Eusociality (Greek eu: “good/real” + “social”) is a term used for the highest level of social organization in a hierarchical classification. Eusociality is characterized by cooperative brood care, overlapping adult generations and division of labor by reproductive and (partially) non-reproductive groups. In analogy with some human societies, groups of specialized individuals are sometimes called castes. Several different levels of sociality have been categorized including presocial (solitary but social), subsocial, parasocial (including communal, quasisocial, and semisocial), and eusocial.
The most familiar examples of eusocial insects are ants, bees, and wasps (order Hymenoptera), as well as termites (order Blattodea) – all with reproductive queens and more or less sterile workers and/or soldiers. Austroplatypus incompertus, a species of weevil native to Australia, is the first beetle (order Coleoptera) to be recognized as eusocial. Mammalian examples include the naked mole rat and theDamaraland mole rat; however, this classification is controversial owing to disputed definitions of ‘eusociality’ as well as the existence of other mammals that satisfy the original definition of Wilson (1971).
Although recently in contention by at least one of its original proponents, the prevailing view is that eusociality with biologically sterile individuals represents the most extreme form of kin selection. A more recent view put forth is that eusociality results not from kin selection so much as selective pressure on parental genes, expressing in offspring as extended phenotypes of the parents. Research about eusociality and its origins is a prolific branch of sociobiology.
The phenomenon of reproductive specialization is found in various organisms. It generally involves the production of sterile members of the species, which carry out specialized tasks, effectively caring for the reproductive members. It can manifest in the appearance of individuals within a group whose behavior (and sometimes anatomy) is modified for group defense, including self-sacrificing (“altruism”).
The term “eusocial” was introduced in 1966 by Suzanne Batra and given a more definitive meaning by E. O. Wilson. It was originally defined to include those organisms (originally, only invertebrates) that had certain features:
Subsequent to Wilson’s original definition, other authors have sought to expand or narrow the definition of eusociality, focusing on the nature and degree of the division of labor, which was not originally specified. A narrower definition specifies the requirement for irreversibly distinct behavioral groups or castes (with respect to sterility and/or other features), and such a definition excludes all social vertebrates (including mole rats), none of which have irreversible castes. A broader definition allows for any temporary division of labor or non-random distribution of reproductive success to constitute eusociality, and some have accordingly argued that even humans may be considered eusocial. Others believe that the hierarchical classification may not serve much purpose.
Eusocial animals have appeared paradoxical to many theorists of the field of evolution, including W. D. Hamilton: if adaptive evolution unfolds by differential survival of individuals, how can individuals incapable of passing on their genes possibly evolve and persist? Since they do not breed, we ought to expect any genes causing this condition to be highly unlikely to persist in the population. In Origin of Species (first edition, Ch. 8), Darwin called this behavior the “one special difficulty, which at first appeared to me insuperable, and actually fatal to my theory.” Darwin anticipated that a possible resolution to the paradox might lie in the close family relationship, but specific theories (e.g. kin selection or inclusive fitness) had to wait for the discovery of the mechanisms for genetic inheritance.
Early ideas on eusociality included suggestions that trophallaxis or food sharing was a basis for sociality.
According to inclusive fitness theory, eusociality may be easier for species like ants to evolve, due to their haplodiploidy, which facilitates the operation of kin selection. Sisters are more related to each other than to their offspring. This mechanism of sex determination gives rise to what W. D. Hamilton first termed “supersisters” who share 75 percent of their genes on average. Sterile workers are more closely related to their supersisters than to any offspring they might have, if they were to breed themselves. From the “selfish gene’s” point-of-view, it is advantageous to raise more sisters. Even though workers often do not reproduce, they are potentially passing on more of their genes by caring for sisters than they would 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 siblings rather than producing offspring is often invoked to explain the multiple independent evolutions of eusociality (arising some 11 separate times) within the haplodiploid group Hymenoptera — ants, bees and wasps.
Reeve and Holldobler’s version of superorganism theory further elaborates this model by considering competition and co-operation between groups as well as within groups. In this case, an individual’s inclusive fitness varies depending on how much it invests in within-group competition (e.g. hoarding a private food cache) versus between-group competition (e.g. contributing to common foraging); and on its relatedness to the other group members. In a hymenopteran colony with one breeder (queen) and many workers as described above, the evolutionarily stable state is for each individual to invest entirely in helping the group, leading to a perfect “superorganism”, which implies the stability of eusociality in this case. This agrees with Hamilton’s model. This is implied even without considering between-group interactions. However, they further show that any group of relatives may show high “superorganismness”, provided that there are many groups competing for the same resources. This may favour eusociality, or a degree of eusociality in non-hymenopterans. Indeed, a non-zero level of intra-group co-operation is predicted, even if the group members are entirely unrelated, as long as there is competition between groups.
Theories of parental manipulation point out that the transition from solitary to eusocial appears to involve intermediate stages where dominance interactions are required to suppress the reproductive tendencies of group members; that is, females are manipulated into acting as workers, even if it is against their own self-interest. This model does not require that individuals be highly related, though high relatedness will reduce expected levels of resistance to manipulation.
However, now many eusocial species have been discovered that are not haplodiploid (in addition to termites, such as a species of platypodid ambrosia beetles, several independent lines of Synalpheussponge-dwelling shrimp and bathyergid mole rats). Conversely the solitary bees are (as all bees) haplodiploid yet are not eusocial. The association between haplodiploidy and eusociality is below statistical significance, all of which suggests that haplodiploidy alone is neither necessary nor sufficient for eusociality to emerge.
Recently, some species of gall-making aphids (order Hemiptera) and thrips (order Thysanoptera) were found to be eusocial, with many separate origins of the state. These species have extremely high relatedness among individuals due to their partially asexual mode of reproduction (sterile soldier castes being of the same clone as the reproducing female), but the gall-inhabiting behavior gives these species a defensible resource that sets them apart from related species with similar genetics. In these groups, therefore, high relatedness alone does not lead to the evolution of social behavior, but requires that groups occur in a restricted, shared area.
A single species of beetle, Austroplatypus incompertus, is known to be eusocial.
Among mammals, eusociality has been observed in both naked and Damaraland mole rats (Heterocephalus glaber & Fukomys damarensis). Therefore, Bathyergidae phylogenies imply convergent evolution of eusociality within the African mole-rat family.
Eusociality has also arisen among some crustaceans that, again, live in groups in a restricted, shared area. In some tropical reefs live Synalpheus regalis, a shrimp that relies on fortress defense; they live eusocially with a single breeding female and a preponderance of male defenders, armed with enlarged snapping claws. Again, there is a single shared domicile for the colony members, and the non-breeding members act to defend it.