Nocturnal and diurnal rhythms in the unstriped Nile rat, Arvicanthis niloticus

In a laboratory population of unstriped Nile grass rats, Arvicanthis niloti cus, individuals with two distinctly different patterns of wheel-running ex ist. One is diurnal and the other is relatively nocturnal. In the first exp eriment, the authors found that these patterns are strongly influenced by p arentage and by sex. Specifically, offspring of two nocturnal parents were significantly more likely to express a nocturnal pattern of wheel-running t han were offspring of diurnal parents, and more females than males were noc turnal. In the second experiment, the authors found that diurnal and noctur nal wheel-runners were indistinguishable with respect to the timing of post partum mating, which always occurred in the hours before lights-on. Here th ey also found that both juvenile and adult A. niloticus exhibited diurnal p atterns of general activity when housed without a wheel, even if they exhib ited nocturnal activity when housed with a wheel. In the third experiment, the authors discovered that adult female A. niloticus with nocturnal patter ns of wheel-running were also nocturnal with respect to general activity an d core body temperature when a running wheel was available, but they were d iurnal when the running wheel was removed. Finally, a field study revealed that all A. niloticus were almost exclusively diurnal in their natural habi tat. Together these results suggest that individuals of this species are fu ndamentally diurnal but that access to a running wheel shifts some individu als to a nocturnal pattern.