NUTRITIONAL ENERGETICS AND DIGESTIVE RESPONSES OF AN HERBIVOROUS RODENT (OCTODON-DEGUS) TO DIFFERENT LEVELS OF DIETARY FIBER

Complex polysaccharides such as cellulose, hemicellulose, and lignin ( fiber) are important structural constituents of plants that often are difficult for small herbivorous mammals to digest. These polysaccharid es may affect the efficiency with which food is digested and with whic h nutrients and energy are transformed and allocated. To determine how small herbivorous mammals cope with such high-fiber food, I used as a model the herbivorous, caviomorph octodontid rodent Octodon degus, th e degu, an inhabitant of the semiarid and Mediterranean environments o f northern and central Chile. When given a choice, degus minimized fib er intake, showing pronounced preferences for food containing low fibe r. Because low-fiber items are not available in the field during the d ry season, I postulated that observations of degus feeding on grass co ntaining a high percentage (nearly 60%) of fiber during summer are mor e likely the consequence of necessity than of choice. I suggested that during nutritional bottlenecks, degus operate according to the princi ples of foraging theory and principles governing digestion. Degus seem ed to compensate for the low digestibility of high-fiber food by incre asing the volume of digesta in the alimentary canal as a consequence o f changes in rates of food intake and, hence, increases in turnover ti me of digesta. The digestive responses allowed them to increase the am ount of energy obtained from fiber and to satisfy their maintenance en ergy costs during temporal exposures to different levels of food fiber .