Dolphins: Darling Or Deadly?

Bottlenose dolphins, the darlings of sea parks and nature tourism worldwide, have now received the distinction of being the first of the dolphin-porpoise-whale group accused of killing youngsters of their own species.
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A research team in Scotland and another in the United States say that young dolphins found on the shores were probably pummeled to death by adult dolphins. The researchers cannot tell which adults attacked or why.

"There is this public ethos that dolphins couldn't do a thing like that," observes Tony Patterson of the Inverness Veterinary Centre in Scotland. Yet five carcasses of youngsters discovered in Moray Firth show a "specific and consistent pattern" of injuries typical of dolphin attacks, he says.

Dale G. Dunn of the Armed Forces Institute of Pathology in Washington, D.C., found the same pattern in nine dead dolphin calves retrieved from the Virginia coast during 1996 and 1997. The bodies did not look badly injured, but when he opened the carcasses, Dunn found broken bones, ripped tissue, and bruised organs.

"It looks like someone had taken a baseball bat and just literally beaten these animals to death," Dunn says. He and his colleagues presented their findings at the Sixth Annual Atlantic Coastal Dolphin Conference in May in Sarasota, Fla.

In 1996, Ben Wilson from the University of Aberdeen in Scotland and H.M. Ross from Inverness, co-authors of the recent article, reported that 60 percent of harbor porpoises found dead on the northeast Scottish coast appeared to have been killed by dolphins. To identify the killers, Wilson compared teeth marks seen on some of the corpses with mammal jaws in museums.

Since then, people have caught some attacks on video, Patterson reports. One to three dolphins chase a porpoise and ram their beaks into it hard enough to toss it into the air. "When [it's] thrown high out of the water, there's massive twisting injury," Patterson says. 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. The dolphins immediately lose interest and just go on their way," Patterson observes. 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. "Probably my favorite is male infanticide," notes co-author Paul Thompson from Aberdeen. If a new suitor approachs a female that already has a youngster, "there's no point in hanging around for two years with the wrong male's calf," he says. Killing the calf might bring the female into a receptive state much sooner.

Murderous stepfathers are certainly a possibility, agrees Susan Barco of the Virginia Marine Science Museum in Virginia Beach, who collaborated with Dunn. "We're still very baffled about what is going on," she says.

In 1997, she found more than four times as many dead young dolphins as she did in 1994, raising the possibility of a boom in infanticide. "I have the feeling that in the past year, it probably became acute," Barco says.

Marine mammal specialist John Harwood from University of St. Andrews in Scotland notes that "these results are remarkably similar to those for large terrestrial carnivores like lions." Young males sometimes jump-start their dynasties by killing off another's offspring, he says. "It suggests that the revolutionary pressures in the marine environment are not so different from those on land."

Written by S. Milius