Divided dolphin societies merge for first time

Sydney, NSW - Two become one: the unification of these two socially distinct groups of bottlenose dolphin demonstrates the intelligence and social adaptability of the species.

A unique social division among a population of bottlenose dolphins in Australia's Moreton Bay has ended, according to a new study. The dolphins lived as two distinct groups that rarely interacted, one of which foraged on trawler bycatch.

But scientists think that a ban on fishing boats from key areas has brought the two groups together. They believe these socially flexible mammals have united to hunt for new food sources. The findings are published in the journal Animal Behaviour.

Bottlenose dolphins have large brains and quickly learn new behaviours. Using a wide range of sounds to communicate with other members of the group, or "pod", they have been observed showing remarkable individual and social intelligence:

The Moreton Bay dolphins were thought to be the only recorded example of a single population that consisted of groups that were not associating with each other in a split dubbed "the parting of the pods". But since the study that discovered the rift, trawlers have been banned from designated areas of the bay leading to a 50% reduction in the fishing effort.

A key area of the bay to the south, where the social split was observed by the previous study, has been protected. The changes gave scientists a unique opportunity to observe the adaptability of dolphin society. The "trawler" dolphins from Moreton Bay had previously fed on the bycatch from boats while the "non-trawlers" found other sources of food.

"There's never been really any experiments looking at social structure... where you can compare what it was like before and what it is like now," said Dr Ina Ansmann, marine vertebrate ecologist, University of Queensland, and the study's lead author.

Analysing how the population interacted before and after trawling meant the team could assess how the dolphins' social network had changed. "The dolphins had basically re-arranged their whole social system after trawling disappeared so they're now actually interacting again," Dr Ansmann told BBC Nature.

The scientists identified individual dolphins by the marks on their dorsal fin and recorded which animals were associating with which.

"Each dolphin has small injuries like nicks and notches, cuts and things like that on the fin so they all have a very unique looking dorsal fin." This technique meant that Dr Ansmann could observe changes in behaviour, in some cases down to the individual dolphins which had been studied in the 1990s to reveal the original division.

The "trawler" dolphins of Moreton Bay benefited from the bycatch thrown back from fishing boats "Presumably they're sharing information, co-operating and things like that." One of those males is now fully integrated into a single community.

Dolphins operate in what is called a fission-fusion society, forming groups and then splitting up to form different groups. Through complex communication and social intelligence, bottlenose dolphins often work as a team when hunting for food and Dr Ansmann believes this may be what lies behind the unification.

"When relying on natural food sources I guess it's more important for them to interact with others, or to learn from others, or to co-operate with others to get to these food sources," she said. The results suggest that a flexible social structure may be an important factor in how dolphins exploit a wide range of resources in the marine environment.