An individual's morphology is shaped by the environmental pressures it experiences, and the resulting morphological response is the culmination of both genetic factors and environmental (non‐genetic) conditions experienced early in its life (i.e. phenotypic plasticity). The role that phenotypic plasticity plays in shaping phenotypes is important, but evidence for its influence is often mixed. We exposed female neonate diamond‐backed watersnakes (Nerodia rhombifer) from populations experiencing different prey‐size regimes to different feeding treatments to test the influence of phenotypic plasticity in shaping trophic morphology. We found that snakes in a large‐prey treatment from a population frequently encountering large prey exhibited a higher growth rate in body size than individuals in a small‐prey treatment from the same population. This pattern was not observed in snakes from a population that regularly encounters small prey. We also found that regardless of treatment, snakes from the small‐prey population were smaller at birth than snakes from the large‐prey population and remained so throughout the study. These results suggest that the ability to plastically respond to environmental pressures may be population‐specific. These results also indicate a genetic predisposition towards larger body sizes in a population where large prey items are more common.

Energy budgets explain how organisms allocate energetic intake to accomplish essential processes. A likely life history trade-off occurs between growth and immune response in juvenile organisms, where growth is important to avoid predation or obtain larger prey and immune response is essential to survival in the presence of environmental pathogens. We examined the innate (wound healing) and adaptive (lymphoid tissue, thymus and spleen) components of immune response along with growth in two populations of the diamond-backed watersnake Nerodia rhombifer raised in a common environment. We found that neonate snakes born to females from populations characterized by different predator and prey environments did not differ in energetic intake, but snakes from the population containing large prey grew significantly faster than those from a population containing small prey. Thymus mass, when corrected for body mass, was larger in snakes from the small prey population than in snakes from the large prey population. Additionally, the snakes from the population containing small prey healed significantly faster than those from the population containing large prey. Thus, we detected a negative correlation between growth (over a 4-month period) and wound healing across populations that is suggestive of an energetic trade-off between growth and immune response. The differences observed in growth and immune response among these two populations appear to suggest different energy allocation strategies to maximize fitness in response to differing conditions experienced by snakes in the two populations.

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.

Mean prey size often varies across landscapes, resulting in predator populations having differing access to energetic resources. Shifts in resource quality are likely to cause differences in energy allocation of reproduction. Thus, additional energy intake may lead to (i) increased offspring size, (ii) increased numbers of offspring, (iii) increased relative proportion of energy allocated to reproduction, (iv) increased absolute amount of energy allocated to reproduction or (v) increased energy allocated to growth of some other nonreproductive function, or (vi) some combination of the above outcomes. To test this hypothesis, we evaluated the reproductive allocation patterns of four populations of Diamond-backed Watersnakes (Nerodia rhombifer (Hallowell, 1852)) that differ in their mean prey size. Snakes at large prey sites produced longer, heavier babies compared with snakes from small prey sites. Statistical interactions among sites confounded our ability to compare differences in litter size, litter mass, and relative clutch mass. We suggest that increased prey size results in populations shifting reproductive allocation to increase offspring size. Effects of prey size on litter size, litter mass, and relative clutch mass remain unclear.

No abstract