The reproductive success of male dolphins is not determined by strength or age, but via social bonds with other males, according to a new study.

The research finds that the better integrated male dolphins are in their social network, the more offspring they produce.

“This kind of male cooperation for the purpose of reproduction is highly unusual in the animal kingdom.”

Male dolphins in Shark Bay, Western Australia live in complex social groups in which they form long-lasting bonds to cooperate with other males. To do this, they join in large, stable alliances. Within these alliances, males form smaller, less stable groups of two to three to mate with females, steal them from other alliances, or defend against attacks.

“This kind of male cooperation for the purpose of reproduction is highly unusual in the animal kingdom. It’s only been observed in a much less complex form in some other primates,” says Livia Gerber, a former PhD student at the anthropology department of the University of Zurich and first author of the paper in Current Biology.

Gerber and colleagues, including professor Michael Krützen, wanted to find out whether the dolphins’ complex social life affects the reproductive success of males, or whether, as in most other species, stronger or more experienced males are more likely to sire offspring.

Researchers analyzed 30 years of behavioral data from 85 male dolphins and used genetic data to conduct paternity analyses for more than 400 dolphins.

The study showed that well-integrated “popular” males with strong social bonds to many alliance partners produce the most offspring. Partner stability within the smaller, variable groups of two to three males and the age difference to alliance members, in contrast, played no role in the animals’ reproductive success.

Previous research had suggested that social bonds improve the animals’ chances of survival, increase their longevity, and lead to better immune responses and health.

While all these effects might contribute to a male dolphin’s lifetime reproductive success, the positive effect of social bond strength on cooperation was probably the key factor, Gerber says.

“Well-integrated males might be in a better position to harness the benefits of cooperation and access crucial resources such as food or mates. They may also be more resilient to partner loss compared to those with few, but closer partners.”

Cooperation among social partners is very common in mammals, but its influence on paternity success has not yet been studied in depth. However, understanding the factors that determine reproductive success and thus individual fitness is at the core of evolutionary biology.

“Our study is the first to show that social bonds among male dolphins positively impact their reproductive success and are, therefore, directly linked to fitness,” says Krützen, the paper’s senior author.

“This had previously only been observed in male chimpanzees and some other primates. Our study expands upon previous findings on land mammals and provides compelling evidence that such highly complex, multi-level social systems also developed independently in the ocean.”

Source: University of Zurich