BODY-MASS AT BIRTH AND GROWTH-RATE OF FAT SAND RAT (PSAMMOMYS-OBESUS)PUPS - EFFECT OF LITTER SIZE AND WATER-CONTENT OF ATRIPLEX-HALIMUS CONSUMED BY PREGNANT AND LACTATING FEMALES

1. Litter size, food quality and water availability can influence grow th rate and survival of offspring of mammals. 2. Fat sand rats have li tters that range in size from one to eight pups. They are able to thri ve and reproduce and lactating females can raise young while consuming only the saltbush Atriplex halimus, a chenopod low in energy content but high in water and electrolyte content. Water content of A. halimus differs greatly among seasons, but energy content and concentration o f electrolytes of dry leaves do not. 3. We hypothesized that birth bod y mass and growth rate of fat sand rat pups are affected by litter siz e and water content of A. halimus leaves consumed by the pregnant and lactating female. We predicted that birth body mass and growth rate of pups will decrease with an increase in litter size and with a decreas e in dietary water content. 4. Our hypotheses were supported. The equa tion describing the effects of litter size (L) and dietary water conte nt (W) on birth body mass (mb-0) took the form: m(b-0)=9.11 L-0.150 W0 .427 (n=38, r2=0.45) We used a modified Gompertz equation to describe the effects of these variables on body mass (M(b)) at different ages ( T). The equation took the form: M(b)=189.7 Exp[-3.82 x Exp(-0.076 T W1 .60 L-0.233)] (n=87, r2=0.98)