Effects of day length, temperature, and endogenous control on annual rhythms of reproduction and hibernation in chipmunks (Eutamias ssp.)
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Male least chipmunks (Eutamias minimus) and yellow pine chipmunks (E. amoenus) were captured in the field and exposed to constant conditions of photoperiod (LD 8∶16, LD 12∶12, or LD 16∶8) and temperature (23° C or 5°C) for as long as 20 months. The seasonal courses of reproductive function and functions associated with hibernation were monitored by repeated measurement of testis size, body mass, water consumption, locomotor activity, and torpor.
In the absence of seasonally changing environmental cues the chipmunks showed reproductive and hibernatory cycles that resemble rhythms of reproduction and hibernation in nature, i.e., the animals exhibited ‘endogenous circannual rhythms’. Cycles were repeated spontaneously and had a period of less than a year.
Cycles of testis size and hibernatory functions persisted in photoperiods of LD 8∶16 and LD 12∶12, but were inhibited by LD 16∶8. Some of the animals exposed to LD 16∶8 in autumn showed brief, slight testicular development followed by regression, suggesting that there was an initial delay in the response of these animals to the negative influence of long photoperiod on testes.
Testicular growth was promoted by warm temperature (23 °C) and retarded by cold (5 °C). Occurrence of torpor was not a requisite for the progression of the seasonal reproductive rhythm. Torpor was promoted by cold and was absent at 23 °C, but other manifestations of hibernation (decreases in locomotor activity, body mass, and water consumption) still persisted at 23 °C. Torpor of animals at 5 °C was partially inhibited by LD 16∶8.
Endogenous annual rhythmicity should initiate preparation by chipmunks for their brief, once-a-year breeding season that follows hibernation. In addition to the peripheral responsiveness of seasonal functions to day length and temperature demonstrated by the present results, there must also be responses in chipmunks to environmental information by which the endogenous rhythmicity is synchronized with the annual environmental cycle.
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