Killing Their Young

The mystery deepened when the bodies of young dolphins bearing the same kinds of wounds also began washing up on Scottish beaches. By 1998, the researchers concluded that bottle-nosed dolphins were killing not only young porpoises but also their own babies.

Such incidents of infanticide (the killing of an infant) among dolphins were not confined to Scotland. In Virginia Beach, Virginia, in the late 1990's, biologists found other battered carcasses of baby bottle-nosed dolphins washed up with the tides. All of the bodies had wounds similar to those seen on the bodies of the porpoises and dolphins in Scotland.

The researchers reported that all of the dead baby dolphins discovered in the United States and Scotland were less than 1 year old, an age at which the infants are normally under the full-time care of their mothers. Instead, somehow, these baby dolphins had been bitten and bludgeoned to death by adults.

Because male and female dolphins look nearly identical, teams of researchers have been unable to determine whether it was males or females who were killing the young. But some biologists have speculated that adult males were doing the killing. They based their conclusion on the fact that infanticide among other animals is more commonly committed by males.

In addition, scientists know that a female dolphin, while caring for her calf, can remain sexually inactive for several years. But a female becomes fertile and ready to mate within one to two weeks of losing an infant. Therefore, researchers speculated that killing a mother's calf might be a male strategy designed not only to destroy the offspring of a rival but also to bring the female into a sexually receptive state.

By mid-2000, the deaths of the porpoises still remained a mystery, since these animals have no obvious connection to any reproductive advantages. Scientists in Scotland, however, theorized that if the male dolphins were responsible for the large number of reported deaths, they may have used the porpoises as “target practice” for later attacks on infant dolphins. Other experts, however, theorized that the attacks might instead have stemmed from simple aggression or from confusing the baby porpoises for baby dolphins in the murky waters.

Researchers agreed that reports of such aggressive behavior should serve as a warning to people who enjoy swimming with dolphins. Furthermore, many scientists said that such studies were just the first step in gaining a better understanding of the nature of bottle-nosed dolphins.