Corticosterone Level Changes throughout Larval Development in the Amphibians Rana sylvatica and Ambystoma jeffersonianum Reared under Laboratory, Mesocosm, or Free-living Conditions

Studies of a few model amphibians continue to advance our mechanistic understanding of the endocrine control of larval amphibian development and metamorphosis, but there are few studies examining steroid profiles across species during larval amphibian development. We used censored regression analysis to address our primary objective, which was to examine baseline corticosterone level changes and responses to a standardized stressor throughout larval development in two amphibian species: one anuran (Wood Frogs, Rana sylvatica) and one caudate (Jefferson Salamanders, Ambystoma jeffersonianum). In addition, we looked at two additional factors that could influence the study of corticosterone during larval development, namely the rearing location of the animals (free-living, mesocosm-held, or laboratory-held) and for A. jeffersonianum, the method of induction of the stress response (ACTH injection or a confinement-agitation [CA] protocol). As has been documented for other anurans, baseline corticosterone content of R. sylvatica increased close to metamorphic climax in all rearing locations, although the absolute level varied with rearing location. Baseline corticosterone content of A. jeffersonianum increased gradually over development, and the increase in corticosterone content following CA mirrored the increase in baseline levels, although the absolute magnitude of the increase with CA varied based on rearing location. In larvae of A. jeffersonianum, both the CA method and ACTH injection significantly increased corticosterone content, with 30 min eliciting the maximum hormonal response level. Our results suggest that rearing location can influence corticosterone levels and the response to a standardized CA protocol, and that care should be taken in extrapolating results from laboratory studies to free-living amphibian populations.