DIETARY AND DIGESTIVE CONSTRAINTS ON BASAL ENERGY-METABOLISM IN A SMALL HERBIVOROUS RODENT

McNab (1986, 1988) has hypothesized that mammals using food with low energy content should exhibit basal metabolic rates (BMR) lower than those expected on the basis of their body mass (m(b)). That is, those species that exploit food with low energy content and/or high cost of digestion tend to have low, mass-independent metabolic rates. To date there is not an experimental test of this pattern. The aim of this work was to examine experimentally the effect of diet quality on BMR, digestive efficiency, and the relationship between digestion and energy expenditure in a small herbivorous mammal. We used as a model the herbivorous caviomorph burrowing rodent Octodon degus (m(b) nearly 200 g), an inhabitant of semi-arid and mediterranean communities of nor-them and central Chile. Individuals maintained during 27 wk with a diet high in dietary fiber showed significantly lower BMRs (28%) than those feeding on low fiber. Daily food intake and ingestion rates (energy and dry matter) of individuals under a high-fiber diet were significantly higher than animals maintained with a low-fiber diet. The same pattern was obtained for total feces production and rate of feces production. The total intake and rate of ingestion of proteins were not significantly different between treatments. However, a significantly higher amount of protein was excreted by the individuals exposed to a high-fiber diet. Apparent digestibility of dry matter, energy, and protein were consistently lower in individuals maintained with high fiber. However, nonsignificant differences were observed between gut contents in the two treatments (P > .58), but a significantly higher digesta turnover rate was observed in animals exposed to a high-fiber diet (P < .05). A significant correlation was found between digestibility and the basal metabolic rate of individuals (r(s) = 0.781, P < .01), suggesting that elevated digestibilities on high-quality diets allow increased basal rates of metabolism. We suggest that, although small mammals like degus may select sparsely distributed plants of high quality in their habitat, their capability to drop their metabolic demands may help them meet their nutritional and energy requirements when nutritional conditions in the environment deteriorate.