to primate activity patterns. This proposal was made in a manuscript submitted to the Journal of Mammalogy, but it was not published at the time as the editor preferred the term 'diel'; Tattersall revived 'cathemeral' in his 1987 publication.

Historical Development of the Concept

The of cathemerality emerged from field observations in the that challenged the traditional of diurnal and nocturnal activity patterns in . During studies in , Tattersall documented irregular activity throughout the 24-hour cycle in lemur such as Eulemur fulvus, noting behaviors that did not fit established classifications and highlighting the limitations of binary temporal categorizations. In , Tattersall proposed the term "cathemeral" to describe this of activity dispersed across both day and night, drawing from Greek meaning "through the day" to encapsulate its flexible . From the 1980s through the 2000s, recognition of cathemerality expanded beyond primates to non-primate mammals, including rodents like microtines and ungulates such as certain bovids, as researchers documented similar irregular patterns in diverse taxa. This period also saw debates on whether cathemerality represented a fixed evolutionary trait or a plastic response to environmental variability, with studies emphasizing its prevalence in species facing unpredictable conditions, though early literature often underestimated its scope due to observational biases. By the early 2000s, syntheses argued that cathemerality might be more common across mammals than previously appreciated, prompting calls for broader comparative analyses. Post-2010 developments integrated cathemerality into wider chronobiology frameworks, viewing it as a distinct temporal niche rather than an anomaly. A 2023 review by Cox et al. synthesized evidence from arthropods, fish, birds, and mammals, positioning cathemerality as a widespread strategy for resource partitioning and niche diversification beyond just primates. Recent advancements from 2021 to 2025 have quantified its prevalence using advanced methods; for instance, a 2022 phylogenetic study of skinks classified 31.3% of species as cathemeral, revealing evolutionary transitions from diurnality and addressing prior underestimations in reptilian literature. Similarly, a 2025 analysis of 445 mammal species employed activity modeling from camera trap data to predict cathemerality probabilities, demonstrating its plasticity across environmental gradients and highlighting how traditional classifications overlooked flexible behaviors in many taxa.

Distribution and Examples

In Mammals

Cathemerality is estimated to occur in approximately 11.8% of mammal species, based on comprehensive analyses of diel activity patterns across diverse taxa. This pattern is particularly prevalent among . In strepsirrhine such as lemurs, cathemerality is a common trait; for instance, all species within the Eulemur exhibit irregular activity throughout the 24-hour cycle, with polyphasic bouts that do not adhere strictly to diurnal or nocturnal schedules. Among , patterns often show flexibility influenced by environmental cues like lunar cycles. Ring-tailed lemurs (Lemur catta), typically diurnal, shift to cathemeral activity during periods of high lunar illumination, increasing nocturnal ranging while maintaining daytime . This contrasts with higher , such as haplorhines, which generally display more rigid diurnal schedules and rarely exhibit cathemerality beyond isolated cases like the (Aotus azarai). In other mammalian orders, cathemerality appears in carnivores with opportunistic activity shifts. Lions (Panthera leo) are classic examples, actively hunting and patrolling across both diurnal and nocturnal periods, adapting to prey availability and social dynamics. Similarly, coyotes (Canis latrans) show cathemeral tendencies, with activity bouts distributed irregularly throughout the day and night, particularly in human-modified landscapes. Herbivores also demonstrate this pattern. Recent studies using GPS tracking and camera traps have refined these classifications, revealing finer-scale cathemeral variations in marsupials and rodents across dynamic environments.

In Non-Mammalian Vertebrates

Cathemerality in non-mammalian vertebrates manifests differently across taxa, often influenced by ectothermy and environmental cues that constrain endothermic flexibility seen in mammals. In birds, this activity pattern is relatively uncommon, primarily documented in select owl species that display irregular bouts of foraging across day and night. For instance, the burrowing owl (Athene cunicularia) exhibits evenly distributed activity over 24 hours, with peaks at dawn, reflecting a cathemeral strategy potentially linked to melanopsin expression variations in the retina that modulate circadian rhythms. Certain tropical passerines may also show irregular nocturnal and diurnal foraging to partition resources, though such cases remain sparsely studied compared to strictly diurnal or nocturnal congeners. In reptiles, cathemerality has evolved notably within Squamata, particularly in skinks (Scincidae) and some lizards, often as a transitional state from ancestral diurnal patterns. A 2022 phylogenetic of skink diel activity revealed that cathemerality arose multiple times, with the highest evolutionary rates for shifts from to cathemerality—nearly twice as high as transitions to diurnality—suggesting it as an adaptive intermediate in diverse habitats. Similarly, certain geckos display cathemeral tendencies. Among fish, cathemerality is more prevalent in reef species, where it facilitates predator avoidance through flexible, bimodal activity rhythms. The golden-lined spinefoot (Siganus lineatus, Siganidae), a tropical reef herbivore, demonstrates high plasticity in diel patterns, shifting between diurnal grazing and nocturnal movements based on predation risk and resource availability, unconstrained by strict circadian clocks. Broader reviews of reef fish confirm cathemerality as a widespread temporal niche, allowing species to exploit both light and dark periods for feeding and shelter, particularly in complex coral environments where interspecific competition drives partitioning. Cathemerality in amphibians is limited and understudied, with examples primarily in anurans exhibiting sporadic activity bouts influenced by lunar cycles. In summit anurans of , cathemerality appears common, enabling exploitation of both diurnal and nocturnal niches amid extreme elevations and predation pressures. Lunar phases often modulate these bouts, with reduced activity during full moons to avoid visual predators, though quantitative on prevalence remains scarce.

