MENDON�A, M T*; WARD, C K; Auburn Univ: Latitudinal variation in stress responsiveness in anurans: preliminary evidence

Tropical vertebrates generally exhibit a different suite of life history traits from their temperate counterparts. Additionally, there are physiological traits that also exhibit a latitudinal cline. For example, tropical birds and mammals have long been documented as having lower basal metabolic rates than temperate counterparts. Basal testosterone (T) and corticosterone (B) levels have also been shown to be lower in tropical vs. temperate birds. It is hypothesized that, due to the high parasite load in the tropics, tropical species would maintain low levels of hormones that might act in an immunosuppressive manner (e.g. B and T). In a survey of existing metabolic literature on anurans, we found that tropical frogs and toads (38 species, 8 families) also exhibit significantly lower standard metabolic rates than temperate species (47 species, 7 families). To test whether tropical anurans also exhibited low baseline and maximal levels of B, we collected blood immediately upon capture from a variety of species from temperate (US) and tropical (Costa Rica) areas. We then challenged these animals with saline or 100 ng/g BWT of ACTH (adrenocorticotropin hormone), which should stimulate a maximum B response, and collected blood 4 hours after injection. We found that the tropical anurans sampled (n=5 species, 4 families) displayed significantly lower baseline B levels than their temperature counterparts (5 species, 3 families). None of the tropical species sampled exhibited a significant increase in B in response to the ACTH challenge (n=4 species, 3 families), while the same challenge significantly increased B in all the temperate species. Although these data are preliminary, they support the hypothesis that tropical animals would display depressed levels of a potentially immunosuppressive steroid hormone.