Eusociality
Eusociality is the highest level of organization of sociality. It is defined by the following characteristics: cooperative brood care, overlapping generations within a colony of adults, and a division of labor into reproductive and non-reproductive groups. The division of labor creates specialized behavioral groups within an animal society, sometimes called castes. Eusociality is distinguished from all other social systems because individuals of at least one caste usually lose the ability to perform behaviors characteristic of individuals in another caste. Eusocial colonies can be viewed as superorganisms.
Eusociality has evolved among the insects, crustaceans, trematodes and mammals. It is most widespread in the Hymenoptera and in termites. A colony has caste differences: queens and reproductive males take the roles of the sole reproducers, while soldiers and workers work together to create and maintain a living situation favorable for the brood. Queens produce multiple queen pheromones to create and maintain the eusocial state in their colonies; they may also eat eggs laid by other females or exert dominance by fighting. There are two eusocial rodents: the naked mole-rat and the Damaraland mole-rat. Some shrimps, such as Synalpheus regalis, are eusocial. E. O. Wilson and others have claimed that humans have evolved a weak form of eusociality. It has been suggested that the colonial and epiphytic staghorn fern, too, may make use of a primitively eusocial division of labor.
History
The term "eusocial" was introduced in 1966 by Suzanne Batra, who used it to describe nesting behavior in Halictid bees, on a scale of subsocial/solitary, colonial/communal, semisocial, and eusocial, where a colony is started by a single individual. Batra observed the cooperative behavior of the bees, males and females alike, as they took responsibility for at least one duty within the colony. The cooperativeness was essential as the activity of one labor division greatly influenced the activity of another. Eusocial colonies can be viewed as superorganisms, with individual castes being analogous to different tissue or cell types in a multicellular organism; castes fulfill a specific role that contributes to the functioning and survival of the whole colony, while being incapable of independent survival outside the colony.In 1969, Charles D. Michener further expanded Batra's classification with his comparative study of social behavior in bees. He observed multiple species of bees in order to investigate the different levels of animal sociality, many of which are different stages that a colony may pass through. Eusociality, which is the highest level of animal sociality a species can attain, specifically had three characteristics that distinguished it from the other levels:
- Egg-layers and worker-like individuals among adult females
- The overlap of generations
- Cooperative care of the brood
Eusociality was then discovered in a group of chordates, the mole-rats. Some researchers have argued that another possibly important criterion for eusociality is "the point of no return". This is characterized by having individuals fixed into one behavioral group, usually before reproductive maturity. This prevents them from transitioning between behavioral groups, and creates a society with individuals truly dependent on each other for survival and reproductive success. For many insects, this irreversibility has changed the anatomy of the worker caste, which is sterile and provides support for the reproductive caste. Other researchers have suggested that cooperative breeding and eusociality are not discrete phenomena, but rather form a continuum of fundamentally similar social systems whose main differences lie in the distribution of lifetime reproductive success among group members. Vertebrate and invertebrate cooperative breeders can be arrayed along a common axis, that represents a standardized measure of reproductive variance. In this view, loaded terms like "primitive" and "advanced" eusociality should be dropped. An advantage of this approach is that it unites all occurrences of alloparental helping of kin under a single theoretical umbrella.
Diversity
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, thrips, aphids, wasps, and beetles.In hymenopterans
The order Hymenoptera contains the largest group of eusocial insects, including ants, bees, and wasps—divided into castes: reproductive queens, drones, more or less sterile workers, and sometimes also soldiers that perform specialized tasks. 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 and wasps are eusocial, nearly all species of ants are eusocial. Some major lineages of wasps are mostly or entirely eusocial, including the subfamilies Polistinae and Vespinae. The corbiculate bees contain four tribes of varying degrees of sociality: the highly eusocial Apini and Meliponini, primitively eusocial Bombini, and the mostly solitary or weakly social Euglossini. 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, where worker bees responded to queen pheromone from the related Apis florea. 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", creating very small social colonies, where both females are capable of reproduction though only the foundress female appears to lay eggs. In Halictidae, 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 clime in which the species is found - they are solitary in colder climates and social in warmer climates.
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.