The neonates of many rodent species survive deep hypothermia (T b = 0-8 °C). In key respects, this hypothermia is more akin to hibernation than was thought during much of the twentieth century, indicating that studies of neonatal hypothermia may usefully supplement studies of hibernation in understanding evolved tissue adaptations to near-freezing T b. To clarify evolutionary diversity in neonatal survival of deep hypothermia, neonates of six species or strains were subjected to a standardized procedure: exposure for 2.5 h to test T bs followed by autoresuscitation. Mus and Peromyscus differed dramatically, the lowest T b survived by all ages studied (3-10 days) being 7-8 °C in Mus and 0-1 °C in Peromyscus. There was, however, no evidence of intrageneric plasticity because feral and laboratory Mus were identical, and Peromyscus species with cold- and warm-climate distribution ranges were identical. When neonates survive deep hypothermia, a key question is whether the experience is benign, meaning neonates tolerate hypothermia. To test the benign nature of deep hypothermia, neonates of Peromyscus leucopus were exposed four times (3 h each) to T b = 1-2 °C when 3-10 days old; controls were same-sex siblings not hypothermia exposed. When 74 such sibling pairs were exposed after weaning to predation by screech owls, the hypothermia-treated and control siblings did not differ in which was caught first. Based on study of deaths in 253 sibling pairs prior to weaning while under parental care, parents cared for hypothermia-treated siblings as attentively as controls. The results indicate that the experience of multiple neonatal deep-hypothermic episodes is benign in P. leucopus.
Keywords: Body temperature; Daily torpor; Diversity; Heterothermy; Owl predation; Predation avoidance; Tolerance.