Daily hypothermia and torpor in a tropical primate: synchronization by 24-h light-dark cycle

To study the temporal organization of daily hypothermia and torpor in a noc turnal Malagasy primate, the gray mouse lemur, body temperature (T-b) and l ocomotor activity were recorded using telemetry on 39 males held in 24-h li ght-dark cycles of different photoperiods. Under free-running condition, th e circadian T-b and locomotor activity rhythms had a period shorter than 24 h. Circadian daily hypothermia started by a rapid drop in T-b (0.24 degree sC/10 min) at the end of subjective night (13 h 25 +/- 20 min) and was char acterized by minimal T-b values 3 h 20 +/- 5 min later. Spontaneous arousal from daily hypothermia occurred at a fixed time (6 h 05 +/- 15 min, n = 7) after the beginning of subjective day. In animals exposed to 24-h light-da rk cycles with night duration varying from 10 to 14 h, locomotor activity w as strictly restricted to dark time, but the temporal organization of daily hypothermia was not modified, although changes in amplitude of T-b rhythm were observed. Daily hypothermia was directly induced by light and lasted 5 h 10 +/- 10 min, with minimal T-b values 3 h 30 +/- 30 min (n = 28) after lights on, on condition that nighttime did not exceed the duration of subje ctive night. However, in animals exposed to 24-h light-dark cycles with nig ht duration varying from 10 to 5 h, the limit of induction of daily hypothe rmia by light was similar to9 h after the beginning of night. Finally, unde r short days (14:10-h light-dark cycle), long bouts (6 h 50 +/- 40 min) of actual torpor (minimum T-b 27.6 +/- 0.9 degreesC) were observed and would i nvolve mechanisms depending on physiological changes induced by short day e xposure.