Porpoise

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Porpoises are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. They are distinct from dolphins, although the word "porpoise" (pronounced /ˈpɔrpəs/) has been used to refer to any small dolphin, especially by sailors and fishermen. The most obvious visible difference between the two groups is that porpoises have flattened, spade-shaped teeth distinct from the conical teeth of dolphins, and shorter beaks.

The name derives from French pourpois, originally from Medieval Latin porcopiscus (porcus pig + piscus fish).

Porpoises, divided into six species, live in all oceans, mostly near the shore. Freshwater populations of the Finless Porpoise also exist. Probably the best known species is the Harbour Porpoise, which can be found across the Northern Hemisphere. Like all toothed whales, porpoises are predators, using sounds to locate prey and to coordinate with others. They hunt fish, squid, and crustaceans.

Porpoises apparently diverged from dolphins about 15 million years ago in the northern Pacific, then spread across the globe much later.

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.

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 coralling 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.