This sexual behaviour was first documented back in 2006, when a fur seal tried to mount a king penguin on the sub-Antarctic Marion Island.
Biologists initially thought it was simply the behaviour of a sexually inexperienced seal, or a predatory act.
However, three new cases on the same island - captured on film for a study in the journal Polar Biology - have led researchers to believe that it's a trend.
In each case, the seal - which is much larger and heavier than the penguin - chases down the flightless bird (which could be male or female), captures it and mounts it. It then attempts to have sex a number of times, with breaks in between.
In some cases the seals seemed to penetrate the penguin's 'cloaca' - the opening through which both male and female birds mate.
In three of the cases the seal let the penguin go after the attack, but in one case the seal ended up eating the penguin (seals are the penguins' natural predator).
“One penguin [of the three that were released] did have visual signs of injury (bloody cloaca) but was able to walk and rejoin a huddle of penguins after the event," researcher Nico de Bruyn, from the Mammal Research Institute at the University of Pretoria, told Mirror Online.
"The others showed no visual signs of injury although they were obviously exhausted and there is no way of telling whether they had internal injuries.”
It's the first time that animals from the seal family have been observed trying to have sex with an animal from a different biological class - in this instance a mammal trying it on with a bird.
It's a mismatched union: Antarctic fur seals grow up to up to 2 metres long and can weight between 91kg and 215kg. King penguins, on the other hand, are much smaller, measuring between 70cm and 100cm tall and weighing up to 16kg.
De Bruyn told the BBC that the behavior may be because male seals see each other sexually harassing penguins and then attempt to do it themselves.
The behaviour could be exacerbated by the sexual frustration experienced by seals as they experience hormonal surges during mating season.
He said that it was very unlikely that it was down to a seal wrongly identifying a penguin as a female seal, but de Bruyn feels that it the behaviour is increasing in frequency.
“The drivers of this behavior are almost impossible to understand, but continued observation and reporting can help,” de Bruyn told us.