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Eusociality

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.^[1] This social structure enables colonies to function as integrated units, often likened to superorganisms, where individual fitness is subordinated to collective success.^[1] 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.^[2] 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).^[1] 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.^[3] The evolution of eusociality represents a major transition in biological organization, 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.^[4] Key hypotheses for its origins include kin selection, proposed by W. D. Hamilton, which posits that altruism evolves when inclusive fitness benefits outweigh direct reproduction, particularly amplified in haplodiploid Hymenoptera where sisters share 75% of genes on average, favoring worker sterility to rear siblings.^[1] Other factors, such as ecological pressures like nest defense and resource stability, and genomic influences including sex determination systems, contribute to its repeated emergence, though debates persist on the primacy of haplodiploidy versus ecological drivers.^[5]

Definition and Characteristics

Core Criteria

Eusociality is characterized by three core criteria originally proposed by Charles D. Michener in 1969 and popularized by E.O. Wilson 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 reproduction while the majority forgo it to support the colony.^[6]^[7] These traits distinguish eusocial species from less complex social systems by emphasizing collective investment in reproduction over individual efforts.^[4] Eusociality can manifest as strict (also termed obligate or advanced) or facultative (also termed primitive), depending on the degree of caste permanence. In strict eusociality, such as in ants, morphological and behavioral castes are irreversibly differentiated early in development, locking individuals into lifelong roles like non-reproductive workers or queens.^[8] Conversely, facultative eusociality, observed in some sweat bees (e.g., species in the genus 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.^[9] This reproductive division of labor sets eusociality apart from subsociality, where parental care 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.^[10] 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.^[4]

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 queens, which focus on egg-laying, and in termites, kings that contribute to reproduction alongside queens.^[11] Non-reproductive castes consist of workers, which handle tasks such as foraging and nursing, and soldiers in some species, which serve as defensive specialists with enlarged mandibles or chemical weaponry adapted for colony protection.^[12] These castes arise through developmental pathways influenced by environmental and genetic factors, ensuring a structured hierarchy 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 foraging.^[13] 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.^[14] Caste dimorphism is evident in morphological differences that reinforce specialization, such as the contrast between alates—winged reproductives equipped for dispersal and colony founding—and apterous workers in ants, which lack wings and are optimized for sustained labor within the nest.^[15] These physical distinctions, including variations in body size, gland development, and sensory structures, align caste members with their primary functions, minimizing overlap in capabilities.^[16] Specialization through castes and division of labor enhances colony fitness by improving overall efficiency in resource allocation and task execution, allowing colonies to process food, defend territories, and rear offspring more effectively than if individuals multitasked.^[17] This structured organization reduces redundancy and energy waste, enabling larger colony sizes and greater productivity, as specialized workers outperform generalists in their assigned roles.^[18] By linking cooperative brood care to these divisions, colonies achieve sustained growth and resilience to environmental pressures.^[4]

