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Dolphin Societies Reunite

A population of dolphins that split into two societies in Moreton Bay, Australia, in the 1990s has come back together. Dolphins are known to engage in very complex shifting alliances, but this was the first known case of an apparently permanent division into two separate societies that never  interacted with each other. And now, just as suddenly as the “falling-out” took place 20 years ago, everything is now back the way it was.

moreton-bay-dolphins-ina-asmann-2-073112The original split was apparently set in motion by humans developing trawler-fishing techniques in the bay. One group of dolphins began following the trawlers , feeding off the unwanted fish and other animals thrown back in the water by the fishermen. The other continued to hunt for fish by themselves. As the two groups stayed away from each other, scientists followed their activities and were able to identify the many individual dolphins through markings, scratches, nicks and notches on their fins.

But new fishing regulations have banned fishing from entire areas of the bay, leading to only about half the amount of fishing as there used to be – and not enough by-catch to supply the needs of the trawler dolphins.

“The dolphins basically re-arranged their whole social system after trawling disappeared so they’re now actually interacting again.”Dr. Ina Ansmann, a marine ecologist at the University of Queensland, explained to BBC Nature what happened next:

“The dolphins basically re-arranged their whole social system after trawling disappeared so they’re now actually interacting again. Presumably they’re sharing information, co-operating and things like that.”

The ins and outs of a dolphin “fission-fusion society” with its extremely complex, shifting alliances has been studied in depth by Dr. Richard Connor, who has led a six-year study of bottlenose dolphins in Shark Bay on the west coast of Australia. As Dr. Connor describes it:

“It seems there is constant drama. I have often thought, as I watched their complicated alliance relationships, that their social lives would be mentally and physically exhausting. It seems there is constant drama. I’m glad I’m not a dolphin!”

We can assume, then, that there was plenty of drama when the dolphins in Moreton Bay decided to get back together.

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,” Dr. Ansmann said.

She suggested that a new alliance like this may have involved one dolphin acting as a kind of peace-broker, who integrated back into the fishing group and then brought the other dolphins with him as the whole pod concluded that it was in all their interests to work together.

The study is reported in Animal Behavior. And Dr. Connor’s work is described here.