DNA testing sheds light on polar bear adoption

Monday, July 17th 2000, 12:00 am
By: News On 6

Two mothers met near the Arctic Ocean, had a drink, and swapped babies.

Along came a group of scientists in helicopters. The team swooped down to the ice, tested some DNA, and revealed the truth. The scientists – shocked to find the mothers living as if nothing was amiss – nevertheless chose to leave the families alone.

After all, the mothers were polar bears. The baby swap was a mistake. And neither mother seemed to know what had happened.

"It's a case of mistaken identity," said biologist Nick Lunn of the Canadian Wildlife Service. "I think if they knew, I don't think they would do it."

When adoption happens in the animal kingdom, it usually involves family members. The more closely related two creatures are, the more of the genetic material DNA they have in common. So by adopting an orphaned cousin or nephew, for example, an animal can keep some of its own genes in the evolutionary action.

But when Dr. Lunn looked at the DNA in three known cases of polar bear adoption in the Canadian Arctic, he and other scientists were surprised to learn that the mothers and their cubs were not related at all.

"It doesn't pay to invest energy in raising somebody else's genes," said Ian Stirling, also a biologist with the Canadian Wildlife Service.

The service has been tracking polar bears in West Hudson Bay, Canada, for decades. Since 1966, researchers have been capturing bears, putting numbered tags on them, letting them go, and then recapturing them to see what they've been up to. The project's goal is to learn more about the animals and to understand how changes in the environment might affect polar bear populations.

During more than three decades of research, scientists have seen eight instances of possible polar bear adoption. In several cases, researchers noticed females roaming with cubs that had lived with different females the year before. In one instance, a mother was nursing two cubs, one a year older than the other. That seemed odd because females don't usually give birth in consecutive years. Scientists suspected that mothers were adopting cubs, but there was no way of knowing for sure whose baby was whose.

Within the last few years, though, the same technological advances that can unravel a hospital's baby-swapping mistake have opened new possibilities for animal research. By comparing segments of DNA called markers, scientists can determine a child's birth parents. Each marker in a child's DNA must match one from one of its parents. Scientists use statistics to figure out how similar the markers need to be to conclude that a parent and child are related.

Out of curiosity, the Canadian scientists used the technique to analyze 16 markers in three cubs and their adoptive mothers. When the test confirmed that all three mothers had adopted cubs completely unrelated to them, the scientists were dumbfounded.

"I double-checked to make sure I was reading the tags right," Dr. Lunn said. "I never expected this to occur." His results appeared in the journal Zoology in June.

Just as surprising was the fact that the adoptions happened at all, Dr. Lunn added. Polar bears spend most of their 20- or 30-year life spans alone, leaving scant opportunity for adoptions to occur.

Males and females come together only to mate. And mothers keep cubs close for a couple of years until the little ones bulk up enough to live on their own. Cubs emerge from the den at 40 pounds, while adult females can weigh 650 pounds, and full-grown males can exceed 1,300 pounds.

Although mother and cub are usually isolated together during the weaning period, sometimes different mothers and cubs gather near a garbage dump or beached whale, Dr. Lunn said. If a male bursts into the area, cubs may get swapped in the confusion.

That may be a reasonable explanation for how polar bear adoption might occur, Dr. Stirling said. But, he noted, "We've all been scratching our heads as to why."

The most likely explanation, Dr. Lunn said, is that the mothers just don't realize their adopted cubs aren't their own. Because polar bears have such limited social lives, they may not need to be especially keen at recognizing their kin.

There is one hitch to this theory, though, Dr. Stirling said. Scientists have failed in every attempt to persuade a polar bear to adopt an orphaned cub. That discrepancy might say something about bear personalities. "As the Inuit say, 'Bears are like people because they're all different,'" Dr. Stirling said. "I think that really sizes it up. You never know what a bear is going to do."

Though the phenomenon is probably very rare, understanding why some polar bears adopt unrelated cubs may eventually help scientists protect polar bears, Dr. Lunn said. Arctic ice sheets are breaking up a few weeks earlier in the spring than they used to, he said. That limits how much time the bears can spend on the ice, hunting for seals. With less fat of their own, female bears are giving birth to lighter cubs. Population numbers haven't suffered from the change, but eventually they might, Dr. Lunn said. And if polar bears run into trouble, understanding polar bear adoption might help scientists save the species.

Dr. Lunn's study emphasizes how useful genetic testing might become for animal conservation, Dr. Stirling said. "One of the things that is particularly neat about it from a scientific perspective is that some of it represents work you could not really know for sure without the use of some sophisticated genetics."

For now, all theories about polar bear adoption remain speculative. "Whatever's going on there, it's safe to say we don't really understand it," Dr. Stirling said. But he finds wisdom in the words of a character intimately familiar with one bear, albeit a bear named Winnie the Pooh.

"As Eeyore says, 'It's the not knowing that makes things interesting.'"