Predation risk and the community organization of montane ground squirrels and a phylogenetic test of the association between diurnal activity and gregarious behavior in mammals.

Abstract

Predation risk strongly influences the ecology and evolution of animal behaviors. However, anti-predator responses differ among species. Tradeoffs between anti-predator abilities and foraging efficiencies across habitats can promote the coexistence of ecologically similar species. In general, larger animals may better exploit riskier habitats; smaller animals, resources at low densities. I studied two montane ground squirrels Spermophilus lateralis and Tamias minimus in Colorado. Using food trays, I determined that the smaller T. minimus foraged more efficiently near meadow edges and was competitively superior to S. lateralis there. In contrast, S. lateralis had no preference for either edge or far habitat, but was competitively superior to T. minimus farther from the edge. I predicted, correctly, the exclusion of S. lateralis from meadows with primarily edge habitat. I proposed three alternatives to explain the observed patterns of habitat use: satiation, travel cost, or perceived predation risk. In a series of manipulations of energetic costs, protective cover, and predators, I eliminated all but the predation risk hypothesis. S. lateralis perceives less risk farther from the edge of the meadow than T. minimus. Faster harvesting and running speeds might decrease perceived predation risk. S. lateralis and T. minimus both experienced diminishing returns while foraging in food trays. Due to a higher encounter rate, S. lateralis harvested seeds significantly faster than T. minimus. S. lateralis also ran faster than T. minimus (3.14 m/s and 2.13 m/s, respectively). Gregarious animals may detect predators before solitary ones. I suggest that due to differences between sensory modes, this advantage occrues only to animals using vision. Thus, I predicted that gregariousness as a predator detection strategy should evolve only in diurnal animals. To test this, I mapped diurnal activity and gregarious behavior onto a phylogeny of eutherian mammals, and than calculated their degree of association. I found strong support for my prediction. This pattern may occur in other mobile animals. The data also suggest an association between larger body size and open habitat use among diurnal, gregarious organisms, paralleling a result from Colorado ground squirrels.