Ecological energetics of Palaearctic Tetraonidae in relation to chemical composition and digestibility of their winter diets

Abstract

Parameters of digestion and energetics of free-living grouse were plotted against body mass to determine whether a general relationship exists between quantitative food intake, digestibility, and energy demands of individuals of different sizes, and to clarify the relationship between chemical composition of the diet and individual energetic demands of different species. Regression analysis showed a positive correlation between each of the parameters (except dry mass of caecal faeces) and body mass, reflecting a relationship between differences in chemical composition of the browse in the diets and its digestibility. Modelling intermediary metabolism in three grouse species revealed that about 50% of metabolizable energy originated from carbohydrate oxidation; the rest was shared among fiber (9–35%), protein (9–14%), and crude fat (12–25%). The more nitrogen in the diet, the higher were estimates of total digestibility, as well as those of fiber and fat. A generalized model of nitrogen demands of individuals is proposed and predictions from the model are compared with particular determinations based upon actual data from different species. The digestibility of natural grouse diets, and consequently available energy, is essentially dependent upon the concentration of protein in the diet. This suggests that it is necessary for grouse to exceed the minimal physiological level of nitrogen intake, not only to create reserves, but also to function effectively in the presumed need to detoxify and digest those fractions of the diet that are difficult to absorb.