Huddling facilitates expression of daily torpor in the large Japanese field mouse Apodemus speciosus

Friends with benefits: the role of huddling in mixed groups of torpid and normothermic animals

“…In most species that nest in groups during torpor, all individuals of one group undergo torpor at the same time. The few species that are known to rest in groups of normothermic and torpid individuals include sugar gliders, the feathertail glider (Acrobates pygmaeus), fat-tailed dunnarts (Sminthopsis crassicaudata) and the Japanese field mouse (Apodemus speciosus), all expressing mainly short bouts of torpor (Morton, 1978;Fleming, 1985;Eto et al, 2014). One reason for the generally rare occurrence of mixed groups might be that normothermic individuals or individuals that enter only short and shallow torpor bouts can disturb other individuals that undergo deep torpor.…”

“…Moreover, the body warmth of conspecifics can reduce the costs of periodic rewarming (Arnold, 1988). Daily torpor in groups also reduces energy expenditure (Fleming, 1980;Eto et al, 2014) and animals undergoing torpor in groups often show an increased torpor bout duration (TBD) compared with solitary individuals (Séguy and Perret, 2005;Jefimow et al, 2011). Therefore, it has been suggested that huddling animals undergoing daily torpor benefit from social thermoregulation by saving more energy for survival, and perhaps can maintain a better body condition for the reproductive season that often follows the torpor season than their solitary conspecifics (e.g.…”

Magnitude of food overabundance affects expression of daily torpor

“…The first group of 7 males and 7 females was supplied with 100 g corn diet per week. Given that individual mice consume an average of 6.2 g of food per day under artificial winter conditions [17], or ca. 42 g per week, the corn diets supplied at 100 g/week must have been ample.…”

“…The mice were considered torpid when their T b fell below 32°C, according to Eto et al [17]. According to this threshold, incidence of torpor (number of torpid mice / number of mice observed), torpor frequency (number of torpid days / number of observation days × 100), torpor bout duration (the time T b remained below 32°C), and minimum torpid T b were determined.…”

“…The mouse caches food as a winter energy resource [15,16], and exhibits spontaneous daily torpor during artificial winter conditions [17], making it an ideal food-hoarding daily heterotherm model for investigating the relationship between torpor expression and food overabundance. The present study examined whether individual mice living under artificial winter conditions such as constant short-day photoperiod and cold T a expressed daily torpor differently when supplied with different quantities of overabundant food, or were able to adjust torpor expression in response to temporal changes in food overabundance.…”