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Many organisms show some form of social behaviour or cooperative activity, ranging from simple bacterial associations in biofilms to the highly complex organisation of eusocial insects. Sociality may serve a reproductive, protective or foraging function.
When the cooperation of individuals improves the chances of capturing prey and results in the food intake of an individual being higher than if hunting solitarily, animals hunt in groups. Cooperative hunting is most familiar from mammals, such as lions, hyenas, wolves, chimpanzees, bottlenose dolphins and orcas. However, it has also evolved in some birds (e.g. Harris' hawks, pied currawongs), fish, insects and spiders. Young of the subsocial spider Amaurobius ferox capture prey ten times their size in a coordinated team effort. Usually, members of the same species hunt cooperatively, but there are a few spectacular examples involving different species. Dolphins indicate the presence of fish swarms to fishermen and groupers hunt together with moray eels in a highly coordinated fashion, initiated by a visual signal from the grouper.
While some animals occur in groups only temporarily (such as insect-eating non-migratory birds that switch to a diet of patchily distributed seeds and accordingly flock together during the winter), others show a permanent social organisation, where different types of societies can be distinguished. Matriarchal societies can be found in elephants, where the matriarch with her knowledge of the territory is vital to the survival of the family clan, but also in sperm whales. Fission-fusion societies that show a mixture of fluidity and stability have evolved repeatedly as well, for example in elephants, dolphins, chimpanzees and some New World Monkeys.
One of the most striking convergences concerns eusociality, an advanced and complex social system that is characterised by reproductive castes, i.e. reproductive division of labour with one or several fertile queens and sterile individuals (e.g. workers). Approximately 13,000 species are truly social, and there are at least 20 independent evolutionary origins of eusociality. The most important eusocial animals, in terms of numbers as well as ecological significance, are hymenopteran insects (ants, bees and wasps). Eusociality confers many evolutionary advantages, and is clearly why these groups are so successful. Almost 9,000 species of ant are eusocial, and amongst the most striking social arrangements are those found in the army ants. They are permanently nomadic, temporarily forming bivouacs but no permanent nest, and act as highly co-ordinated units performing carnivorous raids. Equally remarkable is the independent evolution of agriculture in the attine ants (which is also observed in many other arthropods, e.g. termites and ambrosia beetles). The complex eusocial organisation of the ants unsurprisingly includes many associated aspects of activity that show evolutionary convergence, such as quorum sensing, stridulation, silk production and tool making.
Apart from hymenopterans, also other insects are eusocial, namely termites, aphids, thrips and barkbeetles. Termites are only distantly related to ants and in fact evolutionarily very close to the cockroaches. A key factor in the evolution of eusociality is that termites produce hexamerins, which affect key growth hormones (e.g. juvenile hormone) and help to regulate which caste type (e.g. worker or soldier) each individual develops into. As eusocial forms the aphids are interesting not only because of the well-known production of a soldier caste. Soldiers have evolved multiple times and are typically clonal and equipped with powerful claws or stylets (and in one group even horns). A major difference between aphids and other eusocial insects is that many aphid groups live in the open and not in nests. Some aphids, however, are protected because they live in plant galls (as do e.g. thrips).
Interestingly, outside the insects eusociality is only found in two other groups, crustaceans (alpheid shrimps of the genus Synalpheus) and mammals (naked mole rats and Damaraland mole rats). The sponge-dwelling alpheid shrimps exhibit many of the hallmarks of eusocial organisation, diagnostically a single reproductive female and large numbers of non-reproductive individuals. In addition, they show effective co-ordinated defence indicating social communication. Their eusociality contributes to the observed rise in dominance over other crustacean groups. The burrowing mole rats live in underground colonies with a dominant female suppressing reproduction of all other females via latrine pheromones. Although the well-known meerkats are highly social, they cannot be considered eusocial as some significant mating occurs outside the dominant pair.
Social play is the hallmark of the most intelligent animal species, and there is a striking convergence between birds and mammals, with a one-to-one correspondence to the four principal categories of play (chasing, fighting, invitation and object play). While highly characteristic of young mammals, only those birds with complex cognitive abilities (namely crows, hornbills and parrots, particularly kea and kakapo) engage in social play.