Historical Development

Early Observations

Early observations of eusocial behaviors in insects date back to ancient times, with Aristotle providing one of the first detailed accounts in his History of Animals 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.^[19] Aristotle noted the bees' collective defense of the hive and division of tasks, such as nursing larvae and gathering nectar, portraying the society as a model of natural order. In the 18th and 19th centuries, European naturalists expanded these descriptions through direct fieldwork on ant and bee colonies. Charles Darwin, in On the Origin of Species (1859), highlighted the complexity of ant and honeybee societies, particularly the existence of sterile worker castes that forgo reproduction to support the fertile queen and males.^[20] Darwin viewed these neuter insects as a significant challenge to natural selection, questioning how traits beneficial only to the colony could evolve since sterile individuals could not pass them on directly.^[20] He observed overlapping generations within hives, where workers cared for the queen's offspring, underscoring the altruism in these insect 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 20th century further illuminated interactions within ant societies, particularly through his examinations of parasitism. In works like his 1909 paper on the origins of dulotic and parasitic ants, Emery described how certain ant 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 resource allocation—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 social organization in insects was laid by William Morton Wheeler in his 1928 book The Social Insects: Their Origin and Evolution, which synthesized early observations of communal behaviors in ants, bees, wasps, and termites into a cohesive evolutionary narrative emphasizing polymorphism, division of labor, and colony integration as key social traits.^[21] 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 parental care evolves into group cohesion—providing the conceptual groundwork that later researchers built upon to categorize advanced sociality.^[22] 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.^[23] 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 colony, and a reproductive division of labor with castes that include sterile individuals altruistically aiding reproduction.^[7] Wilson's synthesis integrated behavioral ecology, genetics, and comparative studies of hymenopterans and termites, distinguishing eusociality from lower social grades like subsociality or communal breeding, and emphasized its rarity and adaptive significance in insect societies.^[7] 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 1990s, 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 insects 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 Polistes species.^[24] 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.^[2]^[4] 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.^[25] Characteristic features of eusocial Hymenoptera 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.^[26] Queens in these societies demonstrate exceptional longevity, with some ant queens surviving up to 30 years, far exceeding the lifespan of workers.^[27] Worker sterility is typically enforced through policing behaviors, where workers detect and remove eggs laid by other workers, thereby suppressing reproduction and maintaining the queen's monopoly on fecundity. Variations in eusociality exist across Hymenopteran taxa, ranging from primitive to advanced forms. In primitive eusociality, as seen in halictid bees (family Halictidae), 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.^[28] In contrast, advanced eusociality in honeybees (Apis mellifera) features morphologically distinct, permanently sterile workers that forgo reproduction entirely, dedicating their lives to foraging, nursing, and nest maintenance.^[4] Colony sizes in eusocial Hymenoptera vary dramatically, reflecting ecological adaptations and social complexity. Primitive eusocial wasps, such as those in the genus Polistes, typically form small colonies of dozens of individuals, often starting with a single foundress and growing to 20–100 workers.^[14] 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.^[29]

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.^[30]^[31] 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.^[32] 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.^[31] A distinctive feature of termite eusociality is the diploid inheritance system, which contrasts with the haplodiploidy of hymenopterans, yet supports complex caste differentiation. The colony includes true worker castes—sterile immatures dedicated to foraging, nest maintenance, and brood care—though in some lower termite species, these workers retain developmental plasticity and can become neotenic reproductives (immature secondary reproductives) if the primary pair dies.^[33] Soldiers form a specialized defensive caste, 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.^[34]^[35] 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.^[36] These mounds also provide structural defense, forming a hardened outer barrier against predators and environmental hazards.^[37] 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.^[38]

In Other Insects and Arthropods

Eusociality is exceedingly rare among insects outside the orders Hymenoptera and Isoptera, with fewer than 100 known species exhibiting the full suite of traits including cooperative brood care, overlapping generations, and reproductive division of labor.^[11] This scarcity highlights the exceptional evolutionary constraints and ecological niches required for such complex sociality to emerge independently multiple times. Eusociality has also evolved in thrips (order Thysanoptera), with approximately 7 species exhibiting the trait, primarily in gall-forming species such as those in the genera Oncothrips and Kladothrips. These colonies feature sterile soldier castes that defend against intruders using modified forelimbs, facilitated by parthenogenetic reproduction in enclosed plant galls.^[11] In Coleoptera, the only obligately eusocial species is the Australian ambrosia weevil Austroplatypus incompertus, where colonies inhabit fungal-cultivated galleries in eucalyptus 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 soldier castes, relying instead on behavioral defense by workers, with eusociality likely facilitated by the stable, defended wood-boring habitat and diploid inheritance system.^[39] Among aphids (Hemiptera: Aphididae), eusociality has evolved in several gall-inducing species, particularly in the genus Pemphigus, where clonal reproduction through parthenogenesis enables the production of sterile defender castes. In species like Pemphigus spyrothecae, a foundress aphid induces a root gall on poplar plants, 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 species 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 altruism.^[11] In arthropods beyond insects, eusociality occurs in the crustacean family Alpheidae, specifically sponge-dwelling snapping shrimp of the genus Synalpheus. Eusocial colonies, first documented in the 1990s, consist of a dominant breeding pair (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 sponges. 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 insect systems but adapted to an aquatic context without haplodiploidy.^[40]

