WHEN does the grizzly bear cross the road? Answer: Very rarely 鈥� and that鈥檚 bad news for bears in the northern US and southern Canada.
A team of Canadian biologists used DNA fingerprinting to track the migration of 470 grizzlies near Canada鈥檚 Highway 3, a major east-to-west route through the Rocky Mountains and two adjoining ranges. The team obtained DNA from individual bears via hair samples snagged in barbed-wire strands set around smelly bait. Since bears from different populations carry a different mix of genetic types at 15 highly variable regions in the genome, the researchers were able to separate out the road-crossing migrants from the local populations.
In all three mountain ranges, bears from opposite sides of the highway were genetically distinct. Few bears, and in one range none at all, showed signs of having migrated from one side to the other, and those few migrants were almost all male.
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By contrast, bears on opposite sides of a similar-sized valley with no road running through it showed no sign of genetic segregation (Proceedings of the Royal Society B, DOI: 10.1098/rspb.2005.3246). 鈥淚t doesn鈥檛 take much to cut off grizzly bear populations,鈥� says team member Curtis Strobeck, a geneticist at the University of Alberta in Edmonton.
The fact that the road deters females more than males is significant, says Christopher Servheen, grizzly bear recovery coordinator for the US Fish and Wildlife Service in Missoula, Montana. Although male movement allows genetic mixing, it is the number of females that determines a population鈥檚 growth rate. When females can鈥檛 move freely, the isolated populations are more vulnerable to chance events like a bad winter or disease, and are therefore more likely to die out.
Options for solving the problem are limited, but researchers could transport females across the road, or set up overpasses in areas often frequented by the bears.
