As the world heats up and polar ice melts, different types of bears, whales and seals could meet and mate — but these unions may be far from happy, researchers said Wednesday. In fact, interspecies sex brought on by the melting Arctic ice could lead to the extinction of many endangered Arctic animals, the scientists said in an article published in the journal Nature.
At least 22 species are at risk of hybridizing in 34 different combinations, according to a team led by Brendan Kelly, an Alaska-based evolutionary biologist with the National Oceanic and Atmospheric Administration. The pairings include polar bears and grizzlies, narwhals and beluga whales, and various assortments of seals. Some of those species are listed as endangered or threatened.
Because hybrid offspring — or their offspring, in turn — are often infertile, maladapted or sickly, much of the genetic biodiversity of the Arctic could be lost, the scientists warned.
Kelly said the report "is sort of a call-to-arms to encourage our colleagues around the Arctic to recognize this may be going on." The authors urged the scientific community to begin monitoring mammals living in the Arctic to check for any increase in hybridization events.
Kelly and his coauthors reached their conclusions after reviewing the scientific literature, scouring museums for possible hybrid bones and pinpointing which populations are at risk of running into each other as the Arctic ice shrinks. That ice has separated many sea- and ice-dwelling animals for as long as 10,000 years, maintaining a barrier to animal movement, and thus interbreeding. Some scientists have predicted that the Arctic could become ice-free by the century's end as a consequence of climate change.
Although Arctic species rarely interbreed, many are capable of doing so. In 2006, a bear with a patchy white-and-brown coat was shot in the Canadian Arctic. Scientists suspected — and DNA-typing later confirmed — that the animal was a polar-grizzly hybrid, a creature known variously as a "pizzly" or "grolar bear."
In the 1980s, an apparent beluga-narwhal hybrid skull (one that lacked the narwhal's single tusk) was picked up in Greenland.
And last year, what appeared to be a hybrid of two whales — a bowhead whale and a right whale — was spotted in the Bering Sea.
David Withrow, a marine mammal biologist with NOAA, was the one to spot the hybrid whale, the first he'd ever seen. He recalled his shock as the animal headed toward him.
"It really threw me," said Withrow, who was not involved in the new report. "I felt small, like I should know what this whale is."
If endangered animals increasingly mate with other species, they could be hybridized out of existence, Kelly said. Consider North Pacific right whales. About 200 are known to exist — and only 30 in the area where Withrow spotted the bowhead-right whale hybrid in 2009.
Even though hybrids can often be healthier than either of their parents (a phenomenon termed "hybrid vigor" by scientists), this is often a temporary advantage, said coauthor Andrew Whiteley, a conservation geneticist at the University of Massachusetts. The offspring, should the hybrids be fertile, are generally more sickly.
And first-generation hybrids are often ill-suited for the environments in which they live, the researchers said. Take the polar-grizzly bear hybrid: Its mottled coat no longer traps heat as efficiently as that of a pure polar bear, it is a poorer swimmer, and it does not exhibit the crafty hunting behaviors (such as jumping on a snow cave to pin a hiding seal) that would help it to survive.
The beluga-narwhal hybrid skull discovered in the 1980s lacked the narwhal's characteristic horn, a key asset in competing with other males for rank and mates.
Negative effects may be more subtle too, said Fred Allendorf, a conservation geneticist at the University of Montana who was not involved in the study. Birth cycles may not match up with those scarce months when food is available. Animals may head in the wrong direction during migration periods. And it's not clear that hybrids would be hardy enough to handle certain environmental shocks or diseases.
"With a winter storm … or other kinds of environmental stresses, they may not survive where the native populations would have before," Allendorf said.
Cross-breeding could also bring positive results, Allendorf said. However, he seconded Kelly's call-to-arms. "The most important point is … collecting the samples now so we can see if this happens in the future. If we don't, in the future we'll have no way of knowing if this is something new or has been there the whole time."