Energy requirements of the fat sand rat (Psammomys obesus) when consuming the saltbush, Atriplex halimus: a review.

The fat sand rat (Psammomys obesus; Gerbillinae), a diurnal gerbillid rodent, is herbivorous and able to thrive while consuming only the saltbush Atriplex halimus (Chenopodiaceae), a plant relatively low in energy content and high in ash and water. Before consuming A. halimus leaves, fat sand rats scraped off the outer layer with their teeth. This removed much of the electrolytes but increased gross energy and organic matter content of the leaves by only about 3.1%. Basal metabolic rate of fat sand rats was 168 kJ.kg-0.75.d-1, 57 to 60% of that expected for a eutherian mammal of its body size, and average daily metabolic rate was 499 kJ.kg-0.75.d-1, 88% of that expected for a rodent of its body mass. Field metabolic rate was 565 kJ.kg-0.75.d-1 in summer and 680 kJ.kg-0.75.d-1 in winter. The rate of energy expenditure during summer was 60% of that expected for a herbivore and 83% of that expected for a desert eutherian mammal of its body mass. The difference in rate of energy expenditure between winter and summer was taken as thermoregulatory costs. Fat sand rats appear to be well adapted to high air temperatures but less so to low air temperatures. The efficiency of utilization of energy of A. halimus for maintenance was only 0.32 and for growth only 0.30 and 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 fat sand rats to consume large quantities of forage for maintenance. In spite of this, there are several advantages for consuming mainly this diet, namely: (1) it provides a more stable diet throughout the year than do seeds; (2) fat sand rats have no competition for this food resource from other rodents; and (3) fat sand rat burrows are at the base of the plants and therefore they expend minimal energy for foraging.