An international team of researchers has found that bottlenose dolphins from Shark Bay, Western Australia, form complex male alliances in an open social network, a pattern not described before in comparable animals or dolphin communities.
Bottlenose dolphin Tursiops truncates
The study, published in the journal Proceedings of the Royal Society B, shows that groups of two to three males of bottlenose dolphin form “first-order” alliances that involve close relatives, such as cousins, co-operating to guard or act as consorts to females. Long-running studies by many researchers at Shark Bay have revealed that these small groups may persist for up to 20 years.
“Sometimes one of these first-order alliances is seen to assist another to gain access to a female, but on another day the same two groups may be opposed in a contest over a female,” explained Dr. William Sherwin, a co-author on the study and a geneticist at the University of New South Wales.
The findings also show that teams of four to 14 males cooperate in “second-order” alliances to attack other alliances and to defend against such attacks. Second-order alliances can persist intact for over 15 years and may be considered the core unit of male social organization in Shark Bay.
Two or more “second-order” groups may team up from time to time to form a “third-order” alliance. The study tackled the question of how they choose to make these alliances.
The team was able to exclude the so-called “community defense model” used by chimpanzees, for example, in which semi-closed communities (ones that occasionally accept new members emigrating from other groups) are defended by males ranging across their group’s entire range.
“Nor do they follow the “mating season defense model”, where males defend a tighter geographic range only in the mating season,” Dr. Sherwin said. “At such times, the range of one male alliance would have little overlap with others but would have considerable overlap with the range of certain female individuals.”
The study found that even the largest dolphin alliances do not do this. The key difference with comparable species seems to be that they have an open social network, with a fission–fusion grouping pattern with strongly differentiated relationships, including nested male alliances.
“We have now shown that males, like females, exhibit a continual mosaic of overlapping ranges,” said Dr. Richard Connor, a study co-author and a research fellow at the University of New South Wales.
“Humans, elephants and other mammals live in semi-closed groups with sex-biased dispersal but have relationships with other groups. So humans and elephants differ from the dolphins in that key respect of living in semi-closed groups, but have in common a nested relationship structure. All three have unusually low costs of locomotion, which would have allowed larger ranges, leading to interactions with larger numbers of individuals and groups, further complicating their social lives.”
