Climate Change Is Causing Some Mixed-up Wildlife
Species separated for millennia are coming into contact, and mating. The debate over what, if anything, we should do about their hybrid offspring is heating up.
The more genetically similar two species are--in terms of chromosome numbers or reproductive proteins--the easier it is for them to reproduce. Dogs and cats, for example, or lions and lambs are just too different genetically to produce offspring. But even when interspecies pairing is successful, the hybrid off- spring face a series of unique challenges. Just as most mules--a cross between a male donkey and a female horse--are sterile, many hybrids cannot reproduce and are therefore genetic dead ends, says Mallet. Others inherit traits from their parents that render them ill-equipped to thrive or even survive. Polar-grizzly bear hybrids bred in captivity, for instance, can't swim as well as genetically pure polar bears, which could pose grave risks in an ecosystem where ice sheets--the frozen platforms from which they hunt seals--are smaller and farther apart.
One of the biggest debates is about whether hybrids should be eligible for legal protection, particularly if one or both parent species are threatened or endangered. Currently, the Endangered Species Act doesn't address hybrids. Same goes for the International Union for Conservation of Nature's Red List of Threatened Species. (Hybrids may often be unknowingly protected because they can be difficult to distinguish from their safeguarded parents.) The U.S. Fish and Wildlife Service drafted a hybrid policy in 1996 but ultimately decided not to approve it, says J.B. Ruhl, an environmental lawyer and expert in climate change and the Endangered Species Act at Vanderbilt Law School. The agency instead adopted a policy of dealing with these animals on a case-by-case basis. Neither a Fish and Wildlife Service press officer nor several conservation lawyers could name any hybrids currently protected by the agency.
The problem, Ruhl says, is that the Fish and Wildlife Service's current policy addresses individuals, while the real issue is populations. Scientists and conservation experts are split as to whether these legal policies should be changed to deal with the growing number of hybrids. Some see no value in keeping hybrids around at all. Stuart Pimm, a species extinction expert at Duke University, says that wiping out hybrids is the best way to protect threatened species--though doing so would be tricky, he admits. "An unfortunate aspect of all this is that hybridization is a major cause of species endangerment and disappearance," he says. "This is not one of those circumstances where the choices are easy ones, but these hybrids are a threat to many valued species." Hybrids have value, too, argues Richard Kock, a conservationist and member of the IUCN's Species Survival Commission. "We should see [hybrids] as holding genes, some of which represent original species and therefore are of value," he says. "With modern genetic understanding, breeding back to an original geno-type is not impossible. So they have a place in conservation."
Ultimately, how we deal with hybrids will be decided among lawmakers and wildlife managers, in courtrooms and at inter- national meetings. "It becomes a value question," says Kelly. "Do you like having a white bear that specializes in hunting seals in the ice? That's what's in peril."
Species have a strong instinct to mate with their own kind--they wouldn't last very long as distinct species otherwise. So why do in- dividual animals sometimes make the seemingly unusual choice to hook up with a different species?
In captivity, of course, the dating scene is grim. Those circumstances have produced unexpected pairings, such as the 400-pound female bottlenose dolphin that in 1985 gave birth to a calf fathered by her pool mate at Hawaii's Sea Life Park, a 2,000-pound false killer whale. The "wholphin" is still swim- ming at the park today.
Though it might seem odd, intermingling isn't a rare occurrence in the wild. By some estimates, up to 10 percent of animal species and 25 percent of plant species occasionally mix it up. With birds, some hybridizations involve species whose breeding ranges overlap extensively, such as west- ern kingbirds and scissor-tailed flycatchers, or Anna's and Costa's hummingbirds.
More often, the scarcity of same-species mates might spur the affair. That's likely the case with the "Junkin's warbler," named after David Junkin, who discovered the hybrid in a mist net in Wethersfield, New York, in 2006. Genetic testing determined that it was the offspring of a male mourning warbler and a female Kentucky warbler, whose range doesn't extend that far north. The birds are members of the same genus, and have similar nesting behaviors, which might have encouraged the pairing in a region where Kentucky warblers aren't typically found.
Dick Shideler, an Alaska Department of Fish and Game bear biologist, has seen what happens when grizzlies and polar bears meet. "The grizzly runs the polar bear off," he says. So far polar bear mothers have borne all known hybrids, perhaps because of the grizzly's aggressiveness. (Shideler has seen 300-pound female grizzlies chase off much larger male polar bears.) Timing could also play a part. Male grizzlies emerge from hibernation a month or so before female grizzlies; while their counterparts still slumber, they may encounter female polar bears in estrus. "I think a lot of it has to do with being in the right place at the right time," says Shideler. "Grizzly bear males are driven by their nose, so if there's a female in heat and they sniff her out, they're probably not that selective about which species it is."--Alisa Opar
This story originally ran in the November-December 2013 issue as "Mix-Up."