Most eusocial societies exist in
arthropods, while a few are found in
mammals. Some
ferns may exhibit a form of eusocial behavior.
In insects Eusociality has evolved multiple times in different insect orders, including
hymenopterans,
termites, and
beetles. In the well-studied social wasp
Polistes versicolor, dominant females perform tasks such as building new cells and ovipositing, while subordinate females tend to perform tasks like feeding the larvae and foraging. The task differentiation between castes can be seen in the fact that subordinates complete 81.4% of the total foraging activity, while dominants only complete 18.6% of the total foraging. Eusocial species with a sterile caste are sometimes called hypersocial. While only a moderate percentage of species in bees (families
Apidae and
Halictidae) and wasps (
Crabronidae and
Vespidae) are eusocial, nearly all species of ants (
Formicidae) are eusocial. Some major lineages of wasps are mostly or entirely eusocial, including the subfamilies
Polistinae and
Vespinae. The corbiculate bees (subfamily Apinae of family
Apidae) contain four tribes of varying degrees of sociality: the highly eusocial
Apini (honey bees) and
Meliponini (stingless bees), primitively eusocial
Bombini (bumble bees), and the mostly solitary or weakly social
Euglossini (orchid bees). Eusociality in these families is sometimes managed by a set of
pheromones that alter the behavior of specific castes in the colony. These pheromones may act across different species, as observed in
Apis andreniformis (black dwarf honey bee), where worker bees responded to queen pheromone from the related
Apis florea (red dwarf honey bee). Pheromones are sometimes used in these castes to assist with foraging. Workers of the Australian stingless bee
Tetragonula carbonaria, for instance, mark food sources with a pheromone, helping their nest mates to find the food. Beside corbiculate bees, eusociality is documented within Apidae in xylocopine bees, where only simple colonies containing one or two "worker" females have been documented in the tribes
Xylocopini and
Ceratinini, though some members of
Allodapini have larger eusocial colonies. Similarly, in the
Colletidae, there is only one species reported to exhibit any form of social behavior; occasional nests of the species
Amphylaeus morosus contain a female and a "guard" (a sister or daughter of the female that founded the nest), creating very small social colonies, where both females are capable of reproduction though only the foundress female appears to lay eggs. In
Halictidae (sweat bees), by contrast, eusociality is well-documented in hundreds of species, primarily in the genera
Halictus and
Lasioglossum. In
Lasioglossum aeneiventre, a halictid bee from Central America, nests may be headed by more than one female; such nests have more cells, and the number of active cells per female is correlated with the number of females in the nest, implying that having more females leads to more efficient building and provisioning of cells. In several species with only one queen, such as
Lasioglossum malachurum in Europe, or
Halictus rubicundus in North America, the degree of eusociality depends on the climatic zone in which the species is found - they are solitary in colder climates and social in warmer climates. '' honeypot ants, showing the repletes or plerergates, their abdomens swollen to store honey (top), with ordinary workers (bottom) Reproductive specialization in Hymenoptera generally involves the production of sterile members of the species, which carry out specialized tasks to care for the reproductive members. Individuals may have behavior and morphology modified for group defense, including
self-sacrificing behavior. For example, members of the sterile caste of the
honeypot ants such as
Myrmecocystus fill their abdomens with liquid food until they become immobile and hang from the ceilings of the underground nests, acting as food storage for the rest of the colony. Not all social hymenopterans have distinct morphological differences between castes. For example, in the Neotropical social wasp
Synoeca surinama, caste ranks are determined by social displays in the developing brood. Castes are sometimes further specialized in their behavior based on age, as in
Scaptotrigona postica workers. Between approximately 0–40 days old, the workers perform tasks within the nest such as provisioning cell broods, colony cleaning, and nectar reception and dehydration. Once older than 40 days,
S. postica workers move outside the nest for colony defense and foraging.
In termites s live in large nests, with queen, king, soldier (red heads), and worker (pale heads) castes.
Termites (order
Blattodea, infraorder Isoptera) make up another large portion of highly advanced eusocial animals. The colony is differentiated into various castes: the queen and king are the sole reproducing individuals; workers forage and maintain food and resources; and soldiers defend the colony against ant attacks. The latter two castes, which are sterile and perform highly specialized, complex social behaviors, are derived from different stages of
pluripotent larvae produced by the reproductive caste. Some soldiers have jaws so enlarged (specialized for defense and attack) that they are unable to feed themselves and must be fed by workers.
