Amphibians exhibit extreme plasticity in the duration of the larval pe
riod, Probably the most important environmental variable for a tadpole
is water availability, and some amphibian species have been shown to
be capable of accelerating metamorphosis in a desiccating larval habit
at, Current data support the view that the response to pond drying is
not simply a consequence of unavoidable environmental effects but rath
er an adaptive response to the risk of mortality in the drying pond, T
he acceleration of development by pond drying results in the precociou
s activation of the endocrine systems that control metamorphosis, the
thyroid and the interrenal, Furthermore, a single neurohormonal stimul
us, corticotropin-releasing hormone (CRH) activates both of these endo
crine systems, CRH is the primary vertebrate stress neurohormone; thus
, tadpoles mount a classical endocrinological stress response to accel
erate development and avoid mortality in the desiccating habitat, Comp
arison of recent data in mammals with amphibians suggests that the neu
rohormonal stress response is a phylogenetically ancient developmental
trigger that allows the developing organism to assess habitat quality
and respond accordingly.