Unusual Correlation between Rest–Activity and Body Temperature Rhythms in the Naked Mole Rat (Heterocephalus glaber) as Compared to Five Other Mammalian Species

Body temperature and locomotor activity were recorded over a month using intraperitoneal sensors in the following six mammalian species: naked mole rats ( Heterocephalus glaber , 2♂, 2♀), C57Bl/6j mice (6♂), outbred white rats (4♂), Mongolian hamsters ( Allocricetulus curtatus , 3♂), sables ( Martes zibellina , 2♂, 2♀), forest ferrets ( Mustela putorius , 2♂, 2♀). During experiments, the naked mole rats stayed in their colony under special laboratory conditions: 2 weeks in constant darkness (dim red light), 2 weeks under an alternating light–dark regime (12L/12D). Mice and rats were kept in individual chambers under standard laboratory conditions and a 12L/12D regime; sables and ferrets were kept in their outdoor enclosures under natural light conditions in autumn. All the rodents used in the experiments, except for mole rats, are nocturnal animals. Accordingly, all of them showed a clear circadian rest–activity rhythm: a high level of locomotor activity in the daytime and low at night. The body temperature rhythm showed a synphase correlation with the rest–activity rhythm: temperature rose at night and fell in the daytime (0.9 < r < 1). At the same time, in hamsters, an increase in locomotor activity at night, when the ambient temperature went down, was accompanied by a corresponding increase in the amplitude of body temperature elevations. Small predators, sables and ferrets, demonstrated a more smoothed circadian rhythm. In contrast to all these species, body temperature of mole rats dropped sharply by 3–5°С (down to 28°С) upon activation of their locomotor activity (usually in the morning and evening hours) and jumped (up to 36°С) at rest (in the night hours) both under constant darkness and 12L/12D regime (–0.7 < r < –0.9). Overall, these animals showed an original thermoregulatory system with a possible “set point” at about 33.5°C.