Female dolphins identify aggressive males by unique calls, research suggests

Female dolphins identify males by their unique calls and keep track of their past behaviour, choosing to avoid the most aggressive males during mating season, new research suggests.

Bottlenose dolphin society is complex, and male and female dolphins often know each other for decades, said Prof Stephanie King, an expert in animal behaviour at the University of Bristol.

These relationships could be positive, she said. Males might perform displays to impress the females, or engage in affiliative behaviours like touching or petting, a bit like holding hands in humans.

But during mating season, the males often worked together – in pairs or trios, and larger alliances – to gain access to females by aggressively herding them into “consortships,” mating events that could last anywhere from hours to weeks.

King said the male dolphins would “work together to keep her with them for as long as possible,” adding that sometimes they could be “quite coercive”.

“They’ll restrict the female’s movements. They’ll want to keep the females in the areas that they prefer, because then they’re near other males that can help them defend that female from rivals.”

Female dolphins likely observed these behaviours over time, and avoided the more aggressive males, according to research published in the Proceedings of the National Academy of Sciences.

Consortships could involve biting, hitting or charging – interactions that were costly to females who not only risked physical injury but also lost crucial foraging time, the paper said. Some males engaged in the aggressive behaviour more often than others.

“We now know that males and females can use signature whistles to track individual behaviour over time and use that to inform decision making,” King, an author of the paper, said.

The research involved a population of Indo-Pacific bottlenose dolphins from Shark Bay in Western Australia that have been studied in depth for more than 40 years – providing rich insights into their lives.

This includes each male’s “signature whistle”, individual behaviour, and timing when adult females were ready to mate – known as estrus.

King said research had shown that males relied on these signature whistles to recognise one another and manage their relationships. “So then we were like – well, what about the females?”

The researchers collected 34 signature whistles from male dolphins and played them underwater to 17 female dolphins, using drones to observe their responses.

The reproductively available females showed a significantly stronger avoidance responses to the whistles of male dolphins with higher rates of coercion, King said, suggesting an awareness of their past behaviour.

From the females’ perspective, she explained: “It’s like ‘I keep track of which males might be more likely to herd me. And if I don’t want to be herded, I’m going to show an avoidance response.’ Whereas, unavailable females – those that were older, or with calves and unlikely to be targeted – did not show the same level of response.”

Australian cetacean researcher Dr Mike Bossley, who has studied the lives of individual dolphins in the Adelaide dolphin sanctuary, said it was “an imaginative and valuable field study confirming the complexity of dolphin societies”.

Bossley, who was not involved with this study, said it showed how female dolphins can identify the behavioural characteristics of males.

“In essence, knowing each males’ personality – and using that information to help choose who will father their calf.”