MOSS LANDING, Calif. -- Marine biologists have figured out why a growing number of dead harbor porpoises have been found on California beaches in recent years: dolphin attacks.

Now they're trying to determine a reason for them, KTVU-TV in San Jose reported.

Okeanis, a Moss Landing-based conservation group, shot video of bottlenose dolphins fatally attacking a porpoise. It helps explain why carcasses were found with internal bleeding, rake marks and broken bones.

"We saw severe beatings," said Okeanis chief scientist Daniela Maldini. "It was a mob of dolphins."

The video shows male dolphins corralling the porpoise, ramming it and drowning it, then bringing the carcass to researchers on a boat and swimming away.

Those images, taken for the first time last September, were the first to even depict dolphin-on-porpoise violence.

Dan Costa, a professor of ecology and evolutionary biology at UC Santa Cruz, said such an act of violence is not as unusual as it might seem in the wild.

"Everybody thinks flipper is this nice, mellow animal," Costa said. "In the wild, there's interactions that occur and not all of them are friendly."

Scientists say about 74 dead porpoises washed up last year in the state.

Okeanis biologists are working with the Santa Barbara Museum of Natural History, Marine Mammal Center and other groups to find evidence that will shed more light on the attacks.

One possible explanation for the violence, according to Maldini, could be pent up sexual angst on behalf of male dolphins.

"It could be a little sexual frustration," she said, "and a lot of males around for very few females. That's my favorite in terms of an explanation, but at this point we're fascinated by the behavior and trying to figure it out."

Scientists now want to test the testosterone levels of the male dolphins to see if higher levels could be playing a role.