for National Geographic News
Populations of polar bears can be pushed to the brink of collapse if hunters kill too many males, a new study suggests.
A single male can impregnate several females each breeding season, so wildlife managers generally maintain that fewer males than females are needed to support a healthy population. But the balance can tip too far if there are not enough males to go around.
Hunting has already tilted the balance in some Canadian bear populations, as trophy hunters target males, which can grow to twice the size of females.
A team of biologists has estimated the tipping point for a population of polar bears around Lancaster Sound in the Canadian province of Nunavut. There, hunters are currently allowed to take two males for every female.
The scientists found that the danger point for the sound's polar bears may be closer than was previously believed.
The researchers took into account how long a male is likely to look before finding a mate and how long he spends with her before moving on.
The findings predict that the Lancaster Sound population could reach the brink when the ratio of breeding males to females reaches 2-to-3. The most recent census of bears in the area found a single adult male for each available female.
Once the bears start missing their mates, the number of new cubs would fall rapidly, the team reported.
The minimum proportion of males depends on how crowded an area is with bears. Sparse, scattered populations need more males, they added.
"It just takes so much longer to find those very few females that are running around somewhere," said Péter Molnár of the University of Alberta in Edmonton who led the study.
The work of Molnár's team provides the sort of information managers need as they set limits on hunting, said Scott Schliebe, a polar bear specialist with the U.S. Fish and Wildlife Service in Alaska.
"When we monitor these populations, we're going to have to look at the ratio of males to females to make sure that things aren't tipping."
(Read related story: Polar Bears Proposed for U.S. Endangered Species List [December 27, 2006].)
Molnár's team reported its findings today in the Proceedings of the Royal Society B.
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