Coping with anthropogenic environmental change is among the greatest challenges faced
by wildlife, and endocrine flexibility is a potentially crucial coping mechanism.
Animals may adapt to anthropogenic environments by dampening their glucocorticoid
stress response, but empirical tests of this hypothesis have provided mixed evidence.
An alternative hypothesis is that a non-attenuated stress response and efficient negative
feedback are favored in anthropogenic habitats. To test this idea, we non-invasively
sampled corticosterone release rates of common toad (Bufo bufo) tadpoles in agricultural,
urban, and natural habitats, and quantified their stress response and negative feedback
by a standardized stress-and-recovery protocol. We repeated the same sampling with
tadpoles raised from eggs from the same ponds in a common-garden experiment to infer
if the differences observed between populations in different habitats were due to
individual phenotypic plasticity rather than microevolution or transgenerational effects.
We found that, compared to tadpoles in natural ponds, urban tadpoles had higher baseline
and stressed corticosterone release rates, and tadpoles in agricultural ponds had
similar corticosterone release rates but greater stress-induced change, indicating
stronger stress responses in both types of anthropogenic habitats. As predicted, tadpoles
in both agricultural and urban ponds showed more efficient negative feedback than
did tadpoles in natural ponds. Water pollution levels, as indicated by the concentrations
of carbamazepine and corticoid-disrupting compounds in pond water, contributed to
elevating the stress response regardless of land use. Infection by neither Batrachochytrium
dendrobatidis nor Ranavirus was detected in free-living tadpoles. No habitat-related
glucocorticoid differences persisted in the common-garden experiment. These results
suggest that toad tadpoles in anthropogenic habitats increased their glucocorticoid
flexibility via phenotypic plasticity. The coupling of stronger stress response and
stronger negative feedback in these habitats supports the importance of rapidly "turning
on and off" the stress response as a mechanism for coping with anthropogenic environmental
change. (C) 2020 The Authors. Published by Elsevier B.V.
