Timing of torpor bouts during hibernation in European hamsters ( Cricetus cricetus L.)

Abstract

Body temperature (Tb) of seven European hamsters maintained at constant ambient temperature (Ta = 8 degrees C) and constant photoperiod (LD 8:16) was recorded throughout the hibernating season using intraperitoneal temperature-sensitive HF transmitters. The animals spent about 30% of the hibernation season in hypothermia and 70% in inter-bout normothermy. Three types of hypothermia, namely deep hibernation bouts (DHBs), short hibernation bouts (SHBs), and short and shallow hibernation bouts (SSHBs), were distinguished by differences in bout duration and minimal body temperature (Tm). A gradual development of SSHBs from the diel minimum of Tb during normothermy could be seen in individual hamsters, suggesting a stepwise decrease of the homeostatic setpoint of Tb regulation during the early hibernation season. Entry into hibernation followed a 24-h rhythm occurring at preferred times of the day in all three types of hypothermia. DHBs and SHBs were initiated approximately 4 h before SSHBs, indicating a general difference in the physiological initiation of SSHBs on the one hand and DHBs and SHBs on the other. Arousals from SHBs and SSHBs also followed a 24-h rhythm, whereas spontaneous arousals from DHBs were widely scattered across day and night. Statistical analyses of bout length and the interval between arousals revealed evidence for a free-running circadian rhythm underlying the timing of arousals. The results clearly demonstrate that entries into hypothermia are linked to the light/dark-cycle. However, the role of the circadian system in the timing of arousals from DHBs remains unclear.

Body temperature patterns before, during, and after semi-natural hibernation in the European ground squirrel

“…After hibernation the Turkish hamster shows a gradual re-appearance of circadian rhythmicity under both LD and dim light conditions (Pohl 1981). Waûmer (1998) found circadian arrhythmicity in body temperatures in the European hamster under natural conditions during arousal episodes of several days only after relatively long periods of deep torpor in which body temperature dropped below 10±15°C. The decreased amplitude of daily body temperature rhythm during post-hibernation euthermia in our data set may re¯ect a similar process of internal desynchronisation of the circadian system after deep hibernation.…”

“…Bats often hibernate in open caves,¯y around during arousal episodes, may feed and drink, and even occasionally leave their hibernacula during winter (Daan 1973a). The European hamster (Waûmer 1998) and the pygmy possum (KoÈ rtner and Geiser 1998; KoÈ rtner et al 1998) have also been reported to leave their hibernaculum for above-ground activity. A functional circadian system may well be useful to synchronise such exposures to outside conditions at a preferred phase of the day.…”

“…Several studies have reported circadian patterns in the timing of torpor bouts or arousal intervals under light-dark cycle (LD) or continuous darkness (DD) conditions or entrained patterns under LD conditions (Daan 1973b;Pohl 1987Pohl , 1996Twente and Twente 1987;Canguilhem et al 1994;Grahn et al 1994;Wollnik and Schmidt 1995;Waûmer and Wollnik 1997;. Such patterns are more easily detected and indeed better documented when torpor bouts are short (Daan 1973b;Waûmer and Wollnik 1997;Waûmer 1998). When torpor bouts are long (>5 days) it is often unclear whether the timing of the subsequent arousal is timed by a circadian system.…”

“…Such conditions are characterised by unnatural ambient temperature patterns and dierent thermal insulation properties of the nest site from those in natural hibernacula (Strijkstra 1999). Body temperature data collected during hibernation under natural conditions are scarce (Wang 1979;Barnes 1989;Barnes and Ritter 1993;Waûmer 1998), and do not focus on circadian rhythms of euthermic body temperature patterns before and after the hibernation season. Field data recorded with temperature-sensitive collar transmitters may re¯ect timing of arousal episodes, but are not accurate or collected frequently enough to detect possible circadian body temperature¯uctuations (Young 1990;Michener 1992).…”

Gradual reappearance of post-hibernation circadian rhythmicity correlates with numbers of vasopressin-containing neurons in the suprachiasmatic nuclei of European ground squirrels

“…The functioning of the circadian system during mammalian hibernation is still under debate, and may vary between species, with torpor bout duration, and with body temperature during torpor (KoÈ rtner and Geiser 2000; Hut et al 2001;Oklejewicz et al 2001). Persistence of circadian body temperature oscillations during torpor has been shown in goldenmantled ground squirrels (Grahn et al 1994) and bats (Menaker 1959), and a circadian eect on the timing of arousals has been claimed in several other mammals (Strumwasser 1959;Pohl 1961Pohl , 1967Pohl , 1987Pohl , 1996Daan 1973a;Twente and Twente 1987;Canguilhem et al 1994;Grahn et al 1994;Wollnik and Schmidt 1995;Waûmer and Wollnik 1997;KoÈ rtner et al 1998;KoÈ rtner and Geiser 2000). However, some of the evidence is not convincing and absence of circadian rhythmicity during hibernation has also been demonstrated (Waûmer and Wollnik 1997;Thomas 1992;Pohl 1987;Oklejewicz et al 2001).…”