Reproduction and abundance of the fat sand rat (Psammomys obesus) in relation to weather conditions in Tunisia

The fat sand rat Psammomys obesus is the main reservoir host of cutaneous l eishmaniasis in North Africa and western Asia. In order to understand bette r the transmission of this disease to humans, a longitudinal study of a pop ulation of the rodent was carried out in Tunisia. Rodents were collected in 10 trapping sessions between May 1995 and January 1997, using mole traps a nd cage traps. The structure of the population was established from eye len s weights and the reproductive state was determined on dissection. Reproduc tion took place continuously from autumn 1995 to spring 1996 with sexual ac tivity rates from 60-65% in females and 23-100% in males. The mean litter s ize rose from 3.6 in September to 6.1 in March. Reproduction ceased complet ely in the summer of 1996, and did not start again till January 1997, follo wing an apparent failure in the autumn. Animals born early in the breeding season bred at an early age, but those born later delayed breeding till the following season. At most times the population clearly consisted of two co horts, one born in the most recent breeding season, and one born in the pre ceding one. In response to the reproductive cycle, population numbers showe d an increase in winter and a decline in summer. The peak observed in March 1996 was due to the sexual activity of multiparous females and young breed ing females. At this time there was a female bias in the sex ratio. In a wi nter dry period, favourable habitat for breeding was confined to an area in which several species of Chenopodiaceae were associated. Sexual activity i n females was clearly correlated with rainfall and rain days, with a 1 mont h delay (r=0.78; P=0.007). Male sexual activity was independent of weather conditions. This result is comparable with other rodents of Sahelian or tro pical conditions. The increase in fertility was unrelated to age and is dis cussed in the light of food availability as a proximate factor. The persist ence of the parasite causing cutaneous leishmaniasis in host populations is discussed.