Energy and nitrogen requirements of the fat sand rat (Psammomys Obesus) when consuming a single halophytic chenopod

The fat sand rat (Psammomys obesus; Gerbillinae), a diurnal gerbillid, is herbivorous and is able to thrive while consuming only one halophytic chenopod, in particular the saltbush Atriplex halimus (Chenopodiacae), a plant relatively low in energy yield and high in ash and oxalate contents. Before consuming A. halimus leaves, fat sand rats scrape off the outer layer with their teeth, which removes much of the electrolytes. To overcome the high oxalate content, fat sand rats harbor the bacterium Oxalobacter spp. in their intestines, which is capable of degrading oxalates. When they were fed only on A. halimus, the basal metabolic rate (BMR) of the fat sand rats was 168 kJ kg-0.75d-1, 57 to 60% of that expected for an eutherian mammal of its body mass, while the average daily metabolic rate (ADMR) was 499 kJ kg-0.75d-1, 88% of that expected for an eutherian mammal of its body mass. Field metabolic rate (FMR) ranged between 565 kJ kg-0.75d-1 in summer and 680 kJ kg-0.75d-1 in winter, and in summer was 83% of that expected for a desert eutherian mammal of its body mass. Atriplex halimus is high in crude protein and the fat sand rats were easily able to balance their N intake. Metabolic fecal nitrogen was 70.5 mg kg-0.75d-1 and endogenous urinary nitrogen was about 171.9 mg kg-0.75 d-1 and, therefore, minimal N requirements equalled 242.3 mg kg-0.75d-1, which is 98% of the amount predicted for a placental animal of its body mass. The effi ciency of utilization of energy of A. halimus for maintenance was only 0.32 and for growth only 0.30 and, therefore, their respective heat increments of feeding were 0.68 and 0.70. This low utilization of feed plus its low energy yield and high water content forced the fat sand rats to consume large quantities of fresh and dry matter for maintenance. In spite of these negative aspects, there are several advantages to consuming only A. halimus, namely: (1) it provides a more stable diet throughout the year than do seeds; (2) there is little competition for this food resource from other rodents; and (3) fat sand rat burrows are at the base of the plants and, therefore, minimal energy is expended for foraging.