Activity rhythms and masking response in the diurnal fat sand rat under laboratory conditions.

Daily rhythms are heavily influenced by light in two major ways. One is through photic entrainment of a circadian clock, and the other is through a more direct process, referred to as masking. Whereas entraining effects of photic stimuli are quite similar in nocturnal and diurnal species, masking is very different. Laboratory conditions differ greatly from what is experienced by individuals in their natural habitat, and several studies have shown that activity patterns can greatly differ between laboratory environment and natural condition. This is especially prevalent in diurnal rodents. We studied the daily rhythms and masking response in the fat sand rat (Psammomys obesus), a diurnal desert rodent, and activity rhythms of Tristram's jird (Meriones tristrami), a nocturnal member of the same subfamily (Gerbillinae). We found that most sand rats kept on a 12 h:12 h light-dark (LD) cycles at two light intensities (500 and 1000 lux) have a nocturnal phase preferences of general activity and higher body temperature during the dark phase. In most individuals, activity was not as stable that of the nocturnal Tritram's jirds, which showed a clear and stable nocturnal activity pattern under the same conditions. Sand rats responded to a 6-h phase advance and 6-h phase delay as expected, and, under constant conditions, all tested animals free ran. In contrast with the nocturnal phase preference, fat sand rats did not show a masking response to light pulses during the dark phase or to a dark pulse during the light phase. They did, however, have a significant preference to the light phase under a 3.5 h:3.5 h LD schedule. Currently, we could not identify the underlying mechanisms responsible for the temporal niche switch in this species. However, our results provide us with a valuable tool for further studies of the circadian system of diurnal species, and will hopefully lead us to understanding diurnality, its mechanisms, causes, and consequences.