Sea otters – cute, furry, adorable, clams-wouldn't-melt-in-their-mouths sea otters – have been observed forcibly copulating with, and in the process killing, juvenile harbor seals off California.

Writing in a recent edition of the journal Aquatic Mammals, Heather Harris of the California Department of Fish and Game and colleagues document 19 occurrences of this behavior in Monterey Bay between 2000 and 2002, leading to the deaths of at least 15 seals.

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Harris and her colleagues describe one incident in vivid detail:

A weaned harbor seal pup was resting onshore when an untagged male sea otter approached it, grasped it with its teeth and forepaws, bit it on the nose, and flipped it over. The harbor seal moved toward the water with the sea otter following closely. Once in the water, the sea otter gripped the harbor seal's head with its forepaws and repeatedly bit it on the nose, causing a deep laceration. The sea otter and pup rolled violently in the water for approximately 15 min, while the pup struggled to free itself from the sea otter's grasp. Finally, the sea otter positioned itself dorsal to the pup's smaller body while grasping it by the head and holding it underwater in a position typical of mating sea otters. As the sea otter thrust his pelvis, his penis was extruded and intromission was observed. At 105 min into the encounter, the sea otter released the pup, now dead, and began grooming.

On some occasions, they further note, otters would guard and copulate with the seals long after their victims had died -– as much as seven days afterward, in fact.

Bizarre as this behavior may seem, Harris and her co-authors point out that it is not dissimilar to standard sea otter mating protocol, in which males will often approach the female from behind, grip her around the chest with his forepaws, and grasp her nose or the side of her face with his teeth. Although the female frequently resists, generally the two eventually spin in the water, the male behind the female, until mating is complete. Here, too, the encounters may prove fatal for the female, either as a result of wounds inflicted from the male's bites and scratches or because of drowning. Harris and colleagues describe one instance in which a male otter held a struggling female underwater until her body became limp and then copulated with her several times. Indeed, trauma related to mating was responsible for 11 percent of deaths in fresh southern sea otter carcasses examined between 2000 and 2003.

So what's this all about? It hardly makes sense to mate with a female and kill her; and it makes even less sense to mate with another species of animal, dead or not. Harris and her co-authors offer an explanation.

Sea otters are polygynous; stronger males establish territories with high numbers of females and work to exclude other sexually mature males. These excluded males gather in so-called "male areas", denied the opportunity for contact with females. It's a situation that may have been exacerbated by a change in the demographics of Monterey Bay's sea otters. For reasons that are still far from clear, overall mortality rate in the otter population is increasing, and disproportionately affecting females. As a consequence, an even greater number of mature males are denied mating opportunities, possibly causing sexual encounters to be more aggressive when they do occur. And those males that remain denied an opportunity to mate take out their frustrations on the hapless young harbor seals, an interspecies interaction that has been known to take place, albeit with less dramatic consequences, in other marine mammals.

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All the incidents recorded by Harris and her coauthors took place in Monterey Bay. However, Harris told Discovery News, "Given that we documented interspecific sexual interactions involving at least three different male otters and that similar behavior has been described in many wildlife species, it is certainly possible that this behavior could be occurring elsewhere in the range."

Photograph: A male sea otter guarding the carcass of a juvenile harbor seal. Photo by Stori Oates.