In Non-Arthropods

Eusociality is exceptionally rare among vertebrates, having evolved independently in only two genera of rodents within the family Bathyergidae approximately 25 million years ago during the early Miocene.^[41] These include the naked mole-rat (Heterocephalus glaber) and the Damaraland mole-rat (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.^[42] In naked mole-rat colonies, typically comprising 70–300 individuals, a single queen monopolizes reproduction, often through aggressive suppression of subordinates, while workers of both sexes perform foraging, burrow maintenance, and pup care without reproducing.^[42] Breeding often involves incestuous mating between the queen and a related male, maintaining high genetic relatedness that supports kin selection.^[43] Similarly, Damaraland mole-rat colonies, usually smaller with 10–25 members, feature a dominant breeding pair and non-breeding helpers that contribute to colony tasks, with reproduction likewise skewed toward a single female and occasional inbreeding among close kin.^[44] This mammalian eusociality contrasts with arthropod forms by occurring in diploid organisms adapted to arid, subterranean environments where cooperative digging and defense enhance survival. Beyond vertebrates, eusocial-like organization appears in trematode flatworms (phylum Platyhelminthes), particularly in larval stages within snail 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 combat—and behaviorally, with soldiers comprising about 10–20% of the colony to optimize transmission success in the confined snail environment.^[45] This represents one of the few non-arthropod examples of eusocial traits, driven by intense interspecific competition within hosts.^[46] 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.^[47] 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.^[48] 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.^[49] 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).^[50]^[8]^[51] 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).^[52]^[1] 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 Burmese amber from the Lower Cretaceous, dating to approximately 99 million years ago.^[53] These fossils include soldier termites with specialized morphologies for colony defense and worker ants engaged in foraging or combat, indicating advanced division of labor and cooperative behaviors characteristic of eusocial societies.^[54] Such findings demonstrate that eusociality in ants and termites predates the Cretaceous-Paleogene (K-Pg) extinction event by tens of millions of years, with no earlier unequivocal fossil evidence identified. Genomic phylogenies further refine the timing of eusociality's emergence in Hymenoptera, revealing that it arose in some lineages after the K-Pg boundary (66 million years ago), coinciding with post-extinction ecological opportunities.^[55] For instance, ancestral state reconstructions in bees suggest transitions to eusociality occurred during the Paleogene, following the diversification of flowering plants.^[8] These molecular estimates align with the fossil data, portraying eusociality as an adaptive response to shifting environments across deep time, with recent 2025 analyses indicating even higher numbers of origins within bee families like Halictidae (at least 11).^[56] Eusociality's phylogenetic distribution is heavily skewed toward holometabolous insects—those undergoing complete metamorphosis, such as Hymenoptera and termites—where larval and adult stages enable specialized castes and division of labor.^[1] This pattern is rare outside arthropods, absent in most vertebrates, with the notable exception of the aforementioned eusocial rodents, which represent a singular vertebrate instance.^[4] Overall, the trait's sporadic occurrence underscores its evolutionary lability, confined to taxa with predisposing life-history traits like parental care and nest fidelity.

The Paradox of Eusociality

Eusociality presents a profound evolutionary paradox because it involves the evolution and persistence of non-reproductive castes, such as workers, that forgo personal reproduction entirely, thereby reducing their direct fitness to zero while aiding the reproduction of others in the colony. This apparent conflict with the principles of natural selection, where traits enhancing individual reproductive success are favored, has long puzzled biologists, as sterile individuals seemingly contribute nothing to their own genetic lineage through direct descent.^[1] Charles Darwin first articulated this challenge in 1859, describing the sterile "neuter" insects in ant and bee colonies as "one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory." He noted that natural selection 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.^[57] Empirical studies reveal that while high genetic relatedness within colonies—often resulting from inbreeding or haplodiploid sex determination—facilitates cooperation, 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.^[58] 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.^[59] 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.^[60] 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.^[61]

