Male southern bottlenose dolphins in the Coffin Bay waterways in South Australia have greater sexual success if they work together with their brothers and cousins.

Key points: Two hundred dolphins in SA were studied

Two hundred dolphins in SA were studied Coffin Bay is a competitive mating environment with more males than eligible females

Coffin Bay is a competitive mating environment with more males than eligible females Males found cooperating with cousins, brothers had more success reproducing

And the larger their group, the more likely they are to entice the ladies.

Flinders University analysed the social and genetic structure of 12 groups of dolphins in the Coffin Bay waterways, with the study finding males teamed up in groups of two to five and formed alliances to increase their chances of mating.

Professor Luciana Moller (right), with Professor Luciano Beheregaray from Flinders University, says male dolphins form bonds based on bloodlines in the Coffin Bay waterways. ( Supplied: Flinders University )

Lead researcher Fernando Diaz-Aguirre said the collaborations appeared to help in a competitive mating environment.

"There are not enough females to go around so the males work together to herd a female away from the pod and then court her as a collective," Dr Diaz-Aguirre said.

Dolphin wingmen the key to success

Associate professor Luciana Moller said the population of about 200 resident dolphins in the waterways was about 50 per cent males and 50 per cent females, but available females were fewer because they only gave birth to a single calf every two to five years and that calf stayed with the mother for up to four years.

"There's a lot of competition for males to get a mating opportunity because there are not a lot of females that would be in oestrus [the period when the female is fertile] each given year," Professor Moller said.

"The males just came up with the strategy to cooperate with each other like they were cooperating with their mates to gain access to a female.

"And if you do that with your brother or your cousin, then you might not be the father of the calf yourself but you have genes that you share with your brother or your cousin that will be passed on to the offspring."

Blood is thicker the water

Previous research had shown other dolphin types form similar bachelor groups but this study analysed why bonds were based on blood relations in the Coffin Bay region using photo-identification data and biopsy samples.

Researchers find the strong male bonds in the Coffin Bay area improves their chances of mating, and ensures they stay fit and have stronger family bonds over time. ( Supplied: Flinders University )

"It seems quite successful to form an alliance … they might have two or three animals together or four or five and the ones who were able to have that cooperation with a larger number of males had much more success in getting offspring," Professor Moller said.

"It's quite complex for them to sort out the dynamics of being in an alliance and who is going to mate.

"It seems to vary but the observations that we have are that most of them would have access to mate with the female so they seem to be sharing access."

Dr Diaz-Aguirre said alliances meant they could also defend females to prevent other male groups from mating with them.

Simon Allen, from the School of Biological Sciences, University of Bristol, said similar social patterns occurred in bottlenose dolphin populations elsewhere, such as Port Stephens in NSW and, perhaps most famously, Shark Bay in WA. But he said the more interesting alliances were non-related males.

"In Shark Bay, there is a nested system of alliance formation — so 'alliances of alliances' that work together [and against other alliances] in order to gain access to reproductive receptive females," Dr Allen said.

"These 'second-order' alliances consist of four to 14 males and are the core social unit in the bay, within which two or more pairs or trios of males form very tightly bonded 'first-order' alliances.

"The more unusual circumstances that are somewhat harder to explain are those in which allies are unrelated, so someone else gaining paternity success means none of your genes being passed on," he said.

"Mechanisms other than kinship must play an important role in unrelated alliances."

Decades of research

Dolphins that form family alliances have similar success elsewhere. ( Supplied: Flinders University )

Dr Allen worked on research with University of Massachusetts's evolutionary biologist Richard C. Connor and evolutionary anthropologist Michael Krutzen, from the University of Zurich.

The group have been researching dolphins under the Shark Bay Dolphin Research Alliance, which has 30 years of observations.

Professor Connor and Professor Kruetzen said the Coffin Bay research added to previous work from other locations.

"Given the patterns of philopatry, where females [and even males] continue to use their mothers' home range throughout their lives, it is not surprising that opportunities arise for males to associate with relatives," Professor Connor said.

Professor Connor said their findings showed the Shark Bay dolphins' first-order alliances of two or three dolphins varied, but the larger second-order alliances were stable.

He said the Coffin Bay study of larger first-order alliances might be a combination of first and second-order alliances.