Body temperature and behaviour of golden hamsters (Mesocricetus auratus) and ground squirrels (Spermophilus tridecemlineatus) in a thermal gradient.

Golden hamsters and thirteen-lined ground squirrels were maintained individually in a thermal gradient (14 degrees C to 33 degrees C) for several weeks under a 14L:10D light-dark cycle. Animals of both species showed robust daily rhythms of body temperature and locomotor activity with acrophases consistent with the habits of the species (diurnal acrophases in the diurnal squirrels and nocturnal acrophases in the nocturnal hamsters). Hamsters showed a robust daily rhythm of temperature selection 180 degrees out of phase with the rhythms of body temperature and locomotor activity. Squirrels did not show a daily rhythm of temperature selection. These results raise the hypothesis that a daily rhythm of temperature selection is exhibited by nocturnal but not by diurnal endotherms.