In beetles Austroplatypus incompertus is a species of
ambrosia beetle native to Australia, and is the first beetle (order
Coleoptera) to be recognized as eusocial. This species forms colonies in which a single female is fertilized, and is protected by many unfertilized females, which serve as workers excavating tunnels in trees. This species has cooperative brood care, in which individuals care for juveniles that are not their own. These species have very high relatedness among individuals due to their
asexual reproduction (sterile soldier castes being clones produced by
parthenogenesis), but the gall-inhabiting behavior gives these species a defensible resource. They produce soldier castes for fortress defense and protection of the colony against predators,
kleptoparasites, and competitors. In these groups, eusociality is produced by both high relatedness and by living in a restricted, shared area.
In crustaceans Eusociality has evolved in three different lineages in the colonial
crustacean genus
Synalphaeus.
S. regalis,
S. microneptunus,
S. filidigitus,
S. elizabethae,
S. chacei,
S. riosi,
S. duffyi, and
S. cayoneptunus are the eight recorded species of parasitic shrimp that rely on fortress defense and live in groups of closely related individuals in tropical reefs and sponges. They live eusocially with a single breeding female, and a large number of male defenders armed with enlarged snapping claws. There is a single shared living space for the colony members, and the non-breeding members act to defend it. The fortress defense hypothesis additionally points out that because sponges provide both food and shelter, there is an aggregation of relatives (because the shrimp do not have to disperse to find food), and much competition for those nesting sites. Being the target of attack promotes a good defense system (soldier caste); soldiers promote the fitness of the whole nest by ensuring safety and reproduction of the queen. Eusociality offers a competitive advantage in shrimp populations. Eusocial species are more abundant, occupy more of the habitat, and use more of the available resources than non-eusocial species.
In trematodes The
trematodes are a class of parasitic flatworm, also known as flukes. One species,
Haplorchis pumilio, has evolved eusociality involving a colony creating a class of sterile soldiers. One fluke invades a host and establishes a colony of dozens to thousands of clones that work together to take it over. Since rival trematode species can invade and replace the colony, it is protected by a specialized caste of sterile soldier trematodes. Soldiers are smaller, more mobile, and develop along a different pathway than sexually mature reproductives. One difference is that a soldier's mouthparts (pharynx) is five times as big as those of the reproductives. They make up nearly a quarter of the volume of the soldier. These soldiers do not have a germinal mass, never metamorphose to be reproductive, and are, therefore, obligately sterile. These mole-rats live in harsh, limiting environments, where dispersal is difficult and dangerous and cooperation is required to find food and defend against predators. Most colony members are workers, and they cooperatively care for offspring of a single reproductive female (the queen) to whom they are closely related. These mole-rats are eusocial under any definition of the term. Interestingly, the discovery of male and female dispersers has revealed that there is a mechanism of inter-colony outbreeding in naked mole-rats. Outbreeding reduces intra-colony genetic relatedness, but reduces inbreeding depression. Dispersers are morphologically, physiologically and behaviorally distinct from colony members and actively seek to leave their burrow when an escape opportunity presents itself. These individuals are equipped with generous fat reserves for their journey.
In humans Scientists have debated whether humans are
prosocial or eusocial. Human reproductive labor can be considered as being divided between castes if one treats
menopausal women as a sterile caste. Critics of
group selection theory such as
Richard Dawkins reject this argument. They reject the view that human reproductive labor can be viewed as being divided between castes. it has been suggested that homosexuality in men and menopause could have evolved through
kin selection. This would mean that humans sometimes exhibit a type of
alloparental behavior known as "
helpers at the nest", with juveniles and sexually mature adolescents helping their parents raise subsequent broods, as in some birds,
some bees without sterile castes, and
meerkats. In these species, the helpers reproduce on their own if given the opportunity. Whether these animals are considered
eusocial depends on whether "eusociality" definition requries a sterile caste, which as mentioned above is a subject of disagreement.
In plants '' may display a simple form of eusociality. One plant, the
epiphytic staghorn fern,
Platycerium bifurcatum (
Polypodiaceae), may exhibit a primitive form of eusocial behavior amongst clones. The evidence for this is that individuals live in colonies, where they are structured in different ways, with
fronds of differing size and shape, to collect and store water and nutrients for the colony to use. At the top of a colony, there are both pleated fan-shaped "nest" fronds that collect and hold water, and gutter-shaped "strap" fronds that channel water: no solitary
Platycerium species has both types. At the bottom of a colony, there are "nest" fronds that clasp the trunk of the tree supporting the fern, and drooping photosynthetic fronds. These are argued to be adapted to support the colony structurally, i.e. that the individuals in the colony are to some degree specialized for tasks, a
division of labor. == Evolution ==