Effects of natural deep hypothermia on cognitive abilities in mammals: hibernators and neonates

Mammalian hibernators, when in hibernation, often experience deep hypothermia, defined to be a core body temperature ≤ 10 °C, from which they endogenously recover when they arouse. Similarly, among the many species of small mammals that breed in cold seasons in cold geographical areas, nestlings are vulnerable to experiencing deep hypothermia, from which they can recover by the simple expedient of being rewarmed by their parents. Both hibernation and neonatal tolerance of hypothermia are generally viewed as adaptations for mammals living in cold climates. However, following recovery from these deep-hypothermia states, there is a potential risk of cognitive dysfunction that would be maladaptive. This paper reviews the research that has been conducted on post-recovery cognitive function. Hibernation studies have focused on six rodent and two bat species – and have employed tests of spatial learning, social recognition, retention of operant conditioning, olfactory memory, and open-field habituation. Eight experiments have found that hibernators perform cognitive functions as effectively after hibernation as before, whereas five experiments have revealed degraded cognitive performance after hibernation. The specific question addressed in studies of neonates has been whether individuals exposed to deep hypothermia in the neonatal period exhibit altered cognitive function as post-weanlings or adults. Only three species have been studied, in research focused on spatial learning, taste-aversion learning, and predator evasion. All results point to no effect of neonatal deep hypothermia on later cognitive function, except for evidence that hypothermia on the first day of life (but not later days) slows spatial learning in laboratory rats.