Why Some Animals Try Mating with the Wrong Species

The silent, frosty, lonely Antarctic is not a place that usually conjures sex-related thoughts. But the ice-clad continent recently revealed a wicked side, when biologists reported that on sub-Antarctic Marion Island, doe-eyed seals are recklessly coercing cute penguins into sex.

Scientists at the University of Pretoria in South Africa detailed the phenomenon in an article published in the Polar Biology journal; the report is accompanied by a video showing the affair quite clearly. In it, a male fur seal forcibly humps a hapless king penguin, while other penguins cowardly stay at arm's length from the odd couple. The poor victim attempts at some point to peck at its towering assaulter, but with no result. The whole footage is disturbing and eerily surreal. At the end of it, the question is begged: why on earth is a male seal mating with a small penguin instead of with another seal?

Sex between different species—also called "misdirected mating" or "reproductive interference"—is rare but not unheard of in the animal realm. Besides seals, types of dolphin, bird and big cat have been known to engage in various types of sexual activity with other species. Still, it's a pretty mysterious matter.

"It's quite difficult to explain why it happens, because it doesn't look particularly beneficial," Bram Kuijper, a zoologist from University College of London, told me in a phone call "I mean, in some cases, you obviously can't have offspring as a result of it. But even when that happens between similar species, and there is offspring, it usually doesn't perform very well."

In the Marion Island imbroglio, according to Antarctic marine mammal ecologist Iain Staniland, a tentative explanation is that those seals were just exploding with hormones. "Seals have very rigid breeding system, where a strong, dominant male may end up getting up to 12 females, leaving a lot of other males mateless. So these young males are sexually frustrated, and penguins are an easy target for their frustration," he said. Staniland's interpretation echoes that of the authors of the study, who hypothesised that the incident may be due "mate deprivation."

Staniland also highlighted that seal-on-penguin violence might be on the rise due to some sort of skewed copycat effect: "Seals are smart animals, and males are learning from each other that penguins work for that purpose. Fortunately, this is likely to remain a regional situation, localized on the island," he explained. Marion Island, where the first sexually violent seals were spotted in 2008, looks therefore bound for becoming a sort of Antarctic Gomorrah.

But hormone-fuelled fads are not the worst that can happen when it comes to inter-species sex. Bottlenose dolphins, for example, have been witnessed as sexual assaulters-cum-killers of smaller cetaceans like porpoises, a habit that gave rise to the neologism "porpicide." Their heinous attacks have a disturbing street-cred facet. "With dolphins, it's not only about sexual drive, it's mostly about their social behaviours. When they engage in these actions with other species it's like if they're saying 'I'm in charge, I'm dominant over you,'" Staniland said.

When you move from sea to dry land and air, things become slightly less grim and, in general, more fruitful. Here, "misdirected mating" is typically the result of strong similarities between the species, which just end up mating and having children. Kuijper cited the frequent inbreeding between two birds (the collared flycatcher and the pied flycatcher) but examples are legion, from mules to more fun-sounding hybrids like the zorse or the cama.

Many of these "interferences" happen in captivity, as the two individuals involved would never hook-up if they met outside a zoo. "A lion and a tiger breed only if you put them together in the same cage, and it usually happens because the male has no idea of how a female of his species should look like," Kuijper told me.

In other words, the force that has brought about the so-called liger was the blissful ignorance of a he-lion who couldn't tell who he should be pining for.

Compared to the grotesque seal-on-penguin violence that we have been witnessing on Marion Island, that looks like a minor oversight.