Spontaneous daily torpor and fasting-induced torpor in Djungarian hamsters are characterized by distinct patterns of metabolic rate

Abstract

The Djungarian hamster is a rodent species that expresses both spontaneous daily torpor (SDT) when acclimated to winter conditions as well as fasting-induced torpor (FIT) during summer. In an earlier report we argued that these two thermoregulatory phenomena differ in several parameters. In the present study, we further complete this comparison by showing that metabolic rate patterns differ between both SDT and FIT. SDT bouts were significantly longer and deeper compared to FIT bouts. Additionally, respiratory quotient measures support the view that SDT is entered from a state of energetic balance while FIT appears to be an emergency shutdown of energy demanding thermogenesis due to a shortage of energy sources. In a second experiment, we also confirm that brief periods of food restriction during the hamsters' torpor season increase the frequency of SDT, but do not affect its depth or duration. Although winter-acclimated animals could flexibly alter torpor frequency in order to stay in energetic balance, we also found evidence for torpor expression patterns that resembled FIT, rather than SDT. Consequently, if energetic challenges cannot be compensated with increased SDT expression any longer, the hamsters seem to be driven in a negative energy balance resulting in FIT as a last resort.

Basking hamsters reduce resting metabolism, body temperature and energy costs during rewarming from torpor

“…3). With regard to physiological variables during steady-state torpor, the minimum T b of about 22°C was similar to that measured under similar thermal conditions (Heldmaier and Steinlechner, 1981;Diedrich et al, 2015). However, the mean minimum TMR measured here was only 34% of that measured by Heldmaier and Steinlechner (1981), but similar to those reported more recently at ∼1/3 of BMR (Diedrich et al, 2015;Ruf and Geiser, 2015).…”

“…The resting T b of 34.7°C at T a 15°C was similar to that previously reported under similar thermal conditions (Ruf et al, 1993) and so was the decrease of RMR with T a (Heldmaier and Steinlechner, 1981), although our values were slightly lower. The torpor patterns of hamsters observed here were somewhat unusual because hamsters often expressed more than one torpor bout per day, but it is known that multiple bouts per day may be used by the species, especially when food is withheld (Steinlechner et al, 1986;Diedrich et al, 2015), as in our measurements. The advantage of multiple torpor bouts in our study was that we could compare fully endothermic with radiant heat-assisted rewarming in the same individual on the same day (Fig.…”

Hypothalamic control systems show differential gene expression during spontaneous daily torpor and fasting-induced torpor in the Djungarian hamster (Phodopus sungorus)

“…Therefore single FIT bouts are less effective in saving energy than SDT bouts. The lower energy savings achieved from FIT bouts can be compensated by multiple FIT bouts per day [19,20,44]. In our study, no significant difference in depth or duration between FIT-SD and SDT in winter adapted animals was apparent.…”

“…The food was provided daily at ZT14. During the first experimental week hamsters were fed 30% of their daily food intake followed by four weeks with 60% feeding to induce FIT [19]. The hamsters were weighed twice a week and received additional 10% food, if they exceeded a body weight loss of over 25%.…”

“…This form of torpor can occur not only during any season but also at any time of day. Although SDT and FIT in Djungarian hamsters both involve metabolic depression and decreased T b , they show some distinct physiological characteristics including differences in preparatory time period before the first torpor episode occurs, circadian control, fuel utilization as well as differences in torpor depth and duration [19,20]. The physiological variations between these two forms of torpor suggest that they might be regulated by distinct central control mechanisms.…”