Energy requirements, length of digestive tract compartments and body mass in six gerbilline rodents of the Negev Desert

Energy requirements of an animal are size dependent and, in this study, the average daily metabolic rate (ADMR) of six Negev Desert gerbilline rodents, ranging in body mass (m(b)) from 10 g to over 200 g, scaled to m(b)(0.57). Although gerbilline rodents are considered 'granivores', these rodents consume substantial amounts of green vegetation and the largest gerbil is a strict herbivore. We predicted that the lengths of the compartments of the digestive tract would scale allometrically to mb(0.33) and that ADMR would scale allometrically to the lengths of the compartments to the exponent of 1.73. Using log-transformed data, the length of the colon scaled to m(b)(0.50) = 0.74; p = 0.02), of the caecum to m(b)(0.45) = 0.80; p = 0.01) and of the small intestine to m(b)(0.30) = 0.59; p < 0.05). Therefore, the exponents for the colon and caecum were higher than predicted and were close to the exponent for ADMR generated for the rodents. The absolute lengths of the colon (r(adj)(2) = 0.68; p = 0.03; slope = 0.99) and of the caecum (r(adj)(2) = 0.79; p = 0.01; slope = 1.19) were related significantly to ADMR, but of the small intestine was not (r(adj)(2) = 0.04; p = 0.33; slope = 0.85). The exponents implied that the relationships were isometric and not allometric as predicted and that the rates of increase of the lengths of the intestine compartments were at the same rate as the increase in ADMR. The lengths of the colon and caecum were highly correlated between each other (r(adj)(2 )= 0.98; p < 0.001; slope = 1.12) and explained most of the variation in ADMR. Green vegetation could be a nutritional bottleneck for rodents as it is bulky and, consequently, limits the dietary intake, and fermentation occurs in the caecum and colon, whereas seeds, which are compact and are digested in the small intestine, would limit intake to a much lesser degree. However, when the effect of body mass was eliminated by using residuals of the variables on body mass, only the length of the small intestine was significant (r(adj)(2) = 0.86; p < 0.005; slope = -1.33) and was related negatively to ADMR. Therefore, when effects of body size were removed, most of the variation in ADMR was explained by the length of the small intestine and implied that the length of the small intestine increased with a decrease in ADMR. A higher energy expenditure was related to a shorter small intestine and, therefore, by implication, a higher concentration of metabolizable energy yield of the diet. We also questioned whether there are differences in the morphology of the digestive tract due to differences in dietary consumption. The digestive tracts of the gerbils were not diverse and could be characterized as structurally homogenous. All the gerbils had a uni-locular, hemiglandular stomach and the differences in the digestive tract among species did not seem to be of functional importance, but rather were related to the taxon. However, some important morpho-functional characteristics of the digestive tract emerged that apply to the whole group.