An odd-looking white bear with patches of brown fur was shot by hunters in 2006 and found to be a cross between a polar bear and a grizzly bear. Apparently, grizzlies were moving north into polar bear territory. Since then, several hybrid animals have appeared in and around the Arctic, including narwhal-beluga whales and mixed porpoises.
The culprit may be melting Arctic sea ice, which is causing barriers that once separated marine mammals to disappear, while the warming planet is making habitats once too cold for some animals just right. The resulting hybrid creatures are threatening the survival of rare polar animals, according to a comment published Wednesday in the journal Nature.
A team led by ecologist Brendan Kelly of the National Marine Mammal Laboratory counted 34 possible hybridizations between distinct populations or species of Arctic marine mammals, many of which are endangered or threatened.
"The greatest concern is species that are already imperiled," said Kelly, first author of the Nature comment. "Interbreeding might be the final straw."
Pizzlies and Narlugas
When hunters encountered a hybrid of a polar bear and a grizzly in 2006, Kelly's colleagues remarked that the incident was just a fluke. But as Kelly delved into the issue, he found more evidence of similar anomalies. In 2009, a cross between a bowhead and a right whale was spotted in the Bering Sea, between Alaska and Russia. And a museum specimen in Alaska attests to breeding between spotted seals (Phoca largha) and ribbon seals (Histriophoca fasciata), which belong to different genera, a scientific classification of organisms that is broader than the species level.
Evidence suggests at least five other types of hybrids that may arise from animals of distinct genera, Kelly's team reported. These include:
- Narwhal (Monodon monoceros) and beluga whale (Delphinapterus leucas)
- Ringed seal (Phoca hispida) and ribbon seal (Histriophoca fasciata)
- Bowhead whale (Balaena mysticetus) and right whale (Eubalaena spp.)
- Harp seal (Phoca groenandica) and hooded seal (Cystophora cristata)
- Harbour porpoise (Phocoena phocoena) and Dall's porpoise (Phocoenoides dalli)
Breedings between these marine mammals near the North Pole are likely to result in fertile offspring, because many of these animals have the same number of chromosomes, said comment co-author Andrew Whiteley, a conservation geneticist at the University of Massachusetts, Amherst.
Chromosomes that are unmatched in number cannot pair during meiosis, a special type of cell division necessary for sexual reproduction. During meiosis, chromosomes duplicate and the cell divides to form daughter cells, which split apart to form gametes, or mature sexual reproductive cells. But with unmatched chromosomes, the split into gametes would be uneven, creating sterility.
Over the short term the hybrid offspring from these Arctic animal matings will likely be strong and healthy, because unlike inbreeding, which magnifies deleterious genes, so-called outbreeding can mask these genes. Most of these genes are recessive, meaning the offspring need a pair to actually show the trait. Different species or different genera generally don't have the same bad recessive alleles, and so there's not a high chance of a pair turning up. (Alleles are different versions of the same gene.)
But over time, as the hybrids mate randomly, those harmful genes will come out of hiding and make the offspring less fit and less capable of surviving, Whiteley warned.
Kelly said that breeding between species usually isn't beneficial when it's caused by accelerated environmental change, because the hybrid animals don't have time to evolve survival traits. "This change is happening so rapidly that it doesn't bode well for adaptive responses."
For instance, a cross between a narwhal and a beluga whale spotted in Greenland lacked the narwhal's spiral tusk, which contributes to breeding success. The polar–grizzly hybrid bears in a German zoo showed behaviors associated with seal hunting, but not the strong swimming abilities of polar bears.
Animals already threatened with extinction could take a hit from hybridization. The breedings between the North Pacific right whale, whose numbers have fallen below 200, and the more numerous bowhead whale, could push the former to extinction. (Over time, the hybrids would begin to outnumber the sparse right whales.)Story: Polar bears no longer on thin ice?
"This is one of the consequences of the rapid changes we're inducing in that environment and one more reason to consider whether we really want to continue warming the climate as rapidly as we are," Kelly told LiveScience.
The Arctic Ocean may lack summer ice by the end of the century, "removing a continent-sized barrier to interbreeding," the researchers wrote. As such, Kelly and his collaborators urge scientists to model the prevalence and outcomes of hybridization, genetically monitor at-risk populations, and generate a priority list.
And they're pushing policy-makers to incorporate hybrids into their management and protection plans. Currently, the Endangered Species Act doesn't protect hybrid animals, Kelly said. "It's just not something that has been on people's radar screen, and we think it should be."
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