Plastic juvenile growth rates and offspring size: A response to anthropogenic shifts in prey size among populations

Abstract

Variation in prey characteristics among populations is frequently associated with similar variation in predator body sizes. Increasingly, human-mediated alterations in prey landscapes impose unique ecological pressures on predators that may lead to rapid shifts in predator body size. Here, we ask whether adult body size differences among populations are the product of genetic adaptation or phenotypic plasticity of juvenile growth in response to human-altered prey size differences. Using a common-garden design, we measured growth rates of neonate diamond-backed watersnakes (Nerodia rhombifer) from fish farms populations that vary substantially in prey size distributions. We also investigated the influence of initial offspring size differences on growth rate. We found that juvenile growth rates were faster for snakes from populations with access to larger average prey sizes. Our data suggest that these differences are the product of phenotypic plasticity, mediated through differences in initial size and prey consumption. Population-level differences in growth rate were not detected when initial size and prey mass consumed were included in the model. We propose that larger offspring sizes may favor increased growth rates, mediated through a larger energy processing capacity relative to smaller individuals. This experiment provides evidence supporting the growing body of literature that adaptive plasticity may be a significant driver of rapid phenotypic divergence among populations across a landscape. This mechanism may explain the stability and colonization of populations in the face of rapid, human-mediated, landscape changes.