Antarctic fur seal males have been seen forcing themselves on king penguins multiple times in shocking sexual acts that are radically changing the way animal experts attempt to explain such seemingly bizarre behavior.

At first it was thought that sex between animals of different species was a colossal error -- a result, maybe, of mistaken identity. The occurrences of male seals raping penguins, documented in the latest issue of the journal Polar Biology, suggest otherwise. Instead it may be a learned behavior by hormone-fueled males, which could weaken the overall reproductive fitness of both animals if it gets out of hand.

The seal-penguin rapes "may be learned behavior associated with some sort of reward, or it may be an extreme case of reproductive interference," wrote senior author Nico de Bruyn of the University of Pretoria and his team in the journal.

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"Reproductive interference occurs when individuals of one species engage in reproductive activities with individuals of another species, and when these interactions reduce the fitness of one or both species," Emily Burdfield-Steel, who is a researcher at the University of Jyväskylä in Finland, explained to Discovery News.

The fitness of the birds is definitely reduced, since at least one penguin victim was seen bleeding between its legs. The seal population would also appear to suffer, given that mating requires time and energy that, in this case, would not result in offspring.

The possible reward, however, could be the male's immediate sexual fulfillment.

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As de Bruyn and his team wrote of one seal-penguin encounter: "The seal ran up to the penguin and bumped it down. It lay on top of the penguin and started thrusting its hips in a copulatory fashion. The seal's erect penis was clearly visible."

A disturbing video (warning: it's graphic) that has since gone viral shows one of four such encounters that the researchers observed on Marion Island, located in the Indian Ocean just north of Antarctica.

De Bruyn first saw an Antarctic fur seal attempt to copulate with a king penguin six years ago on the same island. Although this location has been heavily studied for three and a half decades, the behavior had never been noted before. Recently, on a return visit, de Bruyn and his team documented the four seal-penguin rapes, so they think the behavior is "newly emerging."

An Antarctic fur seal seen among king penguins.Corbis

The region has a "bachelors beach" dominated by young male seals, so without many female seals around, the male seals could wind up chasing, for sex, what is normally a dinner item for them: penguins.

The seal-penguin copulations all happened during the seal's breeding season on the island, from late November to early January. On one occasion, a male seal ate a penguin that it had just raped.

In terms of why a predator would suddenly view a prey animal sexually, Burdfield-Steel suggests that raging hormones could reduce an individual's choosiness.

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"For the less choosy sex," she said, "this keenness to mate may lead to individuals being less discriminating when choosing mates and therefore more likely to fail to differentiate between species."

She and colleague David Shuker report that there are at least 167 documented instances of animals having, or attempting to have, sex with other species. This excludes hybridization, where such mating can lead to offspring (think mule). Humans, of course, are included in this group -- everything from the Bible to cave art has shown that some people have practiced bestiality.

In other examples, male sea otters have been documented raping and drowning young seals. Another viral video shows a young chimp raping a frog. Yet another video shows a rabbit attempting to mount a dog. Most dog owners have stories of their pets going at it with a person's leg, arm or other reachable area.

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It remains to be seen what will happen next on Marion Island but, for now, the sex between different species appears to be relatively rare, which is usually the case throughout the animal kingdom.

As animal researcher Ana Amaral of the University of Lisbon told Discovery News, "In most cases found in nature, each species favors its own kind when mating."

Not all mixed species sexual encounters are violent and unproductive. Some pollinating insects regularly attempt to mate with flowers. For example, male orchid dupe wasps find Australian tongue orchids so alluring that they will linger on the orchid's petals, leaving behind blobs of ejaculate.