PHOTOPERIODIC INFLUENCES ON GONADAL DEVELOPMENT AND MAINTENANCE IN THE COTTON RAT, SIGMODON-HISPIDUS

Development of the reproductive system was retarded in cotton rats (Sigmodon hispidus) exposed to short daylengths (10 h of light/day, 10L:14D) from birth-50 days of age. Compared with animals exposed to long daylengths (14L:10D), the seminal vesicles and testes of of short-day reared rats were reduced in weight. However, spermatogenesis and development of the testicular interstitial apparatus were not affected by photoperiod or blinding. Short day exposure and blinding from birth also delayed vaginal opening but were without apparent effect on ovulation or the number of corpora lutea formed. Long term exposure (up to 20 weeks) to short daylengths did not affect the gonads of adult male cotton rats; constant darkness and blinding also exerted few if any substantial effects on the male gonad. The prepubertal cotton rat measures and is sensitive to photoperiod duration and the sexually mature individual becomes relatively free from the dictates of photoperiod as regards reproductive morphology. Overall the reproductive system of this species does not appear to be subject to photoperiodic regulation. It is concluded that proximate factors other than light are important regulators of the seasonal reproductive cycle.