10 April 2012

Shark Bay gangland

Many species of mammal live in complex social groups of close kin, where they work together in defending against predators and sometimes, in gathering food resources. This is particularly obvious in the more intelligent mammals, such as primates and many large carnivores like wolves and lions. Social groups such as these are usually made up of sisters, or closely related females, with one (or a small number) of unrelated males that they mate with. This type of social relationship is called a 'semi-closed group' and normally, the population of a species in a certain area is split up into small groups, where each group holds a specific territory and acts with hostility towards invading and neighbouring groups. 

There are a few species of mammal that have even more sophisticated social networks than this, where each individual group may form co-beneficial affiliations with neighbouring groups for their mutual benefit. Such socialites are called 'open social networks' and are characteristic of the intellectual elite species of mammal, including humans, many of the higher primates, elephants, whales and dolphins. 

Indo-Pacific bottlenose dolphins (hereby referred to simply as dolphins) in Shark Bay, Western Australia, are known to live in open social networks. Recent research however, has found that this is not the full extent of their social domestication and that the dolphins in Shark Bay display social behaviour that is not only unique among mammals, but is unique even amongst other dolphins!

A school of Indo-Pacific bottlenose dolphins (Tursiops species) in Shark bay, Australia. Bottlenose dolphins are top predators within their ecosytem and the upper vertebrae in their neck are fused to prevent their head from moving side-to-side, allowing their spine withstand the extreme speeds that they often reach whilst hunting. 

Dolphins resident of Shark Bay live in a form of fission-fusion grouping, where the bottlenose population are under the control of groups of male dolphins that show closer bonds between other male individuals than in male-female or female-female dolphin relationships. This is highly unusual within the animal world, where males are usually in competition with each other for females and typically, form a social hierarchy where only the head male (or alpha) is allowed to mate. Females on-the-other-hand, often form tight-knit groups for their own protection and protection for their young.

The groups of males form three orders of nested alliances. First-order alliances consist of of 2 or 3 males that cooperate with each other to capture and control groups of females, all of which are allowed to mate and usually act aggressively to protect their group's mating right. First-order groups have often been seen to form second-order alliances, where teams of 10 to 14 males cooperate to defend themselves against attacks from other alliances and to mount their own assaults. Second-order alliances are the core unit of social organisation in Shark Bay and can remain intact for decades! Finally, some second-order alliances merge with others in the second-order to form third-order alliances, which are concerned solely with the possession and acquisition of females. Such groups have been seen to mount large-scale assaults on other other alliances to steal their female members and fiercely guard their own against theft.

It is thought that this strange method of social bonding between male dolphins living in Shark bay has arisen because the affects of kinship, which is the basis of most social grouping, is diluted in their population. Females only give birth to a single calf at a time and have babies infrequently, meaning that the likelihood of a male dolphin having a brother of a similar age is low. This, along with the fact that dolphins leave their mothers at an early age, means that they cannot consistently rely on close kin for allies; and as a consequence, have evolved strong social bonds between other males.

Randic S., Connor R., Sherwin W. B. & Krutzen M. (2012). A novel mammalian social structure in Indo-Pacific bottlenose dolphins (Tursiops sp.): complex male alliances in an open social network. Proceedings of the Royal Society B published online on 28 March 2012.

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