Sociality is a survival response to evolutionary pressures. For example, when a mother wasp stays near her larvae in the nest, parasites are less likely to eat the larvae. Biologists suspect that pressures from parasites and other predators selected this behavior in wasps of the family Vespidae.
This wasp behaviour evidences the most fundamental characteristic of animal sociality: parental investment. Parental investment is any expenditure of resources (time, energy, social capital) to benefit one's offspring. Parental investment detracts from a parent's capacity to invest in future reproduction and aid to kin (including other offspring). An animal that cares for its young but shows no other sociality traits is said to be subsocial.
An animal that exhibits a high degree of sociality is called a social animal. The highest degree of sociality recognized by sociobiologists is eusociality. A eusocial taxon is one that exhibits overlapping adult generations, reproductive division of labor, cooperative care of young, and—in the most refined cases—a biological caste system.
Solitary animals, such as the jaguar, don't associate except for courtship and mating. If an animal taxon shows a degree of sociality beyond courtship and mating, but lacks any of the characteristics of eusociality, it is said to be presocial. Although presocial species are much more common than eusocial species, eusocial species have disproportionately large populations.
The entomologist Charles D. Michener published a classification system for presociality in 1969, building on the earlier work of Suzanne Batra (who coined the words eusocial and quasisocial in 1966). Michener used these terms in his study of bees, but also saw a need for additional classifications: subsocial, communal, and semisocial. In his use of these words, he did not generalize beyond insects. E. O. Wilson later refined Batra's definition of quasisocial.
Subsociality is common in the animal kingdom. In subsocial taxa, parents care for their young for some length of time. Even if the period of care is very short, the animal is still described as subsocial. If adult animals associate with other adults, they are not called subsocial, but are ranked in some other classification according to their social behaviours. If occasionally associating or nesting with other adults is a taxon's most social behaviour, then members of those populations are said to be solitary but social.
Solitary-but-social animals forage separately, but some individuals sleep in the same location or share nests. The home ranges of females usually overlap, whereas those of males do not. Males usually do not associate with other males, and male offspring are usually evicted upon maturity. Among primates, this form of social organization is most common among the nocturnal strepsirrhine species and tarsiers. Some examples of solitary-but-social species are mouse lemurs, lorises, and orangutans.
Sociobiologists place communal, quasisocial, and semisocial animals into a meta-class: the parasocial. The two commonalities of parasocial taxa are: the exhibition of parental investment, and socialization in a single, cooperative dwelling.
In addition to the aforementioned, communal, quasisocial, and semisocial taxa differ in a few ways. In a communal group, adults cohabit in a single nest site, but they each care for their own young. Quasisocial animals, however, also share the responsibilities of brood care. If a population is described as having demonstrated semisocial behavior, it is understood that individuals in said population are not only invested in cooperatively rearing their young in shared locations; furthermore, the society has also divided labor between those able and unable to reproduce, and it has developed a biological caste system.
Beyond parasociality is eusociality. Eusocial insect societies have all the characteristics of a semisocial one, except overlapping generations of adults cohabit and share in the care of young. This means that more than one adult generation is alive at the same time, and that the older generations also care for the newest offspring.
Eusocial societies have overlapping adult generations, cooperative care of young, and division of reproductive labor. When organisms in a species are born with physical characteristics specific to a caste which never changes throughout their lives, this exemplifies the highest acknowledged degree of sociality. Eusociality has evolved in several orders of insects. Common examples of eusociality are from Hymenoptera (ants, bees, sawflies, and wasps) and Blattodea (infraorder Isoptera, termites), but some Coleoptera (such as the beetle Austroplatypus incompertus), Hemiptera (bugs such as Pemphigus spyrothecae), and Thysanoptera (thrips) are described as eusocial. Eusocial species that lack this criterion of morphological caste differentiation are said to be primitively eusocial.
Two potential examples of primitively eusocial mammals are the naked mole-rat and the Damaraland mole-rat (Heterocephalus glaber & Fukomys damarensis, respectively). Both species are diploid and highly inbred, and they aid in raising their siblings and relatives, all of whom are born from a single reproductive queen. They usually live in harsh or limiting environments. However, a study conducted by O’Riain and Faulkes in 2008 suggests that due to regular inbreeding avoidance, mole rats sometimes outbreed and establish new colonies when resources are sufficient.
Eusociality has arisen among some crustaceans that live in groups in a restricted area. Synalpheus regalis are parasitic shrimp that rely on fortress defense. They live in groups of closely related individuals, amidst tropical reefs and sponges. Each group has one breeding female; she is protected by a large number of male defenders who are armed with enlarged snapping claws. As with other eusocial societies, there is a single shared living space for the colony members, and the non-breeding members act to defend it.
E. O. Wilson and Bert Hölldobler controversially claimed in 2005 that humans exhibit sufficient sociality to be counted as a eusocial species, and that this enabled them to enjoy spectacular ecological success and dominance over ecological competitors.
|Degree of sociality||Parental investment||Members of an adult generation
and their young cohabit
|Reproductive division of labour||Caste system||Overlapping adult generations|
|Solitary but social||Yes||Sometimes||No||No||No||No|