In Invertebrates

Cathemerality is widespread among arthropods, serving as a common time-partitioning strategy that allows irregular activity patterns across day and night, often linked to foraging needs and environmental cues. In insects, examples include certain ant species, such as six documented cases exhibiting cathemeral foraging behaviors, and dung beetles with eye adaptations supporting activity in varying light conditions. Spiders also demonstrate this pattern, with five species identified as cathemeral through observational data, enabling opportunistic predation without strict diurnal or nocturnal constraints. Moths provide another illustration, with 23 species showing activity bouts during both daylight and darkness, highlighting the prevalence in diverse insect groups. Beyond insects and arachnids, cathemerality appears in other arthropod relatives like crustaceans, where patchy activity aligns with environmental rhythms such as . For instance, the isopod Caecidotea communis displays cathemeral , remaining active day and night without sensitivity to cues, marking the first documented case in crustaceans. In intertidal zones, some crustaceans exhibit irregular bouts influenced by tidal exposure, allowing whenever conditions permit, regardless of time of day. Recent research, including post-2020 studies utilizing camera traps and eye morphology analyses, has challenged earlier assumptions of cathemerality's rarity in invertebrates, estimating higher occurrence in habitats with variable light and resource availability. For example, trap data from 2021 on small arthropods revealed increased detection of cathemeral patterns, underscoring its role as an adaptive strategy in understudied invertebrate communities. These findings emphasize that, unlike more rigid patterns in larger vertebrates, invertebrate cathemerality often reflects fine-scale environmental entrainment in smaller, exoskeletal organisms.

Ecological Influences

Environmental Factors

Photoperiod, the duration of daylight, plays a significant role in modulating cathemeral activity patterns by influencing the timing and intensity of behavioral shifts in various species. In the Arabian oryx (Oryx leucoryx), for instance, individuals exhibit cathemeral behavior during transitional seasons, shifting from predominantly nocturnal activity in summer to diurnal in winter as day length changes, allowing flexible exploitation of foraging opportunities aligned with light availability. Similarly, in brown lemurs (Eulemur fulvus), photoperiodic variations account for substantial variability in daily activity rhythms, with longer days promoting increased crepuscular peaks. Luminosity, particularly moonlight, further drives adjustments in cathemeral schedules by altering visibility and enabling or constraining movement. Full moon phases often increase nocturnal activity in cathemeral species such as the ring-tailed lemur (Lemur catta), where brighter nights facilitate greater ranging and feeding without strictly adhering to diurnal constraints, overriding endogenous circadian cues. In red-fronted lemurs (Eulemur rufifrons), ambient light levels directly modulate nocturnal excursions, with higher luminosity during lunar cycles leading to elevated activity independent of other zeitgebers. Climate and habitat structure impose seasonal and latitudinal influences on cathemerality, with patterns varying between temperate and tropical zones due to differences in environmental stability. In temperate regions, such as the northern Holarctic above 40°N, cathemeral species richness is elevated, correlating with temperature seasonality (r = 0.62 in shrews, r = 0.67 in Cricetidae), where fluctuating day lengths and milder winters enable bimodal or multimodal activity to optimize resource use. Conversely, tropical environments, characterized by stable equatorial climates with minimal photoperiod variation, support cathemerality in select taxa like lemurs, though overall prevalence is lower compared to temperate zones, as consistent warmth favors more uniform diel partitioning. These climatic drivers interact briefly with thermoregulatory needs, such as avoiding extreme heat, to fine-tune activity windows. Microhabitat features, including light penetration in forest canopies versus open areas, shape the spatial distribution of cathemeral behaviors by affecting perceived safety and foraging efficiency. In dense forest canopies, reduced luminosity promotes fragmented activity across diel phases, as seen in three-toed sloths (Bradypus variegatus), which shift from nocturnal in lowland forests to more diurnal in montane canopies with greater light diffusion. Open habitats like grasslands facilitate broader cathemerality due to uniform light exposure, contrasting with shaded understories where activity concentrates around twilight transitions. Recent analyses link these microhabitat effects to broader global climate patterns, such as altered canopy density from warming trends, potentially expanding cathemeral niches in fragmented landscapes.

