Ocean Watch

Monday, August 3, 1998

Adult male dolphins
may be killing
unrelated babies

Dolphin researchers from the United States and Britain recently reported that baby bottlenose dolphins found dead on the shores of both countries were likely battered to death by adult bottlenose dolphins.

This news is a bit disturbing to many of us because it doesn't fit the image we have of this species. These playful, friendly dolphins are the sweethearts of marine parks, born with smiles on their faces and apparent joy in their hearts.

How can these genial animals kill their babies?

Physically, it's pretty easy. Adult bottlenose males are about 12 feet long and weigh 600 pounds, with females slightly smaller. At birth, baby dolphins are about 3 feet long and weigh only a fraction of an adult.

Besides being big, anyone who has watched a dolphin show can see these animals are also incredibly strong. Dolphins can "walk" upright on their tails, leap far from the water and easily swim with a human or two in tow.

Being towed around by a dolphin looks like fun, and seems safe, but these wild animals don't always know their own strength.

I once heard a story of a researcher who got into a tank with a tame dolphin he knew well. When the dolphin saw its friend in the water, it rushed to greet him, smashing the man against the concrete wall of the tank. Although purely an accident on the dolphin's part, its exuberant welcome broke the man's femur like a twig.

Dolphins didn't develop all this muscle power just for fun and games. Sometimes they attack other species, a behavior people have been able to catch on videotape and study.

In one attack, three bottlenose dolphins chased a harbor porpoise, rammed their beaks into it and tossed it into the air. The porpoise died. When researchers examined the dead animal, they found massive twisting injuries, with blubber and muscles ripped from the bones.

The researchers noted that after the harbor porpoise was dead, none of the three dolphins tried to eat it. They simply lost interest in the whole affair and went about their business.

Workers estimate that about 60 percent of harbor porpoises found dead on the northeast Scottish coast have been pummeled by dolphins. The baby bottlenoses found dead, five in Scotland and nine in Virginia, were also killed in the same way.

It's not hard to understand why bottlenose dolphins might fight with and kill adult members of another species. Such violence in the competition for food and space is common in the animal kingdom. But why would bottlenose dolphins kill their own babies?

No one knows for sure but researchers have a theory: Courting males may sometimes kill offspring not their own because when a female dolphin loses an infant, she becomes fertile again in one to two weeks. In this way, the new suitor can father his own calf and thus pass on his genes of strength and aggression.

Such male infanticide is fairly common in the animal kingdom. Lions do it, as do some monkeys and chimps.

This newfound violent behavior in bottlenose dolphins doesn't mean they've gone bad. They're the same as they always were: wild animals that, like all others, behave according to natural instincts and biological laws.

As Charles Darwin noted: "Each organic being is striving to increase at some period of its life, during some season of the year, during each generation . . . and has to struggle for life and suffer great destruction. The vigorous, the healthy, the happy survive and multiply."

This news about bottlenose dolphins is a reminder that the struggle for life is hard for all species -- even the cute ones.