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 reproduction but also indirect fitness gained by aiding the reproductive success of genetic relatives, weighted by the coefficient of relatedness r. This concept, introduced by W. D. Hamilton, extends classical Darwinian fitness by accounting for the propagation of genes via kin, thereby providing a mechanism for the evolution of altruism where individuals sacrifice their own reproduction to benefit relatives. Hamilton's rule, rB > C, quantifies the condition under which a social behavior evolves, where r is the genetic relatedness between actor and recipient, B is the fitness benefit to the recipient, and C is the fitness cost to the actor. 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 - 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 cost C of sterility is offset if the benefit B to the queen's production of additional sisters, multiplied by the worker's relatedness r to those sisters, yields greater indirect fitness gains than the worker could achieve by reproducing independently. Experimental evidence from honey bees supports kin selection, as worker policing behaviors align with inclusive fitness predictions by preferentially removing worker-laid eggs to favor queen production.^[62] Such findings demonstrate that worker altruism, policed to align with inclusive fitness, directly boosts colony-level fitness by prioritizing queen-reared offspring.^[62] The framework of inclusive fitness and kin selection extends to diploid eusocial taxa like termites, where high paternity skew maintains elevated average relatedness among colony members to satisfy Hamilton's rule despite symmetric sex determination. Termite colonies typically arise from lifelong monogamous pairs, resulting in full-sibling relatedness of r = 0.5, equivalent to a worker's relatedness to its own offspring; however, extreme paternity skew in species with occasional multiple mating ensures most offspring share a single father, preventing relatedness asymmetry and sustaining indirect fitness benefits that favor worker sterility when benefits B from cooperative brood care exceed twice the reproductive cost C. This mechanism underscores kin selection's generality across genetic systems, resolving the paradox of eusociality in diploids through assured high relatedness.^[63]^[64]

Haplodiploidy Hypothesis

The haplodiploid sex determination system, characteristic of the order Hymenoptera (including ants, bees, and wasps), produces males from unfertilized haploid eggs and females from fertilized diploid eggs.^[65] 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.^[65] This asymmetry forms the basis of the haplodiploidy hypothesis, proposed by W. D. Hamilton, which posits that the elevated relatedness between sisters favors the evolution of worker sterility in females, as they gain greater inclusive fitness by raising sisters (reproductives) rather than their own offspring.^[65] Hamilton argued that this predisposition explains the higher incidence of eusociality in haplodiploid insects compared to diploids.^[65] 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.^[1] 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.^[66] Empirical support for the hypothesis 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 inclusive fitness.^[67] This bias has been corroborated in various hymenopteran species, though its consistency varies with mating frequency and colony structure.^[67]

Multilevel Selection

Multilevel selection theory posits that natural selection operates simultaneously at multiple hierarchical levels, including both individuals and groups, to explain the evolution of complex social behaviors like eusociality. In eusocial species, individual-level selection often favors selfish reproduction, but group-level (colony-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.^[1] This framework views the colony as a superorganism where selection favors traits that benefit the collective unit, even at the expense of individual fitness components.^[68] 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 natural selection acting on colony-level traits rather than solely through gene-centered kin selection. 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 unit of selection.^[68] 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.^[69] 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 helpers emerge and persist when migration rates are low, as colony-level benefits from cooperation outweigh individual-level costs, particularly in haplo-diploid and diploid systems.^[70] These models indicate that population viscosity amplifies group selection's effect, allowing eusocial traits to spread faster than under panmictic conditions.^[71] 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.^[72] This combined approach underscores how individual and group selections interact to maintain harmony in highly integrated societies like those of ants and bees.^[4]

