DIGESTION BY BARNACLE GEESE IN THE ANNUAL CYCLE - THE INTERPLAY BETWEEN RETENTION TIME AND FOOD QUALITY

Structural carbohydrates in plants are hard to digest by the animals that eat them, and they hamper digestion of the content of the plant cells. The efficiency of digestion by herbivores is, therefore, closely related to both the retention time of the food in the digestive tract and the proportion of cell walls in the food. This study examined food digestion by free-living barnacle geese Branta leucopsis in relation to food quality and retention time. Over the range of short winter days (8 h light) to continuous light in the arctic breeding area the geese increased the food retention time two- to four-fold. Low throughput rates in summer resulted in enhanced digestion of the food. The organic matter digestibility of graminoids, corrected for differences in protein content, was 37% in winter, and 56% in summer. Enhanced digestion allowed the geese to extend their food spectrum by exploiting mosses (bryophytes), which are, at least temporarily, the only plants available in the summer range. The disadvantage of prolonged food retention time is the concurrent decrease of the amount of food that can be processed per time unit. The digestion pattern in the successive periods of the year can be regarded as an adaptation to differences in energy needs, and to differences in the selective force acting on the geese to minimize feeding time.