Biology Reference
In-Depth Information
3. ROLE OF THE NEUROENDOCRINE SYSTEM IN
MEDIATING ENVIRONMENTAL INFLUENCES ON THE
TIMING OF METAMORPHOSIS
The duration of the larval period varies considerably among and
within amphibian species. The earliest time for the onset of metamorphosis
is established by a genetically determined, species-specific minimum size for
transformation. The time that it takes to reach the minimum size is deter-
mined in part by growth opportunity in the larval habitat (
Werner, 1986;
Wilbur & Collins, 1973
). The better the resource supply, the earlier that
a tadpole can reach its species-specific minimum size for metamorphosis.
Variation in the proximate environment establishes trade-offs between
growth opportunity and risk of mortality (environmental stress, predation
risk, etc.) which ultimately determines the duration that the animal spends
as a tadpole. Species that breed in permanent, predictable habitats can have
relatively long larval periods (i.e., 3 years or greater); whereas those that
breed in unpredictable, ephemeral ponds have short larval periods (as short
as 10 days from hatching) (
Denver, Boorse, & Glennemeier, 2002
). The
proximate mechanisms that govern the timing of metamorphosis involve
the production, metabolism, and actions of hormones. Competence to re-
spond to environmental signals depends on the development and activity of
endocrine glands that produce the hormones that control metamorphosis.
Points of regulation by the environment include the neuroendocrine sys-
tem, peripheral endocrine organs, hormone transport and metabolism, and
hormone action. Thermal, osmotic, and effects related to the gaseous envi-
ronment may be sensed directly by most or all tissues. Signals generated by
other factors, such as photoperiod, resource availability, predator presence,
and crowding are integrated by higher brain centers and transduced by the
neuroendocrine system into changes in peripheral endocrine gland activity.
The activity of the tadpole hypothalamo-pituitary-thyroid axis can be reg-
ulated at multiple levels, and thyroid activity determines when larvae enter
metamorphosis, and the rate at which metamorphosis progresses. Because
the stress hormonal axis is closely linked to the thyroid axis, central nervous
stress pathways play a critical role in transducing environmental information
and regulating metamorphic timing.
Work of
Etkin (1968
) suggested that the “clock” that determines the
timing of metamorphosis is located in the hypothalamus. He showed that
tadpoles in which the pituitary primordium was autotransplanted to the tail
