WARD, C.K.*; WILLIAMS, M; MENDONCA, M.T.; Auburn University; Auburn University; Auburn University: Decreased stress in the tropics: A physiological explanation for the Jimmy Buffet effect?

The stress response is a highly conserved set of physiological and behavioral mechanisms that are initiated when there is a threat to homeostasis. Recently, studies have shown that vertebrates in different areas of the world respond to stress differently. Birds in Artic and Temperate zones exhibit different adrenal responses to stressful stimuli, both in timing and amplitude, than birds in the Tropics. It is possible that this trend extends to other groups of vertebrates. To test whether tropical amphibians also exhibit a more moderate reaction to stressors, we assessed the degree of inter-renal responsiveness on Bufo marinus in Costa Rica. Toads were captured by hand and bled immediately to determine baseline corticosterone levels. We then administered either 1,000 ng/10 g body weight adrenocroticotropic hormone (ACTH) or saline to subsets of toads (n=12/group) and bled them 10 hrs after injection. We compared these results with the stress responsiveness of the southern toad, Bufo terrestris, a temperate toad. In comparison to the temperature toads response, B. marinus exhibited significantly lower baseline plasma corticosterone (2.2 + 0.76 ng/ml vs 8.4 + 2.3ng/ml, p=0.01) levels and did not display the typical large increase in plasma corticosterone response to the ACTH challenge (22.6ng/ml + 6.5 vs. 45 ng/ml + 9, p=0.01). Along with stress responsiveness, we examined the standard metabolic rates of bufonids over a latitudinal gradient and found that toads in tropical areas exhibit significantly lower standard metabolic rates than toads in temperate zones (p=0.026). The combined stress responsiveness and metabolic data raise questions about how other life history traits may vary with latitude.