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.^[73] This mechanism is particularly evident in primitively eusocial wasps and bees, where offspring remain philopatric, enhancing colony persistence amid unpredictable habitats.^[74] Group living in eusocial species also provides robust defense against predators and parasites, reducing individual risk through collective vigilance and specialized castes. In ants and termites, colony members form barriers, alarm systems, and soldier castes that deter intruders, such as rival ants or vertebrate predators, far more effectively than solitary individuals could.^[4] For instance, termite soldiers use chemical signals and physical aggression to protect nests from kleptoparasites and competitors, minimizing per capita mortality in dense, high-risk habitats.^[75] 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.^[4] The spatial distribution of resources further influences eusocial colony formation, with clumped food sources promoting aggregation and cooperative exploitation. In termites, access to concentrated wood resources necessitates durable nests defended by workers, fostering division of labor and overlap of generations.^[76] Similarly, ants thrive in environments where prey or seeds are patchily distributed, allowing colonies to monopolize and store these resources through foraging teams, which solitary insects could not efficiently secure.^[76] 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.^[77] 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.^[78] 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.^[79]

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 pheromone (QMP), a blend of volatile fatty acids primarily consisting of 9-ODA, 9-HDA, HOB, and methyl p-hydroxybenzoate, is secreted from the queen's mandibular glands and distributed throughout the colony via worker retinue behavior. QMP inhibits ovarian development in workers, preventing vitellogenin synthesis and oogenesis, thereby enforcing sterility in the worker caste. 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%.^[80] Juvenile hormone (JH) regulates caste-specific physiology and behavior in eusocial Hymenoptera, 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 foraging, enhancing locomotor activity and pollen 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 oogenesis 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.^[81]^[82] Nutritional cues during larval stages drive caste determination in termites through differential feeding by workers, influencing developmental trajectories without fixed genetic predestination. In species like Reticulitermes spp., larvae destined for queen (reproductive) development receive protein-rich diets from worker trophallaxis, elevating JH synthesis and promoting alate (winged reproductive) morphogenesis with extended growth periods and larger ovaries. In contrast, underfed larvae default to worker or soldier castes, characterized by abbreviated development and sterile morphology. This plasticity is evident in colony-founding experiments, underscoring nutrition's role in modulating JH-mediated polyphenism. Recent research from the 2020s has revealed that stress-related hormones, such as biogenic amines including octopamine and corazonin, modulate caste flexibility in facultatively eusocial species, allowing adaptive shifts between solitary and cooperative phenotypes. Similarly, corazonin signaling influences caste identity transitions in ants like Harpegnathos saltator, where stress-induced upregulation promotes gamergate (worker-reproductive) reversion, increasing colony resilience. These findings indicate that stress hormones integrate ecological pressures with physiological plasticity, enabling reversible caste expression in species with optional eusociality.^[83]^[51]

Molecular and Genomic Basis

The molecular and genomic basis of eusociality involves intricate gene regulatory networks that underpin caste differentiation and social behaviors in insects. In honeybees (Apis mellifera), transcriptomic studies have identified vitellogenin (vg) as a key molecular signature for queen caste 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 caste fate, promoting queen determination under high-nutrient conditions while suppressing it in workers.31569-2) These pathways interact with epigenetic modifications, such as m6A RNA methylation, to fine-tune gene expression during larval development, ensuring stable caste-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 Hymenoptera, modulating gene expression without altering DNA sequences. In species like ants and bees, long noncoding RNAs (lncRNAs) and microRNAs (miRNAs) regulate developmental transitions by targeting transcription factors involved in caste polyphenism.^[84] 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 DNA methylation in germ cells and somatic tissues.^[84] 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.^[85] Phylogenomic analyses of eusocial insect genomes reveal signatures of adaptive evolution, including expansions in gene families associated with sensory and immune functions essential for colony life. In ants, bees, and termites, olfactory receptor genes have undergone significant duplications, enabling enhanced chemical communication for task allocation and nestmate recognition.^[86] Similarly, immunity-related gene families, such as those encoding antimicrobial peptides and Toll-like receptors, show expansions in social species, reflecting the selective pressures of dense colony living and pathogen exposure.^[86] These patterns, identified through comparative phylogenomics across Hymenopteran and Isopteran lineages, indicate that eusociality drives convergent genomic innovations for social integration.^[87] Genomic evidence also illuminates mechanisms resolving cooperation and conflict in eusocial societies, particularly in termites where policing behaviors suppress worker reproduction to maintain colony stability. Studies identify candidate policing genes, such as those involved in egg recognition and aggression, with caste-specific expression patterns that enforce reproductive altruism and mitigate intragenomic conflicts over resource allocation.^[88] In Reticulitermes flavipes, genomic scans reveal signatures of selection on these loci, supporting their role in conflict resolution and the evolution of worker sterility.^[89] Such findings underscore how genomic adaptations promote harmony in diploid eusocial systems like termites, complementing kin selection in haplodiploid Hymenoptera.

