One to three dolphins chase a porpoise and ram their beaks into it hard enough to toss it into the air. Blubber and muscle rip away from the bones. The attackers make no attempt to eat the victims. Once the porpoise is dead, it's like flicking a light switch. What prompts the attacks remains a fountainhead of speculation.
Hypotheses range from rough play to sexual frustration.
When Patterson and his colleagues found the same kind of injuries as they had seen in porpoises in young bottlenose dolphin carcasses, they fingered older dolphins as the culprits. The attacks on dolphins less than a year old have spawned numerous theories. Killing the calf might bring the female into a receptive state much sooner. From dolphin studies in Shark Bay, Australia, Richard C. Connor of the University of Massachusetts at Dartmouth and his colleagues predicted killer males. Although they observed no infanticide, they noted that females become fertile within 1 to 2 weeks of losing an infant.