Brown bears (Ursus arctos) have been reported to be primarily diurnal throughout their range in North America. Recent studies of black bears during salmon migration indicate high levels of nocturnal foraging with high capture efficiencies during darkness. We investigated the extent of nocturnal foraging by brown bears during a salmon spawning migration at Knight Inlet in coastal British Columbia, using night-vision goggles. Adult brown bears were observed foraging equally during daylight and darkness, while adult females with cubs, as well as subadults, were most prevalent during daylight and twilight but uncommon during darkness. We observed a marginal trend of increased capture efficiency with reduced light levels (day, 20%; night, 36%) that was probably due to the reduced evasive behaviour of the salmon. Capture rates averaged 3.9 fish/h and differed among photic regimes (daylight, 2.1 fish/h; twilight, 4.3 fish/h; darkness, 8.3 fish/h). These results indicate that brown bears are highly successful during nocturnal foraging and exploit this period during spawning migration to maximize their consumption rates of an ephemeral resource.

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.

One of the most distinctive colour polymorphisms of any mammal occurs in black bears (Ursus americanus kermodei Hornaday) of the Great Bear Rainforest of coastal British Columbia. We use carbon and nitrogen stable isotope values and C/N ratios along segments of hair shafts obtained from 14 white and 12 black individual bears to quantify dietary niche of the morphs as a test of the multi-niche model for a polymorphism. On Gribbell Island, where the white bear reaches 30%, 15 N is significantly (GLM repeated measures) more enriched (more marine-derived nitrogen) in the white morph than in the black morph in each season (spring, summer, autumn). On the adjacent Princess Royal Island, where the white morph is less common, both morphs are highly enriched during autumn (~δ 15 N = +11‰), but there are no isotopic differences between morphs in any season. On both islands, C/N ratios (~3.1) of the black morph decrease from spring to autumn, converging on the lower average values for the white morph. Our data suggest that niche of the white morph involves increased use of a marine-associated diet and that ecological segregation between the morphs has facilitated the historical persistence of the polymorphism. ADDITIONAL KEYWORDS: C/N ratios-Great Bear Rainforest-hair shaft-marine-derived nutrients-multiniche polymorphism-salmon-Spirit bear-stable isotopes-Ursus americanus kermodei.

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