Sexual frustration for young dolphins may explain fatal attacks on porpoises

Seemingly random attacks on porpoises might be caused by sexual frustration among young male bottlenose dolphins.

Cases of the cetaceans’ killing other creatures for no apparent reason have been noted in British waters. Now bottlenose dolphins have been seen attacking harbor porpoises in the Pacific Ocean. These observations show for the first time that the aggressors are young males.

Mark Cotter at Okeanis, a nonprofit conservation organization in Moss Landing, Calif., and colleagues observed three assaults by dolphins on lone porpoises. The dolphins chased the porpoises at high speed, rammed and then drowned them.

In one particularly violent attack, three dolphins corralled their victim before seven others joined them to ram the porpoise to death. Cotter found it most shocking that two dolphins remained behind to play with the carcass before pushing it toward his boat. “It was almost like they said: ‘We’re done playing with it, here you go.’ ”

Competition for food does not seem to explain the attacks, as the dietary overlap between the two species is small, says Cotter.

But the fact that 21 of the 23 attackers were males may be revealing. He believes that the attacks are “object-oriented play” during the breeding season by young males that cannot get access to females because of competition from older males. “They are taking out their frustrations,” he says.

— Shaoni Bhattacharya, New Scientist magazine