Why do grouse have ceca? A test of the fiber digestion theory

Two predictions deduced from the theory that grouse ceca function primarily to provide a suitable environment for the fermentation of plant fiber were tested. Digestion of neutral detergent fiber in conifer needles by blue grouse (Dendragapus obscurus) varied from 7.5 to 17.3% across tree species, but, contrary to prediction, the energy potentially derivable from this fiber digestion was insufficient to make ceca energetically profitable. Variation in digestibility of fiber among birds and across species was caused by differential accessibility of fiber to the ceca as essentially all (average, 96.8%) fiber entering the ceca was digested. An allometric analysis of grouse gut lengths suggested, and a correlation analysis confirmed, that cecal lengths among species of grouse decrease in length as dietary fiber increases, also soluble and suspended particulate food components from insoluble. Ceca do act as fermentative and absorptive reservoirs for fiber, but also (and predominantly) for urine and cell soluble or suspended components previously undigested because of high rates of passage. I suggest that the adaptive significance of this strategy is to maximize the rate at which energy is obtained from poor quality foods.