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Causes and consequences of sexual habitat segregation in grizzly bears Wielgus, Robert B.

Abstract

I studied habitat use and population dynamics of 2 grizzly bear (Ursus arctos) populations to test 3 competing hypotheses of sexual habitat segregation (no avoidance, food competition, and sex competition) and 3competing hypotheses on the effects of adult male mortality on female reproduction (additive, compensatory, depensatory). Twenty bears were radio-monitored from 1980 to 1984 in Kananaskis, Alberta and 28 bears were radio-monitored from 1985 to 1990 in the Selkirk Mountains of Idaho and British Columbia. The Kananaskis population had high mortality of older adult males and a corresponding influx of younger immigrant males. That population had a low reproductive rate and appeared to be declining. The Selkirk population had low mortality of older adult males and few younger immigrant males - that population had a high reproductive rate and appeared to be stable. Sexually mature adult females avoided food-rich, male-occupied habitat in Kananaskis where there were many potentially infanticidal, immigrant males; but not in the Selkirks where there were few such males. Sexually immature subadult females did not avoid food-rich, male-occupied habitat in either study area. Mean litter size of cubs was smaller in Kananaskis than in the Selkirks despite earlier age at first reproduction. Age of mothers, food supply, and population density did not explain the smaller litters but adult female avoidance of immigrant males and food-rich, male-occupied habitats appears to explain the smaller litters. My results reject the "no avoidance" and "food competition" models but support the "sex competition" model of habitat segregation. My results also reject the "additive' and "compensatory" models but support the "depensatory" model of adult male mortality. Trophy hunting of adult males was not compensatory for reproduction and beneficial for population growth as commonly believed, but was actually depensatory for reproduction and contributed to population decline.

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