Cross porpoises: Aggrieved dolphins hold grudges, scientists find

Dolphins hold grudges, scientists have discovered, as they will only help those that have come to their aid in the past.

The marine mammals, believed to be one of the most intelligent animals, form social groups and friendships based on a history of co-operative behaviour which is similar to humans, according to University of Bristol researchers.

If a dolphin helps a fellow group member when it is in danger, that comrade is likely to return the favour, they found. This goodwill extends to dolphins that have helped a fellow group member.

However, if a dolphin shirks its responsibility and swims away, and the dolphin in danger survives to remember the incident, they will not help that negligent member in the future - suggesting they hold a grudge.

The findings are based on 30 years of observations of Bottlenose dolphins living in Shark Bay, Western Australia.

Drones were flown above dolphin groups and their behaviour was recorded.

Dr Stephanie King, from the University of Bristol's School of Biological Sciences, said: "Social animals can possess sophisticated ways of classifying relationships with members of the same species.

"In our own society, we use social knowledge to classify individuals into meaningful groups, like sports teams and political allies.

"Bottlenose dolphins form the most complex alliances outside humans, and we wanted to know how they classify these relationships."

Dolphins work together for a number of reasons, such as to carry out an attack on competitors or to protect their group.

Biologists analysed how male dolphins responded to the calls of other males.

They discovered that the males responded strongly to all of their allies that had consistently helped them out in the past, even if they weren't currently close friends.

However, they didn't respond strongly to males who hadn't come to help in the past, even if they were friends.

"Such concepts develop through experience and likely played a role in the co-operative behaviour of early humans," said Dr King.

"Our results show that cooperation-based concepts are not unique to humans, but also occur in other animal societies with extensive co-operation between non-kin."

The findings were published in the journal Nature Communications.