Influence of Photoperiod and Partial Contact on Estrus in the Desert Pocket Mouse, Perognathus penicillatus1

Abstract

The effects of photoperiod and proximity of a male on the incidence of estrus were studied in the desert pocket mouse (Perognathus penicillatus). Groups of 8–20 females were maintained for two years in one of three photoperiods: (1) seasonally increasing and decreasing daylength; (2) constant long (16 h) daylength; (3) constant short (10 h) daylength. Within each photoperiod one half of the animals were housed singly, the other half were subjected to male stimulation by being housed with a male in a screen-divided cage. Unpaired mice in the seasonal photoperiod showed the expected annual cycle of a high frequency of estrus in spring and summer and quiescence in fall and winter. Constant photoperiod, either long or short, caused a disruption of this periodic behavior. The mice exhibited discrete periods of higher and lower estrous frequency of several months’ duration, which did not coincide with natural seasons and differed in timing between individual mice. Olfactory and possibly other stimulation by the constant proximity of a male led in screen-paired mice to an increased level of sexual activity. This increase was much more pronounced in animals maintained under a constant long or constant short photoperiod than in those under normal seasonal illumination. These results are consistent with the hypothesis that exposure to abnormal, constant photoperiods will cause, in the pocket mouse, an escape from the entrainment of the circannual gonadal rhythm by annual seasons and that the proximity of a male exerts an enhanced stimulation on their sexual activity under these conditions. These animals may therefore provide a good model for the study of the interaction of the two modes of stimulation and their effects on the neuroendocrine regulation of estrous behavior.