Reproductive Scenescence in the Brown/Grizzly Bear

Photo by Jeff Keay

Schwartz, C. C., H. V. Reynolds III, V. G. Barnes, Jr., R. Sellers, J. Swenson, S. D. Miller, B. McLellan, J. A. Keay, R. McCann, M. Gibeau, W. Wakkinen, R. Mace, W. Kasworm. 2003. Reproductive senescence in the brown/grizzly bear. Ursus 14(2):109-119.

ABSTRACT: Changes in age-specific reproductive rates can have important implications for managing populations, but the number of female brown (grizzly) bears (Ursus arctos) observed in any one study is usually inadequate to quantify such patterns, especially for older females and in hunted areas. We examined patterns of reproductive maturation and senescence in female brown bears by combining data from 20 study areas from Sweden, Alaska, Canada, and the continental United States. We assessed reproductive performance based on 4,726 radio-collared years for free-ranging female brown bears (age ≥ 3); 482 of these were for bears ≥ 20 years of age. We modeled age-specific probability of litter production using extreme value distributions to describe probabilities for young- and old-age classes, and a power distribution function to describe probabilities for prime-aged animals. We then fit 4 models to pooled observations from our 20 study areas. We used Akaike’s Information Criterion (AIC) to select the best model. Inflection points suggest that major shifts in litter production occur at 4–5 and 28–29 years of age. The estimated model asymptote (0.332, 95% CI = 0.319–0.344) was consistent with the expected reproductive cycle of a cub litter every 3 years (0.333). We discuss assumptions and biases in data collection relative to the shape of the model curve. Our results conform to senescence theory and suggest that female age structure in contemporary brown bear populations is considerably younger than would be expected in the absence of modern man. This implies that selective pressures today differ from those that influenced brown bear evolution.