The harbor porpoise that washed ashore last month at San Francisco's Fort Funston was clearly a victim of foul play.

It had two badly broken jawbones, fractured ribs on both sides and a broken scapula, evidence of a sadistic attack. Worst of all, the female porpoise, which had been seen twice before and identified by researchers in Monterey Bay, was lactating when she was killed, according to marine biologists.

It was a clear case of what scientists are calling "porpicide," the deliberate slaying of a harbor porpoise by a surprising and, to most people, unlikely culprit.

"We suspect that it was a bottlenose dolphin," said Bill Keener, a researcher for Golden Gate Cetacean Research.

The brutal battering wasn't an isolated incident. Scientists say there has been a dramatic increase in dolphin attacks on harbor porpoises along the California coast over the past few years, including an attack Wednesday off Half Moon Bay.

"I've lived here 15 years, and I've never seen anything like that," said Alvaro Jaramillo, a biologist at the San Francisco Bay Bird Observatory, who was jogging on the beach near his home in Half Moon Bay when he looked out in the water and saw two dolphins pummeling a porpoise. "They would chase the porpoise, and it looked like they were sandwiching it between them, then they would throw it up in the air. The porpoise just couldn't get away from them."

Porpoise deaths

Since Aug. 12, six dead harbor porpoises have washed ashore at Ocean Beach, Fort Funston and Stinson Beach. It is not yet clear how they all died, but most of the injuries are consistent with bottlenose attacks, said Maureen Flannery, the collections manager for the California Academy of Sciences.

Well over 50 harbor porpoise deaths are believed to have been caused by bottlenose dolphins along the California coast since 2005, when the first fatal attack ever recorded in California was confirmed. A study published this year in the Marine Mammal Science journal documented 44 fatal attacks between 2007 and 2009, mostly off San Luis Obispo and Santa Cruz and in Monterey Bay.

Mark Cotter, a marine biologist and lead author of " 'Porpicide' in California," said the confirmed dolphin attacks probably represent only a fraction of the actual number of porpoise killings.

"There have been lots of dead porpoises on the beach that were probable attack victims," Cotter said. "There have been at least 15 to 20 animals in the last two summers."

Reasons for killing

Scientists do not know why the highly intelligent species made famous by the 1960s television show "Flipper" would suddenly start battering its ocean brethren, but scientists said the perpetrators are probably young, sexually frustrated toughs defending their turf.

The elusive harbor porpoise, Phocoena phocoena, prefers colder water and ranges south only as far as Point Conception, near Santa Barbara. Between 1,000 and 2,000 are believed to live along the California coast. The porpoises, which grow to about 5 feet in length, were seen only infrequently in San Francisco Bay over the past century until three years ago, when they returned in large numbers.

The bottlenose dolphin, Tursiops truncatus, is partial to warm, tropical regions and was never seen north of Point Conception until 1982-83, when a strong El Niño weather pattern caused the sudden warming of ocean currents. The dolphins, which can reach 10 feet long, have since become residents of Monterey Bay and, since at least 2008, have been spotted frequently in San Francisco Bay. About 500 or 600 live along the coast of California.

"We have been fascinated by not only the return of harbor porpoises to San Francisco Bay, but by the nearly simultaneous discovery that bottlenose dolphins have returned to the latitude of San Francisco to use the bay," Keener said. "We've been photographing the dolphins in San Francisco Bay, and they match the ones in Monterey, so they go back and forth from Monterey to San Francisco. You've got two marine predators who are sharing the bay again. That's really interesting, but when they come into contact, there is going to be some conflict and the loser is going to be the porpoise."

Sex-related assaults

Scientists have documented attacks by bottlenose dolphins on estuarine and Atlantic spotted dolphins and on harbor porpoises on the East Coast and in Europe since 1991. Fatal attacks have been recorded recently on porpoise species and even a juvenile pilot whale in the United Kingdom, but nobody had ever heard of such a thing along the Pacific Coast until the porpoises began washing up on beaches.

"Until about three years ago, there didn't seem to be evidence that harbor porpoises were being attacked," said Thomas Jefferson, co-author of the porpicide study. "In the past three or four years, there have been dozens of cases of documented attacks of porpoises by dolphins."

The killings coincide with the expansion of dolphin territory, but Cotter and Jefferson believe the primary motive is not territorial. The two species primarily feed on different things and do not generally frequent the same places at the same times, according to the porpicide study.

Jefferson said the killings appear to be sex-related.

"In some of the incidents recorded in the past, there has been activity focused on the genital area of the porpoise," like bite marks or scraping, he said. "These bottlenose dolphins, we call them 'porpitrators,' were either known males or suspected males and mostly younger males."

Cotter said the attacks do not appear to be sexual assaults as much as they are aggression caused by sexual frustration, much as one might find in a gang of teenage boys.

Male bottlenose dolphins are known to practice infanticide as a way of inducing estrous in females. It is possible, according to the study, that they are practicing their baby-killing techniques on porpoises, which are about the same size as young dolphins.

Cotter doesn't believe that is the case.

"All the animals are subadults that are reaching sexual maturity, but they aren't high ranking yet in the dolphin social structure and therefore don't have access to females," Cotter said. "All of these attacks seem to happen in the summer and into the fall, which coincides with the peak breeding time, so we lean toward sexual frustration not infanticide behavior."

Reality of nature

The witnessed attacks all went down the same way. Two or three dolphins broke away from a larger pod and worked together to chase down a porpoise. They pummeled the victim, launched it out of the water, and used their noses to raise the tail, preventing the porpoise from lifting its head out of the water to breath. The sex of the victims did not seem to matter.

Disturbing as the killings may be, Jefferson said, they are part of a natural process.

"We are just seeing dolphins more realistically," he said. "Like most species they have rules in their society and sometimes they act aggressively too, but they are doing what they need to do to survive."

If anything, Jaramillo said, witnessing an attack has inspired him to come out to the beach more often, next time with binoculars and a camera.

"You see all kinds of interesting things on the beach," he said, "but this one takes the cake."

This article has been corrected since it appeared in print editions.