Predation and Avoidance Strategies

Cathemerality serves as a risk-minimizing strategy by enabling animals to exhibit unpredictable activity patterns that confound predators constrained to fixed diurnal or nocturnal schedules, thereby reducing overall exposure to predation. In lemurs, for instance, this flexibility allows shifts in activity to avoid diurnal raptors during the day and nocturnal carnivores at night, achieving temporal crypticity that lowers encounter rates with specialized predators. Empirical evidence supports that cathemeral species experience lower predation rates compared to strictly diurnal or nocturnal counterparts, as their variable timing dilutes the predictability exploited by predators. A 2025 framework for mammalian diel activity quantifies this by modeling diel risk across activity phenotypes, revealing that cathemeral mammals occupy intermediate risk profiles that buffer against peak predation windows, with plasticity enabling adaptive shifts in response to varying threat levels. In multi-species communities, cathemerality facilitates temporal partitioning, where prey species stagger activity to minimize overlap with predators and competitors. For example, spinefoot rabbitfish (Siganus spp.) on coral reefs display variable diel activity to evade jacks (Carangidae), expanding their temporal niche when predator density decreases and thereby reducing vulnerability during high-risk periods.

Thermoregulation and Energetic Constraints

Cathemeral animals often adjust their activity patterns to mitigate thermal stress, particularly in environments with pronounced diurnal temperature fluctuations. In hot climates, endotherms like red kangaroos (Macropus rufus) shift toward increased nocturnal activity to reduce daytime heat exposure, relying on evaporative cooling mechanisms such as licking their forearms during peak temperatures above 40°C, which helps maintain core body temperatures around 36-39°C. This flexibility allows them to avoid solar radiation while foraging at night when ambient temperatures drop, optimizing heat dissipation without compromising energy intake. Similarly, ectothermic cathemeral species, such as the weasel skink (Saproscincus mustelinus), balance thermoregulation by engaging in nocturnal thigmothermy on warm surfaces after sunset, supplementing daytime basking to achieve preferred body temperatures of 28-32°C and sustain activity across light-dark cycles. Energetic constraints play a pivotal role in shaping cathemeral rhythms, influenced by body size and metabolic demands. Larger-bodied cathemerals, such as megaherbivores exceeding 1000 kg (e.g., African elephants), exhibit sustained activity bouts throughout the 24-hour cycle due to lower mass-specific metabolic rates, enabling continuous low-intensity foraging on fibrous vegetation without the need for prolonged rest periods. In contrast, small-bodied mammals under 10 g, like shrews in the genus Crocidura, face elevated metabolic costs—up to 10 times the basal rate during activity—necessitating patchy, hunger-driven cathemeral patterns to frequently replenish energy stores and prevent hypothermia in cooler nights. These patterns reflect metabolic scaling principles, where relative energy expenditure decreases with increasing body mass, allowing larger species greater temporal flexibility while constraining smaller ones to intermittent bursts. Cathemerality imposes trade-offs between foraging and rest to maintain energy balance, particularly under thermoregulatory pressures. In primates like ring-tailed lemurs (Lemur catta), extended daylight hours facilitate increased ground foraging for diverse, nutrient-rich foods, reducing energetic costs associated with climbing and enhancing overall intake by up to 20-30% during warmer seasons, as opposed to energy-intensive arboreal pursuits at night. This strategy provides dual nutritional and thermoregulatory benefits, allowing activity when temperatures align with thermal neutral zones (typically 25-30°C) while minimizing heat stress. Recent syntheses affirm these insights, emphasizing how metabolic scaling exacerbates such constraints in variable thermal environments, where small animals prioritize short, efficient activity windows to offset high baseline expenditures.

