Geographic variation in sexual-size dimorphism of the American black bear (Ursus americanus) in areas of western North America

Abstract

Patterns of geographic variation in cranial characters of the American black bear (Ursus americanus) were examined for 11 localities in the western US and Canada. Forty-four dimensions were recorded from skulls of 290 (149 males, 141 females) adult (≥4.5 years old) animals using digital calipers. Results showed that males are generally larger than females and the degree of sexual-size dimorphism for characters among localities is relatively consistent. Most averages (84.1%) of mean values across characters varied from 7.0% to 18.0%. The majority (90.9%) of character ranges (difference between highest and lowest value for a trait at each locality) of mean values for sexual dimorphism varied from 5.0% to 14.0%. Overall, there was no significant statistical difference in the degree of sexual dimorphism across localities in 43 of 44 characters. Measurements for males were significantly correlated with those of females for 38 of 44 characters. Results indicated that the level of sexual-size dimorphism remained relatively low and constant across localities. No statistically significant relationship existed between degree of sexual-size dimorphism and selected environmental variables. Although overall causes for levels of sexual differences in morphological features of black bears remain uncertain, bioenergetics appears to be a determining factor associated with sexual-size dimorphism in black bears.