An Antarctic fur seal has been observed trying to have sex with a king penguin.
The South African-based scientists who witnessed the incident say it is the most unusual case of mammal mating behaviour yet known.
The incident, which lasted for 45 minutes and was caught on camera, is reported in the Journal of Ethology.
The bizarre event took place on a beach on Marion Island, a sub-Antarctic island that is home to both fur seals and king penguins.
At first glimpse, we thought the seal was killing the penguin
Nico de Bruyn, University of Pretoria
Why the seal attempted to have sex with the penguin is unclear. But the scientists who photographed the event speculate that it was the behaviour of a frustrated, sexually inexperienced young male seal.
Equally, it might be been an aggressive, predatory act; or even a playful one that turned sexual.
"At first glimpse, we thought the seal was killing the penguin," says Nico de Bruyn, of the Mammal Research Institute at the University of Pretoria, South Africa.
The brazenness of the seal's behaviour left those who saw it in no doubt as to what was happening.
De Bruyn and a colleague were on Trypot beach at Marion Island to study elephant seals when they noticed a young, adult male Antarctic fur seal, in good condition, attempting to copulate with an adult king penguin of unknown sex.
The 100kg seal first subdued the 15kg penguin by lying on it.
The penguin flapped its flippers and attempted to stand and escape - but to no avail.
The seal may have been frustrated in its attempts to find a partner
The seal then alternated between resting on the penguin, and thrusting its pelvis, trying to insert itself, unsuccessfully.
After 45 minutes the seal gave up, swam into the water and then completely ignored the bird it had just assaulted, the scientists report.
Why a fur seal would indulge in such extreme sexual behaviour is unclear.
Sexual coercion among animals is extremely common: males of many species often harass, coerce or force females of their own kind to mate, while animals are also known occasionally to harass sexually a member of a closely related species.
Harassment is common among pinnipeds, the group of animals that includes seals, fur seals, and sea lions; and occasionally it happens between related species.
Male grey seals have been known to harass and mate with female harbour seals, for example, producing hybrids.
"Sexual harassment is often more commonplace in non-monogamous mating systems, and in species where males are physically much larger than the other sex and thus physically capable of coercion or harassment," says de Bruyn.
But this is thought to be the first recorded example of a mammal trying to have sex with a member of another class of vertebrate, such as a bird, fish, reptile, or amphibian.
Chinstrap penguins occasionally indulge in homosexual behaviour, and adelie penguins sometimes "prostitute" themselves to get stones for nest-building; while one in seven emperor penguins will change partners from one year to the next.
But generally, king penguins lead straightforward sex lives: males and females pair up for years on end.
Marion Island is the only place in the world where Antarctic fur seals are known to hunt king penguins on land, so the idea that the fur seal was trying to eat the object of its attention made sense.
"But then we realised that the seal's intentions were rather more amorous."
The researchers speculate that the male seal was too young to win access to female seals, and in a state of sexual excitement, looked elsewhere.
But the mating season was nearly over when the incident took place, leading the scientists to also wonder whether the seal's natural predatory aggression toward the bird became redirected into sexual arousal.
Equally, the incident may have arisen because the seal was "play-mating".
"It was most certainly a once-off and has never previously or since been recorded anywhere in the world to our knowledge," says de Bruyn.The penguin did not appear to have been injured by the seal, the scientists report.