Searching for the physiological mechanism of density dependence: does corticosterone regulate tadpole responses to density?

Density-dependent population regulation is important in many natural populations, and in those populations, high population density is a likely stressor. In amphibians, previous laboratory studies with tadpoles suggest that corticosterone, the main glucocorticoid stress hormone, is one of the key regulators of density-dependent growth. To test this relationship in natural settings, we manipulated wood frog (Rana sylvatica) tadpole density at three levels in outdoor mesocosms and used a capture stress protocol to examine the hormonal stress response. In addition, we used the same capture protocol in six natural ponds (three high density and three low density). In the mesocosms, there was an increase in corticosterone levels in tadpoles following 1 h of confinement at weeks 1, 2, and 5. However, while tadpoles maintained at higher densities were smaller after metamorphosis, density did not alter mean levels of corticosterone obtained during confinement, and baseline levels of corticosterone did not differ between the densities. Similarly, in natural ponds, density did not correlate with baseline corticosterone or mean corticosterone levels obtained during confinement. We suggest that the physiological response to density may vary across the range of natural densities and that the role of corticosterone may be limited to periods of extreme high density, such as during pond-drying events.