Distance-dependent effect of the nearest neighbor: spatiotemporal patterns in brown bear reproduction.

In mammals, reproductive synchrony and reproductive suppression usually are found in social, group-living species, which often display hierarchical relationships among related animals. Some individuals, particularly younger, philopatric females beyond the age of sexual maturity, may not raise offspring because they are suppressed by other individuals. Although brown bears (Ursus arctos) are a solitary species, the existence of socially induced delayed primiparity of philopatric females has been documented. Here we show further evidence for interactions of a population-regulatory nature that are typically associated with social species. We found that an adult female's probability of having cubs in a given year was influenced by whether or not her nearest neighboring adult female had cubs. At short distances (< or = 10 km) between the home range centroids of neighboring females, females with cubs had a negative effect on their neighboring female's probability of having cubs of the year. At distances >10 km and < or = 20 km, the effect reversed, and it disappeared beyond 20 km. We argue that reproductive suppression is probably caused by resource competition among females living close to each other. Previously, similar population regulation mechanisms have been found only in group-living mammals. Thus, social interactions and behavior in solitary carnivores may be more flexible than usually assumed.