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 Hymenoptera, 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 Halictidae), where phenotypic plasticity allows switches between solitary and social phenotypes in response to environmental cues, highlighting the reversibility of early social stages.^[90]^[91] Fossil records provide snapshots of these transitions, with Cretaceous amber inclusions from Myanmar (ca. 99 million years ago) preserving early eusocial ants and termites 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 Mesozoic 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 Polistes gallicus, helpers contribute disproportionately to colony founding, enhancing productivity beyond solitary efforts and stabilizing the shift toward reproductive division of labor.00088-9)^[53]^[92]^[93] 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.^[94]^[92]^[11] Initial barriers to eusociality, including reproductive conflicts among co-foundresses over nest inheritance, were likely overcome through the evolution of kin recognition 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 discrimination of relatives, mitigating potential infanticide or resource theft in nascent groups and facilitating the stable emergence of helping roles.^[95]^[96]

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.^[60]^[97]^[8] In ants, reversals are less common but manifest through the evolution of social parasitism, where eusocial ancestors lose key social structures like the worker caste. For instance, in the genus Formica, permanent inquiline parasites such as certain Formica species have evolved from temporary parasitic ancestors by completely forgoing worker production, relying instead on host colonies to rear sexual offspring; this represents a degeneration to a solitary-like reproductive strategy within a parasitic context. Facultative eusociality in some bee species further illustrates conditional reversals at the population level, where individuals shift to solitary behavior in environments with abundant resources, reducing the fitness advantages of group living.^[98] 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.^[99] 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.^[100] 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.^[97]

Eusociality in Humans and Culture

Analogies in Human Societies

In his 2012 book The Social Conquest of Earth, biologist E.O. Wilson 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 ants and bees. 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 ecological niche.^[101] 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, human reproduction is broadly distributed, and roles are fluid and culturally influenced rather than genetically rigid.^[101] Furthermore, cultural evolution—through learned behaviors, norms, and institutions—dominates human social dynamics, overriding the instinctual, kin-based mechanisms typical of eusocial insects and rendering direct analogies problematic. Supporting evidence for partial parallels includes observations from hunter-gatherer societies, where cooperative child-rearing and non-reproductive roles foster group cohesion; for instance, among the Hadza of Tanzania, 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 "grandmother hypothesis" illustrates alloparenting and division of labor in human bands, akin to worker assistance in eusocial colonies, though without the irreversible sterility seen in insects.^[102] Post-2020 scientific debates have increasingly emphasized that human social complexity has arisen through convergent evolution with insect eusociality, via distinct genetic and cultural pathways rather than shared genomic mechanisms. As of 2025, 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.^[103]^[104]

Representations in Popular Culture

In H.G. Wells' short story "The Empire of the Ants" (1905), giant ants in the Amazon basin are depicted as exhibiting advanced collective intelligence, organizing coordinated attacks on humans and suggesting an emergent societal structure akin to a conquering empire.^[105] This portrayal draws on early 20th-century fascination with tropical entomology, framing the ant colony as a model of efficient, hierarchical social organization that challenges human dominance.^[106] The 1998 animated films Antz and A Bug's Life both anthropomorphize ant societies to explore themes of caste systems and internal conflicts within eusocial structures. In Antz, the worker ant protagonist Z rebels against a rigid, militaristic colony hierarchy, highlighting tensions between individual desires and collective duties enforced by soldier and royal castes.^[107] Similarly, A Bug's Life presents the ant colony as a cooperative yet oppressed community under external threats from grasshoppers, with characters embodying diverse roles like inventors and foragers to emphasize communal problem-solving and caste interdependence.^[108] Documentaries in the 2020s have increasingly focused on honeybee societies, underscoring threats to eusocial order from environmental stressors. For instance, The Pollinators (2020) examines commercial beekeeping operations and the impacts of habitat loss and rising temperatures that disrupt foraging patterns and increase colony collapse risks.^[109]^[110] 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.^[111] 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.^[112]

