Sexually selected infanticide (SSI) has been documented in some species with a mating system in which males have almost exclusive breeding rights with 1 or more females. When the dominant male is removed, the new male kills the offspring sired by the previous male to enable the mother to be bred earlier. It has been suggested that this immigrant male hypothesis of SSI operates in grizzly bears (Ursus arctos) and that removing dominant males by hunting results in high cub mortality due to killing by immigrant males, or in low reproductive rates because of a female counterstrategy of using suboptimal habitat to avoid potentially infanticidal immigrant males. I tested 2 predictions of the immigrant male hypothesis in a hunted area adjacent to protected areas with high densities of grizzly bears that could supply immigrant males. These predictions were not supported. Over 25 years, 134 grizzly bears were captured: most of the 77 male and 57 female grizzly bears were ≤3 years of age when first captured (54.5% and 52.6%, respectively), and 22.1% of the males and 19.3% of the females were 4–6 years of age when first captured. Similarities of these sex ratios suggest that there was not a substantially greater influx of subadult males than females into the hunted area. Cub survival to the end of the breeding season was high (0.93 or 0.95; n = 87), as was annual cub survival (0.85, n = 81); 15% of the 39 litters monitored for an entire year were completely lost. Yearling survival was 0.95 to the end of the breeding season, when SSI should cease. These results do not support the immigrant male hypothesis of SSI but suggest that grizzly bears either do not exhibit SSI, or that SSI exists in a different form. I propose a second hypothesis of how SSI may operate in bears. This mate recognition hypothesis of SSI is that males of any age may, if they are able, kill cubs that they believe they did not sire the previous year and try to mate with the mother. I use a simulation model to evaluate factors that may influence the existence and likely form of SSI in bears. Results of this study suggest that killing some adult males under a sustainable management regime does not decrease cub survival.
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Vol. 16 • No. 2