A test of energetic trade-offs between growth and immune function in watersnakes.

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.