Sexual differences in over-winter torpor patterns of Richardson's ground squirrels in natural hibernacula

Abstract

Over-winter torpor patterns of Richardson's ground squirrels hibernating in southern Alberta were monitored with temperature-sensitive radiocollars to determine if these patterns differed between males and females in a manner related to the greater costs of mating effort by males than females. The hibernation season (from immergence to emergence) was composed of three periods: post-immergence euthermy, heterothermy, and pre-emergence euthermy. The hibernation season was shorter for juveniles than adults both among males (< 150 versus 234 days) and females (185 versus 231 days), a reflection of the later immergence into hibernation by juveniles. However, regardless of the absolute duration of hibernation, heterothermy accounted for a smaller proportion of the hibernation season of males (93±5%) than females (98±1%) and, within the heterothermal period, males had shorter torpor bouts and longer inter-torpor arousals. Overall, males spent a smaller proportion of the hibernation season in torpor (85±6%) than females (92±1%). This sexual difference was largely attributable to the longer duration of preemergence euthermy for males than females. Males terminated torpor in January and February, when hibernacula were at their coldest, then remained euthermic for 8.8 days (range 0.5-25.0 days) before emergence. In contrast, females terminated torpor in March, when hibernaculum temperatures were increasing, then remained euthermic for only 1.1 days (range 0.5-2.0 days) before emergence. Males lost less mass per euthermic day during hibernation than females (7.0 versus 9.3 g/day). Males and females hibernated at similar depths (56 cm), but males had larger chambers than females (18 versus 16 cm/g). Many males, but no females, cached seeds in the hibernaculum. Males met the costs of thermogenesis and euthermy from a combination of fat reserves and food caches, whereas females relied solely on fat. Access to food caches permitted males to terminate torpor several weeks in advance of emergence, during which time they recouped mass and developed sperm in preparation for the forthcoming mating season.

Body temperature and above‐ground patterns during hibernation in European hamsters (Cricetus cricetus L.)

“…1). In accordance with Michener (1992), the final retreat into the burrow (immergence) was defined as the onset of hibernation. Consequently, the period of hibernation was defined as the time hamsters continuously stay below ground.…”

“…This first TAG is called emergence and was defined as the end of hibernation (again following Michener, 1992). However, some animals stay underground for some time.…”

“…Finally, both variables, changes in T b and TAG were used before to define the period of hibernation (cf. Cabanac & Simon, 1987;Michener, 1992). Comparing both patterns should provide justification for these definitions or advice on other parameters to be taken into account, leading to a more open definition of hibernation.…”

Torpor, arousal and activity of hibernating Greater Horseshoe Bats (Rhinolophus ferrumequinum)

“…It is therefore important to understand the patterns of torpor and arousal under natural conditions. Such information on arousal patterns is lacking for most hibernators (but see Wang 1978;Young 1990;Michener 1992), and has not previously been documented for free-living bats.…”

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

“…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). Laboratory studies indicate a gradual loss of internal synchronisation of the circadian oscillator at temperatures below 15°C (Pohl 1981) and persistence of circadian patterns may depend on the duration and the level of minimum brain temperatures reached during torpor.…”