Geographic physiological variation and northern range limits in the cottonmouth (Agkistrodon piscivorus leucostoma)

Studies of the factors that influence the distribution of organisms are central to ecological research. Previous work on limiting factors in the establishment of an organism's geographic range has traditionally focused on survivorship. In my approach, I consider sublethal range-limiting mechanisms. Cottonmouths are excellent model organisms for studying sublethal northern range-limiting factors. Within their range, cottonmouths are often common and locally abundant. Despite their apparent adaptability, cottonmouths do not range above 38°N latitude. The lack of a more northerly distribution is unusual for several reasons and I tested several possible explanations for the current southern distribution of cottonmouths.;In order to assess the possibility of reproduction as a sublethal range-limiting mechanism, I investigated the effects of a natural and experimental temperature/photoperiod regime on testosterone cycles of male cottonmouths. Testosterone concentrations were measured in three groups. The first was a field group. The second was a lab control group, which received approximated natural temperature and photoperiod conditions for Northwestern Arkansas. The third was an experimental lab group, which received approximated temperature and photoperiod conditions of a region above the cottonmouth's northern range limit. Both the field and lab control groups demonstrated a single testosterone peak in August. The experimental group also demonstrated a single peak, but the peak was phase shifted by one month to September. Through the coupling of energetic calculations with time and environmental temperature, I demonstrated how male cottonmouths may eliminate the breeding cycle and how reproductive failure may serve as a sublethal range-limiting mechanism.;To address the possibility of metabolism as a potential sublethal range-limiting mechanism, I investigated geographic variation in resting metabolic rates of cottonmouths along a latitudinal gradient in central (Louisiana) and peripheral (Missouri, Arkansas) populations. I detected a significant effects and interactions of body mass, temperature, time of day, and latitude of origin. Metabolic cold adaptation appears to operate in cottonmouths from northern populations. Energetic calculations show that while hibernation metabolism is more costly near the northern range limit, it is most likely not a northern range-limiting factor.