Animals try to mate with those of other species - but why?

Sex between animals of different species is well documented, with some creatures' mate-recognition radar off kilter; scientists aren't sure why

PUBLISHED : Sunday, 02 December, 2012, 12:00am
UPDATED : Sunday, 02 December, 2012, 3:21am

One day during field observations last year at Marion Island, a remote nature preserve in the southern Indian Ocean, something bizarre caught Tristan Scott's eye: on a rocky beach, a sleek young male Antarctic fur seal was trying to mate with a king penguin.

The fur seals normally hunt penguins and eat them. But this seal was wrestling with the bird, chasing it as it repeatedly tried to escape.

Baffled at first, Scott, a wildlife researcher, realised that the seal "was trying to court the penguin as if it were a female seal".

When that failed, he "tore the bird to shreds and ate it", Scott recalled.

Disturbing as it may sound, such wayward mating behaviour is not unheard of. An earlier episode of seal-on-penguin sexual violence, also on Marion Island, was reported in 2008 by Nico de Bruyn and colleagues at the University of Pretoria, South Africa, where Scott is a graduate student.

The phenomenon is called misdirected mating, and it extends to other marine mammals. Wildlife experts say sea lions and sea otters have occasionally been seen forcing themselves on other types of seals and killing them.

Indeed, some researchers say that misdirected mating is not abnormal.

"These things happen in wildlife," said Heather Harris, a veterinarian who has studied sea otters in Monterey Bay. "We think that it is within the spectrum of possible normal behaviour."

And Axel Hochkirch, an evolutionary biologist at the University of Trier in Germany, called the behaviour "simply a bad mistake," and added: "Nature is not perfect."

Nor is such mating limited to marine mammals. Insects, spiders, worms, frogs, birds and fish do it, too, Hochkirch said. The behaviour is a form of reproductive interference, in which an animal's mate-recognition radar is imperfect; the encounters do not necessarily end fatally.

Some couplings between closely related species result in familiar hybrids, like the mule. But when mixed matings result in no viable offspring, scientists say, the behaviour is difficult to understand from the standpoint of evolution.

Why, for example, would a fur seal try to mate not just with a different species but with an entirely different class of animal?

De Bruyn speculated that the episodes started out as normal penguin hunting but that "wires somehow got crossed" and set off a sexual response.

Both incidents happened near the end of the seal breeding season - a time when males experience "huge testosterone boosts" but when mating opportunities are monopolised by a few dominant males, leaving lower-ranking males with no outlet for their sexual excitement. As a result, the researchers say, the two frustrated male fur seals may have turned on the penguins.

De Bruyn pointed out that sexual aggression was common within many marine mammal species; for instance, male fur seals often bite females on the neck during mating. Sexual conflict occurs to varying degrees across the animal kingdom, and in extreme cases, males' coercive behaviour may spill over to forcing themselves on other species, said Janet Mann, a Georgetown University field biologist.

Another example comes from Monterey Bay in California, where the local news media give expansive coverage to stranded California sea otter pups rehabilitated by the Monterey Bay Aquarium.

One graduate of that programme had an infamous reputation: Morgan, who was rescued as a pup in 1995, released and then recaptured in 2001 after being spotted forcibly copulating with Pacific harbour seal pups, five of which did not survive.

Observers documented 19 cases of attacks on harbour seals by Morgan and at least two other male otters. The aggressors were "harassing, dragging, guarding and copulating with harbour seals" for up to a week after the pups were killed, according to an analysis published in 2010.

That study, conducted at the California Department of Fish and Game in Santa Cruz, was led by Harris, the sea-otter vet, who was then a research assistant; Melissa Miller, a veterinary pathologist; and Stori Oates, a biologist at Moss Landing Marine Laboratories. Their autopsies of seal corpses found bite marks and lacerations on the nose and face, along with tearing consistent with sexual trauma.

Diagnostic tests found nothing awry in Morgan, who thereafter lived at the Santa Cruz facility; he participated in research conducted by the University of California's Long Marine Laboratory until he died of old age in March, at 17. (Biologists remember him fondly: Mike Murray, a veterinarian at the Monterey aquarium, said that during 11 years in captivity, Morgan taught scientists a tremendous amount about "what makes sea otters tick".)

Harris, Miller and their colleagues suspect the attacks were fostered by a recent demographic shift that resulted in more male otters than females. The species is polygynous - mating is dominated by a few males, as with the Antarctic fur seals - and Elkhorn Slough, an estuary in the bay, had become a bachelor pad for many non-territorial male otters shut out of the mating game. The researchers think that as a result, Morgan and the other misbehaving otters redirected their sexual responses towards the harbour seal pups, born at a large rookery in the same area.

That hypothesis is plausible, said Hochkirch, the German biologist. Something about how the seals looked or moved may have attracted the otters. For a wild male animal, "if you don't find a good mate, you might try to copulate with something which is as close to a mate as possible", he said.

Here, too, the seal abuse is reflective of the sexual violence that is typical among sea otters.

"Everyone thinks they're cute and cuddly," said Mark Cotter, a biologist with Okeanis, a nonprofit marine research organisation in Moss Landing. But when otters mate, he went on, the male bites the female on the face so she can't get away. Female sea otters often die from mating trauma.

The New York Times