Eusociality (Ancient Greek εὖ 'good' and social) is the highest level of organization of sociality. It is defined by the following characteristics: cooperative Offspring care (including care of offspring from other individuals), 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 .
Eusociality has evolved among the , , trematoda and . It is most widespread in the Hymenoptera (, , and ) and in Isoptera (). A colony has caste differences: queens and reproductive Male 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 : the naked mole-rat and the Damaraland mole-rat. Some , such as Synalpheus regalis, are eusocial. E. O. Wilson and others have claimed that have evolved a weak form of eusociality. It has been suggested that the colonial and Epiphyte staghorn fern, too, may make use of a primitively eusocial division of labor.
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 (Apoidea) 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:
E. O. Wilson extended the concept to include other social insects, such as ants, wasps, and termites. Originally, it was defined to include organisms (only invertebrates) that fulfilled the same three criteria defined by Michener.
Eusociality was then discovered in a group of Chordata, 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 (e.g., Hamilton's rule). Thus, cooperatively breeding vertebrates can be regarded as eusocial, just as eusocial invertebrates are cooperative breeders.
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
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 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.
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.
Some mammals in the Carnivora and Primates have eusocial tendencies, especially ( Suricata suricatta) and ( Helogale parvula). These show cooperative breeding and marked reproductive skews. In the dwarf mongoose, the breeding pair receives food priority and protection from subordinates and rarely has to defend against predators.
Though controversial, it has been suggested that male homosexuality and female 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, Carpenter bee, and . These species are not eusocial: they do not have castes, and helpers reproduce on their own if given the opportunity.
In haplodiploid species, females develop from fertilized eggs and males develop from unfertilized eggs. Because a male is haploid, his daughters share 100% of his genes and 50% of their mother's. Therefore, they share 75% of their genes with each other. This mechanism of sex determination gives rise to what W. D. Hamilton first termed "supersisters", more closely related to their sisters than they would be to their own offspring. Even though workers often do not reproduce, they can pass on more of their genes by helping to raise their sisters than by having their own offspring (each of which would only have 50% of their genes). This unusual situation, where females may have greater fitness when they help rear sisters rather than producing offspring, is often invoked to explain the multiple independent evolutions of eusociality (at least nine separate times) within the Hymenoptera.
Further, not all eusocial species are haplodiploid: termites, some snapping shrimps, and mole rats are not. Conversely, non-eusocial bees are also haplodiploid, and among eusocial species many queens mate with multiple males, resulting in a hive of half-sisters that share only 25% of their genes. The association between haplodiploidy and eusociality is below statistical significance. Haplodiploidy is thus neither necessary nor sufficient for eusociality to emerge. Relatedness does still play a part, as monogamy (queens mating singly) is the ancestral state for all eusocial species so far investigated. If kin selection is an important force driving the evolution of eusociality, monogamy should be the ancestral state, because it maximizes the relatedness of colony members.
Climate too appears to be a selective agent driving social complexity; across bee lineages and Hymenoptera in general, higher forms of sociality are more likely to occur in tropical than temperate environments. Similarly, social transitions within Halictidae, where eusociality has been gained and lost multiple times, are correlated with periods of climatic warming. Social behavior in facultative social bees is often reliably predicted by ecological conditions, and switches in behavioral type have been experimentally induced by translocating offspring of solitary or social populations to warm and cool climates. In H. rubicundus, females produce a single brood in cooler regions and two or more broods in warmer regions, so the former populations are solitary while the latter are social. In another species of sweat bees, L. calceatum, social phenotype has been predicted by altitude and micro-habitat composition, with social nests found in warmer, sunnier sites, and solitary nests found in adjacent, cooler, shaded locations. Facultatively social bee species, however, which comprise the majority of social bee diversity, have their lowest diversity in the tropics, being largely limited to temperate regions.
All reversals to solitarity have occurred among primitively eusocial groups; none have followed the emergence of advanced eusociality. The "point of no return" hypothesis posits that the morphological differentiation of reproductive and non-reproductive castes prevents highly eusocial species such as the honeybee from reverting to the solitary state.
The levels of two of the aliphatic compounds increase rapidly in virgin queens within the first week after eclosion, consistent with their roles as sex attractants during the mating flight. Once a queen is mated and begins laying eggs, she starts producing the full blend of compounds. In several ant species, reproductive activity is associated with pheromone production by queens. Mated egg-laying queens are attractive to workers, whereas young winged virgin queens elicit little or no response.
