Adaptive coat colour polymorphism in the Kermode bear of coastal British Columbia

Abstract

We investigated potential ecological attributes of the striking coat colour polymorphism in the black bear (Ursus americanus kermodei Hornaday) occurring on several small islands off the mid-coast of British Columbia, where a white morph (Kermode), fully recessive to the black morph, reaches 10–25% of the population. During three autumn field seasons (2000–2002; 697 h of observation time), we monitored salmon capture behaviour of individual bears (black, N = 37; white, N = 4). Both colour morphs foraged on salmon throughout daylight and darkness but with twice the rate during darkness. Log-linear analysis of capture efficiency (success/attempts) and average capture success per bout of the white morph was marginally lower than the black morph during darkness (22.8%, N = 158 versus 25.8%, N = 279 respectively; P < 0.04), although it was significantly higher during daylight (34.1%, N = 132 versus 25.3%, N = 896, respectively, P < 0.02), with similar trends in three different pursuit modes. Replicated (N = 10–14) field experiments involving a colour dimorphic simulated predator (i.e. human in a black or white cloak) showed that salmon were one-half as evasive during darkness than during daylight, with no differences (P > 0.4) in response to either the white or black models; however, during daylight, salmon were twice as evasive to the black compared to the white model (P < 0.001). The persistence of this coat colour polymorphism may be facilitated by increased salmon accessibility to the Kermode bear and diel foraging differences between morphs. These results are consistent with multi-niche models of adaptive variability.