A daily rhythm in mating behavior in a diurnal murid rodent Arvicanthis niloticus

The time of day at which mating occurs is dramatically different in diurnal compared to nocturnal rodents. We used a diurnal murid rodent, Arvicanthis niloticus, to determine if inverted rhythms in responsiveness to hormones contribute to this difference. Male and hormone-primed female grass rats were tested for mating behavior at four different times of day (ZT 5, 11, 17, 23; ZT 0 = lights-on). In females, there was considerable inter-individual variability with respect to patterns of responsiveness to hormones. Overall, the lordosis quotient (LQ) was rhythmic with a single peak just before lights-on (ZT 23); however, while roughly half of the females (7/15) exhibited this clear daily rhythm, the remaining animals (8/15) had relatively high LQs that did not change as a function of time. Males had their shortest ejaculation latencies and their highest number of ejaculations at ZT 23. Rhythms in mount frequency and post-ejaculatory refractory period were bimodal, with peaks around lights-on and -off (ZT 23 and 11). This temporal pattern of mounting behavior closely parallels previously documented patterns of general activity, whereas rhythms in the more reflexive components of sex behavior (LQ and ejaculation) had more restricted peaks that coincided with just the onset of rhythms in general activity. These rhythms in sexual behavior are essentially reversed relative to those previously documented in lab rats.