Role of short photoperiod and cold exposure in regulating daily torpor in Djungarian hamsters

Summary

• 1. Male and female Djungarian hamsters (Phodopus sungorus) were gonadectomized or sham-operated after 12 weeks of exposure to short photoperiods (10L∶14D). Half of the animals were single housed and transferred to a cold environment (7 °C) at week 13 of short days and half were transferred to cold at week 21. The time courses of short photoperiod induced seasonal changes in body weight, pelage color stage, and daily torpor were monitored periodically until the experiment was terminated after 34 weeks of short days.

• 2. The total duration of short photoperiod exposure was of primary importance compared to the duration of cold exposure in regulating seasonal changes in the frequency of daily torpor, body weight and pelage color exhibited by male and female Djungarian hamsters (Figs. 1–3); that is, the change from long to short days was much more effective as a seasonal time cue than was the onset of cold exposure.

• 3. Gonadectomy did not prevent the occurrence of seasonal torpor in hamsters of either sex, indicating that these cycles are regulated by a time measuring mechanism (seasonal clock) that is largely independent of the gonadal cycle. However, castration did influence certain aspects of the body weight and torpor cycles exhibited by male hamsters.

• 4. Some castrated animals showed a delay in terminating the torpor season (Figs. 1–2) lending further support to the hypothesis that the spontaneous recrudescence of the testes which occurs toward the end of the torpor season may play a role in the termination of torpor in males. The seasonal body weight gain was diminished in castrate as compared to testis-intact hamsters (Fig. 3).

• 5. The frequency of daily torpor was lower in females as compared to males (Figs. 1–2) and was not significantly changed by ovariectomy. The latter result suggests that the annual gonadal cycle has less influence on the torpor season in females than in males.