Date of Award
Master of Science
Human activities and unpredictable changes to environmental habitats impose a suite of stress and challenges to animal homeostatic function. Stress responses are often controlled by the release of glucocorticoids to mobilize energy, primarily corticosterone in small mammals, to help regulate homeostatic function such as heterothermy and changes to body condition, the latter of which serves as a proxy for energy reserves. Adaptive heterothermy is influenced by daily and seasonal patterns, heat produced from daily activity, and has been shown to increase in small mammals during times of environmental stress to conserve energy budgets. Body condition also changes in response to environmental perturbations, mobilization of energy by corticosterone, seasonal changes, and activity. My study aimed to disentangle the effects of environment and activity on homeostatic responses by pharmacologically manipulating corticosterone in kangaroo rats. Kangaroo rats are ecosystem engineers, heterothermic, and their activity periods are functions of their thermoregulatory patterns as well as environmental conditions thus making the species a great candidate for this form of study. I conducted two in-situ field experiments to assess for the effects of stress responses on thermoregulation, activity, and body condition. In my first experiment, I used body temperature (Tb) as a proxy for activity time and examined how pharmacologically increased corticosterone influenced kangaroo rat heterothermy responses to the moon phases and ambient temperatures. I also examined their fat, lean mass, and water content at the end of the study. Moon phase was a significant predictor of activity period as animals typically waited longer during the waxing moon phase to become active and cooled down below activity earlier in the night during the waning moon phase. As nights shortened, activity decreased despite environmental conditions becoming warmer. Corticosterone also significantly decreased total activity time and thus steadily increased heterothermy across the length of my experiment. These results indicate activity, not environment, are stronger drivers of heterothermy patterns. Total fat content (energy content) at the end of the study was not affected by corticosterone. The lack of change in fat content was presumably because kangaroo rat body condition was measured once at the end of the study and likely not a reflection of changed body condition over time. In my second experiment, I examined body condition across a longer period by measuring lean mass, fat content, and total body water across a 2-month period in the summer by pharmacologically increasing corticosterone to test the relationship between stress and body condition and to dissociate the two from environmental factors. Body condition indices generally increased across the summer, but corticosterone implantation did not significantly affect body any of the indices. The loss of heterothermic control and decrease in activity time across the summer suggests that animals are likely conserving energy budgets leading to preservation of condition. Further, banner-tailed kangaroo rats generally breed in the spring where body condition falls due to stress induced by increased competition and then increases across the summer as late summer monsoons promote the growth of primary resources utilized by animals thereby preserving body condition. This study suggests that body condition is driven more by life-history traits, activity time, and environment rather than stress responses.
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