Effect of energy availability, seasonality, and geographic range on brown bear life history

Life-history theory allows predictions of how changes in environmental sele ction pressures along a species' geographic distribution result in discrete shifts in life-history traits. We tested for spatial patterns of 24 popula tions of brown bears Ursus arctos across North America that grouped accordi ng to the following environmental and population parameters: evapotranspira tion as a correlate of primary productivity of vegetation, coefficient of v ariation of monthly evapotranspiration values as a measure of seasonality, population density, and adult female weight. Cluster analysis grouped brown bear populations into two regions: Pacific-coastal populations characteriz ed by high population density and large females that lived in areas of high primary productivity and low seasonality, and inland and barren-ground pop ulations characterized by relatively low density and small bears that lived in areas of low productivity and high seasonality. For each region, we tes ted whether life-history traits (age at maturity and interbirth interval) r elated to primary productivity or seasonality. High altitude (interior; > 1 000 m) and high latitude (barren-ground; > 65 degrees N) populations respon d to extremes in seasonality with risk-spreading adaptations. For example, age at maturity and interbirth interval increased with greater seasonality. In contrast, pacific-coastal populations living on the western edge of bro wn bear geographic range respond to intraspecific competition at high densi ties by maximizing offspring competitive ability. For trample, age at matur ity increased with greater primary productivity and high population density . In each region, the female parent decided on the life-history trade-offs required to reduce the risks of offspring mortality depending on the enviro nmental pattern.