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Reproduction in worker honey bees is associated with low juvenile ...
Three experiments were performed to determine the role of juvenile hormone (JH) in worker reproduction in queenless colonies of honey bees.Missing: key study
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Feb 22, 2005 · In honey bees, where JH increases with worker age, relatively high JH is associated with foraging behavior, and JH, although not required for ...
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The Role of Biogenic Amines in Social Insects - PubMed Central - NIH
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The molecular substrates of insect eusociality - PMC - PubMed Central
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The eusocial non-code: Unveiling the impact of noncoding RNAs on ...
The phenomenon of eusociality is characterized by the presence of distinct castes within a species, with significant examples of its emergence seen in the ...Review Article · 4. Noncoding Rnas: Types And... · AcknowledgmentsMissing: reversible | Show results with:reversible
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In this article, we review current knowledge on the genomic signatures of eusocial evolution in insects and discuss future avenues of research. ... Genomic ...
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Genomic signatures of eusocial evolution in insects - PubMed
Nov 3, 2023 · Abstract. The genomes of eusocial insects allow the production and regulation of highly distinct phenotypes, largely independent of genotype.Missing: gene families olfaction immunity phylogenomic analyses 2023-2025
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Cooperative policing behaviour regulates reproductive division of ...
In eusocial Hymenoptera (ants, bees and wasps), workers or the queen act aggressively towards fertile workers, or destroy their eggs. In many termite species ( ...
[83]
Cooperation and conflict in termite societies - ScienceDirect.com
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Social trait definitions influence evolutionary inferences
Subsocial versus parasocial routes to eusociality. The subsocial route to eusociality posits the following transitions: solitary → subsocial → eusocial ...
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Evolution: Plastic Sociality in a Sweat Bee - Cell Press
This experiment indicates that phenotypic plasticity underlies an ongoing evolutionary transition between solitary parental care and eusociality in this group.
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Genomic signatures of evolutionary transitions from solitary to group ...
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Early-season helping yields increasing returns to scale at the onset ...
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Genomic signatures of evolutionary transitions from solitary to group ...
Abstract. The evolution of eusociality is one of the major transitions in evolution, but the underlying genomic changes are unknown.
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Eusociality through conflict dissolution - PMC - NIH
Eusociality is commonly defined as involving groups with reproductive division of labour, overlapping generations and cooperative work [4]. Additionally, an ...Missing: barriers recognition
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(PDF) Kin recognition in eusocial waSPS - ResearchGate
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Convergent and complementary selection shaped gains and losses ...
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Pathways to parasitic strategies in ants - PNAS
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The risk-return trade-off between solitary and eusocial reproduction
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Revisiting “Grandmothers and the Evolution of Human Longevity ...
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driven not by ...
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SFE: Ants - SF Encyclopedia
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Sense per Simplicity: Ant Societies as a Self-description Formula of ...
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'It's only important if you eat food': inside a film on the honeybee crisis
Jun 17, 2020 · And the bees face increasing risks of disaster; in Nelson's film The Pollinators, a 90-minute documentary on commercial beekeeping and its ...<|separator|>
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“The Pollinators”: New Film Shows How Decline of Bee Colonies ...
Nov 7, 2019 · The film is called The Pollinators and features swarms of yellow-black jacketed honey bees, whose existence may determine the future of human ...
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Star Trek and Collectivism: The Case of the Borg - FEE.org
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