By BBC\Matt Walker
The South African-based scientists who witnessed the incident say it is the most unusual case of mammal mating behaviour yet known.
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.
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 have 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 penguin flapped its flippers and attempted to stand and escape - but to no avail.
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.