Among ants, the queen pheromone system of the fire ant Solenopsis invicta includes both releaser and primer pheromones. A queen recognition (releaser) pheromone is stored in the poison sac along with three other compounds. These compounds elicit a behavioral response from workers. Several primer effects have also been demonstrated. Pheromones initiate reproductive development in new winged females, called female sexuals. These chemicals inhibit workers from rearing male and female sexuals, suppress egg production in other queens of multiple queen colonies, and cause workers to execute excess queens. These pheromones maintain the eusocial phenotype, with one queen supported by sterile workers and sexually active males (drones). In queenless colonies, the lack of queen pheromones causes winged females to quickly shed their wings, develop ovaries and lay eggs. These virgin replacement queens assume the role of the queen and start to produce queen pheromones. Similarly, queen weaver ants Oecophylla longinoda have exocrine glands that produce pheromones which prevent workers from laying reproductive eggs.
Similar mechanisms exist in the eusocial wasp Vespula vulgaris. For a queen to dominate all the workers, usually numbering more than 3000 in a colony, she signals her dominance with pheromones. The workers regularly lick the queen while feeding her, and the air-borne pheromone from the queen's body alerts those workers of her dominance.
The mode of action of inhibitory pheromones which prevent the development of eggs in workers has been demonstrated in the bumble bee Bombus terrestris. The pheromones suppress activity of the endocrine gland, the corpus allatum, stopping it from secreting juvenile hormone. With low juvenile hormone, eggs do not mature. Similar inhibitory effects of lowering juvenile hormone were seen in halictine bees and polistine wasps, but not in honey bees.
In primitively eusocial bees (where castes are morphologically similar and colonies are small and short-lived), queens frequently nudge their nest mates and then burrow back down into the nest. This draws workers into the lower part of the nest where they may respond to stimuli for cell construction and maintenance. Being nudged by the queen may help to inhibit ovarian development; in addition, the queen eats any eggs laid by workers. Furthermore, temporally discrete production of workers and (actual or potential queens) can cause size dimorphisms between different castes, as size is strongly influenced by the season during which the individual is reared. In many wasps, worker caste is determined by a temporal pattern in which workers precede non-workers of the same generation. In some cases, for example in bumblebees, queen control weakens late in the season, and the ovaries of workers develop. The queen attempts to maintain her dominance by aggressive behavior and by eating worker-laid eggs; her aggression is often directed towards the worker with the greatest ovarian development.
In highly eusocial wasps (where castes are morphologically dissimilar), both the quantity and quality of food are important for caste differentiation. Recent studies in wasps suggest that differential larval nourishment may be the environmental trigger for larval divergence into workers or gynes. All honey bee larvae are initially fed with royal jelly, which is secreted by workers, but normally they are switched over to a diet of pollen and honey as they mature; if their diet is exclusively royal jelly, they grow larger than normal and differentiate into queens. This jelly contains a specific protein, royalactin, which increases body size, promotes ovary development, and shortens the developmental time period. The differential expression in Polistes of larval genes and proteins (also differentially expressed during queen versus caste development in honey bees) indicates that regulatory mechanisms may operate very early in development.
The 1973 novel Hellstrom’s Hive by Frank Herbert revolves around a secret society made up entirely of a race of bioengineered insect-like humanoids that is modeled after the behavior of social insect species.
Diversity
In insects
In hymenopterans
/ref> 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.Hearn, Lucas R., Stevens, Mark I. and Schwarz, Michael P. (2023) The presence of a guard vicariously drives split sex ratios in a facultatively social bee. Biol. Lett. 19: 20220528. doi:10.1098/rsbl.2022.0528 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 termites
In beetles
In gall-inducing insects
In crustaceans
In trematodes
In nonhuman mammals
In humans
In plants
Evolution
Phylogenetic distribution
Paradox
Inclusive fitness and haplodiploidy
Argument that haplodiploidy favors eusociality
Argument that haplodiploidy does not favor eusociality
Evolutionary ecology
Multilevel selection
Reversal to solitarity
Physiology and development
Pheromones
Other mechanisms
In popular culture
See also
External links
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