Temperature and water conditions mediate the effects of day length on the breeding cycle of a Sahelian rodent, Arvicanthis niloticus

Abstract

Laboratory studies of variations in testicular activity (testicular weight and plasma testosterone concentration) were carried out on two populations of Arvicanthis niloticus, a Sahelian rodent that displays the 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 (water-deficient diet, 20% atmospheric relative humidity) and at low temperatures (20-25°C) or high temperatures (30- 35°C). The results show that humid conditions or low temperatures stimulate testicular activity in Arvicanthis niloticus whereas dry conditions or high temperatures inhibit breeding. 1) Humid conditions coupled with low temperatures caused the most marked stimulation of testicular activity and maintained sexual activity at its highest level. 2) Humid conditions coupled with high temperatures, or dry conditions coupled with low temperatures, brought about mild sexual activity in animals that were sexually inactive and a regression of testicular weight and plasma testosterone in animals that were sexually active at the beginning of the experiment. In the latter, the results show that testicular activity was maintained and animals remained capable of breeding. 3) High temperatures and dry conditions inhibited short- day gonadal stimulation. On the other hand, in animals maintained under humid conditions or at low temperatures, gonadal stimulation occurred only under a short photoperiod. In animals captured in Burkina Faso, the gonado- facilitating effect of low temperature and humidity, observed in animals maintained on a short-day regimen (11L:13D), was considerably weaker in animals maintained 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 testicular activity in this species but that ambient temperature and water conditions are also involved in controlling the length of the breeding period.