Patterns of morphological variation following colonization of a novel prey environment

Colonization of new areas is accompanied by a variety of novel pressures, which can lead to rapid phenotypic change. We compared morphology of diamond-backed watersnakes (Nerodia rhombifer) among populations of recently colonized fish farms to examine responses to a potential selective pressure, prey size and evaluated intersexual differences in phenotypic responses. Our data suggest not only have these populations experienced morphological change but also that males and females might be responding differently to the shared selective pressure. We found that male snakes from two sites raising primarily small fish had smaller cranial elements than males from one of the sites raising large fish and did not differ from the other large-fish site. Similar to their male conspecifics, we found that females from both large-prey sites had longer quadrates than one of the small-prey sites. In addition, females from large-fish populations reached greater snout-vent lengths than females from small-fish sites. These findings are consistent with an adaptive response to prey size, and also a differential response between the sexes. Our study demonstrates the potential for rapid phenotypic response to a strong selective pressure following colonization.