Stress and Adaptive Responses in Anuran Larvae
Tyrone B. Hayes
Laboratory for Integrative Studies in Amphibian Biology
Museum of Vertebrate Zoology
Group in Endocrinology, and
Dept. of Integrative Biology
University of California, Berkeley, CA
Although considered a stress hormone, responses to corticosterone can be adaptive. In anuran larvae, several environmental factors can cause increases in corticosterone that result in physiological/developmental changes: Dessication, temperature, increased larval density, and exposure to predators are a few environmental stressors that may cause changes in corticosterone levels. In several species, increasing corticosterone in late larval stages can accelerate metamorphosis. Exposure to exogenous corticosterone during early larval development, however, inhibits growth and may delay metamorphosis in the Western toad Bufo boreas. Recently, we have examined the role of endogenous corticosterone in this process by exposing crowded tadpoles to the corticoid synthesis inhibitor (metyrapone). This compound blocks endogenous corticoid production and results in an increase in growth and development of larvae relative to similarly crowded, untreated animals. This early effect of corticosterone may still be adaptative over all: Bufo are known to school and several species have been shown to recognize kin and may preferentially school with kin. It is possible that smaller sibs that show inhibited growth and development decrease competition with larger sibs, who can then metamorphosis allowing the smaller sibs to recover and metamorphosis, thereby increasing overall inclusive fitness. We tested this idea, in part, by rearing crowded groups of pure sibs for comparisons with mixed sibs, but showed no differences in growth and development.
Finally, we examined the dual effect of corticosterone (inhibiting development in early stages and accelerating in later stages) across several species in our laboratory and through literature searches. The data suggest that corticoids are stimulatory in late development across most species studied, except Xenopus laevis in which corticosterone appears to only inhibit growth and development in vivo. We examined the effects of corticosterone (at multiple doses) alone or in combination with thyroid hormones throughout the larval period. We found that corticosterone inhibited development under all conditions. In vitro studies identified a synergistic effect of corticosterone on thyroid hormone induced tail resorption, however. Thus, corticosterone plays a different role in Xenopus laevis compared to other species and acts through mechanisms yet unidentified.