Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Biology (PhD)

Degree Level



Biological Sciences


Steven J. Beaupre

Committee Member

John D. Willson

Second Committee Member

Christian Tispmark

Third Committee Member

David Kreider


Allocation, Corticosterone, Mate search, Reproduction, Reptile, Testosterone


In this dissertation I examined the relationship between individual energetic status, hormone production, and life history trait expression in field-active Timber Rattlesnakes, Crotalus horridus. In chapter one I reviewed what is known regarding these relationships in snakes and defined major research goals. In chapter two I described the seasonal profile of testosterone (T) and corticosterone (CORT) in relation to the breeding season and to individual energetic status in males. Results showed that the seasonal pattern of T production in C. horridus was different than other pit viper species with similar mating patterns. Testosterone was elevated in the months leading up to the breeding season and levels returned to baseline during the months of peak breeding. Testosterone concentrations were positively related to individual energetic status, but only in the months leading to the breeding season when concentrations were elevated. Annual variation was also observed in both the magnitude of T production and the seasonal profile. Corticosterone concentrations were not related to any measured variable. In chapter three I examined the relationship between individual T and CORT concentrations at the onset of the breeding season, energetic status, and male time-energy allocation. Male C. horridus with greater reserves of stored energy at the onset of the breeding season had higher T concentrations and allocated more time and energy toward reproduction compared to snakes with lower stored energy reserves (estimated by body condition index). Both mate search area and time allocated to behaviors other than foraging were directly related to both energetic status and T concentrations. My results suggest that male C. horridus hedge investment of time and energy towards current reproduction against potential costs in terms of survivorship and future fecundity and that testosterone may play a role in mediating this tradeoff. Results are descriptive and future experiments should be conducted to establish causal links. Annual variation was observed in both T concentrations and male time-energy allocation. In a year with low T and no seasonal variation in T, snakes did not appear to allocate time and energy towards mate search and reproductive behavior. In chapter four I described the relationship between female body condition and reproductive allocation and behavior. Additionally, a combination of long-term behavioral monitoring and analysis of microsatellite DNA markers was used to describe the mating system of female C. horridus. Receptive/attractive females were in better body condition compared to the general population, but maternal snout vent length and not body condition was positively related to litter size and mass. Behavioral and molecular data showed that individual females engaged in mating behaviors with multiple males in a single breeding season and that some litters were sired by multiple males. Behavioral data also showed that females will associate with males even when the likelihood of producing a litter the following year is unlikely. Taken together, my results demonstrate that reproductive life history trait expression varies according to natural variation in energetic status in C. horridus, and suggest that the testosterone may be important in mediating this relationship in male snakes.