Climate, trophic factors, and breeding patterns of the Nile grass rat (Arvicanthis niloticus solatus): a 5-year study in the Sahelian region of Burkina Faso (formerly Upper Volta)

Abstract

1994. Climate, trophic factors, and breeding patterns of the Nile grass rat (Arvicanthis niloticus solatus): a 5-year study in the Sahelian region of Burkina Faso (formerly Upper Volta). Can. J. Zool. 72: 20 1 -2 14. From 1984 to 1989, five populations of the Nile grass rat, Arvicanthis niloticus solatus, living in five different habitats of the Sahelian region were studied in northern Burkina Faso (Oursi, 14"N). The following were investigated: (i) seasonal variations in the percentages of sexually active males and females and the percentage of the population less than 6 weeks old, and (ii) seasonal variations in testis and seminal vesicle masses and plasma testosterone levels in adults. Both the large form ofA. niloticus occupying habitats with stable trophic resources and the small form of the species occupying habitats with fluctuating resources showed seasonal breeding, mainly in the dry season, in contrast with what we have observed elsewhere in other species in this region. In dry years with normal rainfall (rains between June and September), breeding occurred from mid-October to mid-July in habitats with constant trophic resources and from mid-October to mid-April in habitats with fluctuating resources. In 1986, a year with exceptional rainfall (additional rains in January), breeding was continuous in habitats where this climatic disturbance caused a reappearance of primary production and there was a population outbreak of A. n. solatus. Relationships between (i) breeding cycles, (ii) long-day periods, which have a gonadoinhibitory effect, (iii) seasonal variations in relative humidity and temperature, and (iv) dietary characteristics of A. n. solatus suggest that this subspecies is capable of breeding throughout the year. This is due to its adaptable dietary habits, which allow it to take advantage of resources that are either constant (insects, harvest wastes, and the water-rich bark of certain woody plants) or seasonal (rain-dependent herbaceous plants and farm vegetables). The end of breeding appears to be related to the relative timing of the stimulatory effect of certain foods and the inhibitory effects of photoperiod, temperature, and humidity. Under this hypothesis, the gonadoinhibitory effect of long days was masked in 1986 by an earlier stimulatory effect related to the reappearance of vegetation following the exceptionally heavy rainfall in January. SICARD, B., MAUREL, D., FUMINIER, F., et BOISSIN, J. 1994. Climate, trophic factors, and breeding patterns of the Nile grass rat (Arvicanthis niloticus solatus): a 5-year study in the Sahelian region of Burkina Faso (formerly Upper Volta). Can. J. Zool. 72 : 201 -214. De 1984 i 1989, cinq populations d'Arvicanthis niloticus solatus de cinq milieux differents du Sahel ont kt6 etudikes dans le nord du Burkina Faso (Oursi, 14"N). Les aspects suivants ont et6 examines plus particulikrement : (i) les variations saisonnikres du pourcentage de miiles et de femelles actifs sexuellement et du pourcentage d'individus de moins de 6 semaines dans la population ...

Temperature and Water Conditions Mediate the Effects of Day Length on the Breeding Cycle of a Sahelian Rodent, Arvicanthis Niloticus

“…In both cases, after 50 days of exposure to these conditions, a slight stimulation of sexual activity was observed in initially sexually inactive animals; and although the treatment resulted in decreased TW (900-1400 mg/100 g BW) and T in animals that were sexually active at the beginning of the experiment, the values nevertheless provide evidence that testicular activity was maintained and allowed the animals to continue breeding. Field observations [7] showed that the presence in a given environment of A. niloticus males with a TW of 900 mg/ 100 g BW is a good indication that the population is breeding, since pregnant females and juveniles were captured at the same time. Nevertheless it should be stressed that the gonadal stimulation observed in animals maintained in either humid conditions or low temperatures occurred only under the short photoperiod.…”

“…On the other hand, testicular regression occurs when day length reaches more than 11 h 30 min in Burkina Faso and more than 12 h 15 min in Mali. In Burkina Faso, testicular activity remains intense throughout the cold dry season (beginning of December to the end of January) and continues at the same level during the first part of the hot dry season, due to the advantageous feeding habits of this species (consumption of insects and of the bark of Acacia seyal, which has a high water content) [7,16]. Testicular activity subsequently regresses due to the combined effects of the long photoperiod, high temperatures, and drought.…”

“…Male sexual activity was assessed by testis size and position (scrotal or abdominal). The testes of sexually active males are in the scrotal position, and their development can be verified by palpation [7]. Trapping campaigns were carried out in August during the hot humid season (period 1, sexually inactive phase) and in December during the cold dry season (period 2, sexually active phase) [7].…”

“…This subdivision was carried out for the following reason. An outbreak of A. niloticus that occurred in Burkina Faso in 1986 [7] resulted from a lengthening of the breeding period, i.e., the population continued to breed throughout the year during both short-day and long-day periods. Thus in an attempt to explain this reproductive phenomenon, we decided to maintain the animals from Burkina Faso (and only these animals) in two different photoperiods that occur naturally at the latitude where the animals were captured, i.e., a short photoperiod of 11L:13D and a long photoperiod of 12.5L:11.5D.…”

“…This phenomenon was observed as soon as light coincided with the photosensitive phase [4]. The fact that the photosensitive phase starts 12 h 15 min after dawn in the Malian population and 11 h 30 min after dawn in the Burkina Faso population allowed us to explain the earlier regression in the Malian population, which lives in a habitat with seasonally fluctuating food supplies [6,7]. A study of seasonal variations in the immunoreactivity of LHRH fibers projecting into the external median eminence also showed that in Burkina Faso habitats under human influence (villages, nomad camps, intensively cultivated areas in wet locations), constant food supplies were capable of inducing-as in the case of Malian populations-more lasting stimulation of the LHRH hypothalamic system [8].…”