Daily torpor is a physiological adaptation in small mammals and birds, characterised by drastic reductions in metabolism and body temperature. Energy-constraining conditions, such as cold and starvation, are known to cause the expression of daily torpor. However, the reason for high degrees of inter- and intra-individual variation in torpor expression (TE) in similar situations is not clear. As littermates of altricial animals are exposed to an uneven allocation of maternal resources from conception to weaning, we tested whether early nutritional experiences have long-term effects on TE in adults. We used full-sibling littermates of laboratory mice that as adults were starved overnight to induce torpor. We measured body mass from birth until adulthood as an indicator of nutritional status, and calculated the relative body mass (RBM) as an indicator of the difference in nutritional status within a litter. After maturation, we subjected mice to five repeated torpor induction trials involving 24 h of fasting and 5 days of recovery. Half of the female mice displayed great individual variation in TE whereas male mice rarely exhibited daily torpor. In females, RBM at birth influenced TE, irrespective of body mass in adulthood; thus, female mice born with low RBMs displayed high TE in adulthood. In conclusion, we provide evidence that TE in mice differs among littermates, and that this variation is linked closely to heterogeneous nutritional experiences during the fetal period.
Daily torpor is a strategy used by some overwintering small endotherms to aid in energy conservation. However, the pattern of torpor varies among individuals within species and populations, even under the same environmental conditions, with significant implications for survival rate and reproductive success. Body mass is one factor that may influence this variation, especially in some small mammals that accumulate fat stores prior to overwintering. However, to our knowledge there has been no previous study examining the detailed relationships between torpor expression and body mass change in small mammals that hoard food as an energy resource during winter. The large Japanese field mouse, Apodemus speciosus, whose winter survival strategy depends on food caches instead of fat stores, displays daily torpor under artificial winter conditions (short-day photoperiod and cold). The present study clarifies the characteristics and patterns of daily torpor and body mass change in this species in the laboratory. Although expression of daily torpor was facilitated progressively as in other species, the observed patterns of torpor expression and body mass change showed considerable individual variation. Moreover, there was no obvious correlation between body mass and daily torpor expression. Therefore, it is suggested that in A. speciosus body mass may not contribute to individual variation of daily torpor during winter. Daily torpor during winter may be adjusted by not only mechanisms common to other small mammals, but also species-specific factors relating to the external or internal reserves of energy in small mammals.
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