Geographic variation and behavorial plasticity: Vigilance in desert bighorn sheep (Ovis canadensis nelsoni) populations.

摘要

Antipredator behaviors such as vigilance are often costly because they must be traded off with other activities, such as feeding. To assess geographic patterns of vigilance behavior in bighorn sheep, I tested the large-scale hypothesis that vigilance in bighorn sheep would differ between populations as a result of spatial heterogeneity in predation regimes. Mean vigilance as well as the covariation of vigilance with environmental factors such as group size and distance from escape terrain were predicted to differ among regions with and without predators. Populations under high predation risk were three populations that had originally been translocated from a mountain range uninhabited by mountain lions to habitat with lions. Populations in habitat without resident mountain lions were three native populations in the Mojave Desert. Results indicated no difference in overall mean vigilance between populations of sheep in habitat with mountain lions verses those without lions. Instead, there were differences in vigilance between terrain types expressed at different group size. Although behavioral response to terrain type of large group size was consistent with adaptive vigilance behavior, I rejected the hypothesis of large-scale adaptation and concluded that lack of population-level differences in overall vigilance was not consistent with predicted patterns of adaptive behavior.; To determine conditions that have prevented large-scale adaptation, norms of reaction for vigilance of native populations were examined to determine whether vigilance in these populations reflects predicted patterns of adaptive behavioral plasticity due to the costs of maintaining vigilance in the absence of predators. Vigilance in translocated populations was examined to determine whether populations under relaxed predation pressure retained sufficient adaptive plasticity to adjust vigilance behavior in response to increased predation risk. Results indicated that the observed variability in adaptive responses among populations of native desert bighorn sheep with relaxed selection, combined with a nonadaptive response of sheep in one of the translocated populations, contributed to the overall pattern of non-adaptive variation in vigilance. Given that some populations exhibited adaptive norms of reaction for vigilance, it appears that there are a variety of processes and contingencies that combine to generate the observed variability in behavior of bighorn sheep.