Diagnostic tests found nothing awry in Morgan, who thereafter lived at the Fish and Game wildlife facility; he participated in research conducted by the University of California’s Long Marine Laboratory until he died of old age last March, at 17. (Biologists remember him fondly: Mike Murray, a veterinarian at the Monterey aquarium, said that during 11 years in captivity, Morgan taught scientists a tremendous amount about “what makes sea otters tick.”)

Dr. Harris, Dr. Miller and their colleagues suspect the attacks were fostered by a recent demographic shift that resulted in more male otters than females. The species is polygynous — mating is dominated by a few males, as with the Antarctic fur seals — and Elkhorn Slough had become a bachelor pad for many nonterritorial male otters that were shut out of the mating game. The researchers think that as a result, Morgan and the other misbehaving otters redirected their normal sexual responses toward the harbor seal pups, born at a large rookery in the same area.

That hypothesis is plausible, said Dr. Hochkirch, the German biologist. Something about how the seals looked or moved may have attracted the otters. For a wild male animal, “if you don’t find a good mate, you might try to copulate with something which is as close to a mate as possible,” he said. Sperm is cheap, so wasting it on the wrong species would probably not hurt the male’s reproductive success.

Here, too, the seal abuse is reflective of the sexual violence that is typical among sea otters.

“Everyone thinks they’re cute and cuddly,” said Mark P. Cotter, a biologist with Okeanis, a nonprofit marine research organization in Moss Landing. But when otters mate, he went on, the male bites the female on the face so she can’t get away. Female sea otters often die from mating trauma.

Another sexually aggressive species is the bottlenose dolphin. In the Bahamas, bottlenose dolphins are routinely seen sexually coercing smaller spotted dolphins, Dr. Mann said.

And on the West Coast, from 2007 to 2010, the California Marine Mammal Stranding Network recovered and autopsied 50 dead harbor porpoises that were apparently beaten up by dolphins, said Frances Gulland of the Marine Mammal Center in Sausalito, Calif. In one of three porpoise-bullying episodes in Monterey Bay that they filmed, Mr. Cotter and his colleagues saw a school of male dolphins batter a male porpoise to death.

The “porpicides,” as Mr. Cotter and colleagues called them in a paper last year, are mystifying: they confer no clear advantage to the dolphins, which seldom compete with porpoises for the same prey off California.