Polar Bears Threatened By Hunter's Choice of Males
ScienceDaily (Nov. 23, 2007) — Policies that encourage hunters to go after male polar bears in order to conserve females, could make it harder for the animals to find mates. University of Alberta researchers determined there is a critical threshold in the male-to-female ratio. Below it, their model predicts a sudden and rapid collapse in fertilization rates.
Harvesting regulations vary around the world, notes PhD student Péter Molnár of the Department of Biological Sciences, the study’s lead author. For example, Canada uses a quota system, permitting some hunting to a predetermined limit, while Norway bans any hunting of polar bears.
Current Canadian management polices encourage hunters to go after male bears in order to conserve females while maximizing the number of bears that may be harvested. But harvesting based on sex selection has reduced the number of males compared to females in polar bear populations across the Canadian Arctic, Molnár says.
Molnár’s work was supervised by Professor Andrew Derocher, a noted polar bear expert, and Professor Mark Lewis, who holds the Canada Research Chair in Mathematical Biology. The study was conducted in collaboration with Dr. Mitchell Taylor from the Government of Nunavut.
The U of A team warns that under certain conditions, this practice of sex-selective harvesting could lead to trouble when it comes to polar bears finding a mate - based on a biological mathematical model they developed, and data from the polar bear population at Lancaster Sound off Cornwallis Island (northwest of Baffin Island) in the High Arctic.
The mathematical model has given the researchers the ability to predict how many male bears are needed in an area compared to the number of female bears that are around, to maintain successful mating.
They determined that there is a critical threshold in the male-to-female ratio. If the sex ratio drops below this threshold, the model predicts a sudden and rapid collapse in fertilization rates. This threshold depends on whether the area has a high-density or low-density bear population.
Each bear population must be evaluated separately, since results will be different depending on the population characteristics of the area. But the U of A scientists can say that in regions with fewer bears travelling a larger area (low-density population), there actually needs to be more males than breeding females to maintain high fertilization rates.
Given this important population threshold, the authors concluded that polar bear populations should be closely monitored and carefully harvested, as severe negative effects could arise either from overharvesting male bears or from population decline.
These findings appeared November 21 in the online edition of the Proceedings of the Royal Society.