In principal, frequency-dependent selection is one of the most powerful selective forces linking demographic and evolutionary changes, but little is known how such frequency-dependent selection influences predator-prey dynamics in nature. In this study, we used the oak-squirrel system to test the hypothesis that tree squirrels as seed predators/dispersers utilize white oak acorns based on the relative frequency of germination phenotype (dormancy vs. nondormancy), subsequently influencing variation in dispersal patterns of white oaks. Following our new finding that Père David’s Rock squirrel (Sciurotamias davidianus) frequently removes the embryos of white oak acorns in Central China, we manipulated the relative frequency of dormant (delayed germinating) and non-dormant (immediately germinating) acorns available to free-ranging squirrels and subsequently determined the animals’ feeding and hoarding responses as well as the fate of individual acorns. We provide clear evidence that squirrels can cause positive frequency-dependent selection in the wild: dispersal success of each germination phenotype increased as it became more common, and germination phenotype frequency can account for as much as 40.5% and 21.4% of the observed dispersal success in white oaks tested here, respectively. This is the first study to demonstrate that behavioral preferences of seed predators/dispersers could cause frequency-dependent selection in nature, and such selection may help maintain phenotype variation in prey species and also promote the coexistence among prey species with contrasting or similar seed phenotypes.

Xiao Z-S, Gao X, Steele M. A., Zhang Z-B. 2009. Frequency-dependent selection by tree squirrels: adaptive escape of nondormant white oaks. Behavioral Ecology, doi:10.1093/beheco/arp169.