TEMPERATURE AND WATER CONDITIONS MEDIATE THE EFFECTS OF DAY LENGTH ONTHE BREEDING CYCLE OF A SAHELIAN RODENT, ARVICANTHIS-NILOTICUS

Laboratory studies of variations in testicular activity (testicular we ight and plasma testosterone concentration) were carried out on two po pulations of Arvicanthis niloticus, a Sahelian rodent that displays th e particularity of being able to breed in the dry season. The animals were captured during phases of sexual activity or inactivity and were maintained in the laboratory for 50 days under humid conditions (water -rich diet, 90% atmospheric relative humidity) or dry conditions (wate r-deficient diet, 20% atmospheric relative humidity) and at low temper atures (20-25-degrees-C) or high temperatures (30-35-degrees-C).The re sults show that humid conditions or low temperatures stimulate testicu lar activity in Arvicanthis niloticus whereas dry conditions or high t emperatures inhibit breeding. 1) Humid conditions coupled with low tem peratures caused the most marked stimulation of testicular activity an d maintained sexual activity at its highest level. 2) Humid conditions coupled with high temperatures, or dry conditions coupled with low te mperatures, brought about mild sexual activity in animals that were se xually inactive and a regression of testicular weight and plasma testo sterone in animals that were sexually active at the beginning of the e xperiment. In the latter, the results show that testicular activity wa s maintained and animals remained capable of breeding. 3) High tempera tures and dry conditions inhibited short-day gonadal stimulation. On t he other hand, in animals maintained under humid conditions or at low temperatures, gonadal stimulation occurred only under a short photoper iod. In animals captured in Burkina Faso, the gonado-facilitating effe ct of low temperature and humidity, observed in animals maintained on a short-day regimen (11L:13D), was considerably weaker in animals main tained on a long-day regimen (12.5L:11.5D). The results are discussed in relation to the annual cycle of testicular activity in Arvicanthis niloticus living in natural habitats. These findings show that annual variations in day length play an important role in controlling testicu lar activity in this species but that ambient temperature and water co nditions are also involved in controlling the length of the breeding p eriod.