Evolutionary Aspects

Origins and Evolutionary Transitions

Cathemerality in mammals is thought to have originated as an expansion from a predominantly nocturnal ancestral state, reflecting the deep evolutionary of synapsids and early mammals that were adapted to nighttime activity over 300 million years ago. Following the Cretaceous-Paleogene (K-Pg) extinction event approximately 66 million years ago, which removed non-avian dinosaurs and opened ecological opportunities, mammals underwent rapid temporal niche diversification, with cathemerality emerging as an intermediate strategy allowing activity across both day and night periods. In primates, this transition likely occurred early in their evolutionary , as ancestral primates were nocturnal, and cathemerality served as a flexible bridge toward diurnality in later lineages, supported by phylogenetic reconstructions showing multiple shifts in activity patterns post-K-Pg. In non-mammalian vertebrates like reptiles, cathemerality has similarly evolved multiple times from nocturnal or diurnal ancestors, often as a stepwise intermediate state. A 2022 phylogenetic analysis of skinks (Scincidae), encompassing nearly 600 species, reconstructed the ancestral state as nondiurnal—likely nocturnal or cathemeral—with cathemerality arising at least five times independently from nocturnal baselines, particularly in fossorial clades associated with limb reduction and underground habits. These transitions highlight cathemerality's role as a labile trait in reptiles, enabling adaptation to variable light environments without committing to strict diurnality or nocturnality. Phylogenetically, cathemerality shows concentrated distribution within specific clades, such as the Lemuridae, where it is ubiquitous across genera like Eulemur, with all exhibiting activity throughout the 24-hour cycle as an ancient trait predating the of around 9–13 million years ago. In broader mammalian phylogeny, however, independent origins of cathemerality are relatively rare and transient, occurring in only about 12% of sampled across diverse orders like and Rodentia, with high rates of evolutionary to strict or limiting its persistence. A 2023 macroevolutionary study across 3,013 confirmed multiple but infrequent origins, attributing this pattern to cathemerality's lower diversification stability compared to unimodal activity rhythms. Fossil evidence for cathemerality remains indirect, inferred from paleoecological contexts of early mammals and in the and Eocene, where post-Cretaceous environmental — including fluctuating regimes due to dense forests and variable climates—likely favored flexible activity patterns over rigid . Earliest fossils from ~66 million years ago, such as those of , suggest small, arboreal insectivores with nocturnal adaptations like large eye orbits, implying cathemerality as a response to emerging diurnal competitors and changing photoperiods in the wake of the K-Pg . This evolutionary shift underscores cathemerality's as a derived trait in recovering mammalian lineages adapting to novel post-dinosaur ecosystems.

Adaptive Advantages and Constraints

Cathemerality confers several adaptive advantages as an evolutionary strategy, primarily by enhancing access to resources and providing flexibility in dynamic environments. By distributing activity across the full 24-hour cycle, cathemeral animals can extend foraging windows, allowing continuous exploitation of food sources that may vary temporally, such as in megaherbivores with high energy demands that feed without strict diurnal or nocturnal restrictions. This temporal partitioning also reduces interspecific competition, as cathemeral species bridge diurnal and nocturnal communities, aligning their activity with prey availability or avoiding overlap with specialized competitors, as observed in various carnivores and herbivores. Furthermore, the facultative nature of cathemerality enables rapid adjustments to environmental variability, such as regional differences in human pressures or resource distribution, exemplified by wild boars that shift activity patterns across habitats. Despite these benefits, cathemerality imposes notable constraints that can limit its evolutionary persistence. Obligate cathemerality, where activity lacks strong circadian entrainment, may disrupt internal rhythms, leading to potential health costs similar to those in animals forced into arrhythmic patterns, including reduced performance and survival due to mismatched physiological processes like hunger-driven cycles in shrews. In stable ecological niches, cathemerality is often less efficient than specialized chronotypes, as the lack of focused activity phases increases vulnerability without commensurate gains in resource acquisition or safety. Additionally, while not extensively quantified, the flexible scheduling inherent to cathemerality likely demands greater cognitive resources for ongoing decision-making on activity timing, potentially elevating energetic overheads in species with limited neural capacity. The evolutionary trade-offs of cathemerality center on balancing predation avoidance against energy expenditures, with optimality varying by context. This pattern allows partial evasion of temporal predators by spreading risk across day and night, but it can incur higher overall energy costs from prolonged vigilance or suboptimal thermoregulation during mismatched phases, as seen in herbivores that increase daytime activity under nocturnal predation pressure at the expense of heat stress. Recent studies highlight this context-dependence, particularly in invertebrates like ants and moths, where cathemerality optimizes foraging under fluctuating predation and resource cues without universal superiority over strict chronotypes. In small mammals, similar trade-offs reveal no clear scaling of cathemerality with body mass, underscoring its role as a situational strategy rather than a universal adaptation.

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