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THE DIGESTIVE SYSTEM OF
VERTEBRATES: CD 
3. Energy and Nutrient Requirements:
Introduction:
The digestive system provides for the assimilation of energy and nutrients required for growth, maintenance, and reproduction. From a quantitative standpoint, the energy derived from carbohydrate, lipid, and protein metabolism is the most important. Energy requirements are determined by the rate of metabolism, which differs widely across taxons and varies with the degree of homeothermy (temperature regulation), the physiological state (maintenance, growth, reproduction), the environment, and the habitat of an animal. Fish, amphibians, and reptiles are ectotherms, whose activities are dependent on their ambient temperature. However, birds and mammals are endotherms, which maintain a relatively constant body temperature. Although endothermy increases their range of distribution, it requires a much greater and more consistent intake of nutrients.
Despite the wide range of energy consumption among different animal groups, their rate of metabolism can be expressed by the equation:
R = aMb
The R is the metabolic rate under basal or standard conditions, a is a coefficient that varies with species, and b is an exponent that expresses the rate of change of R with changes in the body mass (M). The basal metabolic rate (BMR) is the rate of metabolism of endothermic animals in a resting, post-absorptive state and thermoneutral environment, and in the absence of any physiological or psychological stress. Standard metabolic rate is the equivalent minimal rate of metabolism of ectothermic animals at a given body temperature.
Metabolic Rate and Body Temperature:
The average BMR of endotherms varies, with passerine birds > most nonpasserine birds > eutherians > marsupials > monotremes (Table 3.1), but there is a considerable degree of variation within each taxa. For example, the BMR of sloths, hyraxes, and ratites are much lower than that of the average eutherian mammal and the BMR of wombats is much lower than that of the average marsupial. Some of these differences in metabolic rate can be attributed to influence of body temperature on coefficient a. Most chemical and biological activities show a 2 to 3-fold increase with each 100C rise in body temperature, which is referred to as the Q10 effect. A Q10 of 2.5 accounts for differences between the average BMR of eutherian mammals and most nonpasserine birds. However, even when corrected to a common temperature of 380 C, the average BMR of passerine birds was substantially higher and the standard metabolic rate of an ectothermic lizard was considerably lower than that of eutherian mammals (Table 3.1). At a corrected temperature of 200C the metabolic rates of fish ranged over an order of magnitude (Withers 1992).
Table 3.1. Basal or Standard Metabolic Rates at Normal Body Temperatures and Recalculated to a Uniform Body Temperature of 38oC
Metabolic rate in kJ.kg –0.75. day-1. (Modified from Schmidt-Nielsen 1984).
The metabolic rate and body temperature play significant roles in the rates of food intake, digesta passage, digestion, and absorption of nutrients. When mammals and reptiles of equal weight were fed the same diet, the rate of food intake was ten times greater in the mammals, and digesta retention time was eight times greater in the reptiles (Karasov and Diamond 1985). However, a decrease in ambient temperature had little effect on the digestible energy of herbivorous lizards (Zimmerman and Tracy 1989; Baer 1996) and the digestive efficiency of herbivorous reptiles was similar to that of herbivorous mammals (Parra 1978, Hamilton and Coe 1982, Troyer 1984b). Therefore, the lower metabolic requirement and slower rate of digesta passage of the reptiles appeared to compensate for the lower rates of food intake, digestion, and absorption.
Metabolic Rate and Body Mass:
Metabolic requirements increase with body mass. Withers (1992) gives the following mean values for the relationship between BMR and SMR (kj/hr) and the body mass of eutherians, marsupials and lizards:
Eutherian mammals (380C) BMR = 63.0M76
Marsupial mammals (360C) BMR = 48.0M75
Lizards @ 380C SMR = 6.8M80
Lizards @ 200C SMR = 1.5M80
However, the rate of metabolism per unit body weight (mass-specific metabolic rate) decreases with an increase in body mass. (Fig. 3.1). Therefore, small animals such as the carnivorous shrew must process food much more rapidly than larger animals, which limits their gut capacity and digesta retention time. This sets limits on the minimum body mass of vertebrates and especially the herbivores, which must extract their nutrients from large quantities of plant material.
Figure 3.1. Relationship between mass-specific metabolic rate (ml O2/g.h) or metabolic intensity and log of body mass for eutherian mammals ranging from 6 g shrews to 1,300 kg elephants. Note the inverse relationship between mass-specific metabolic rate and body mass. (From Schmidt-Nielsen 1984).
Therefore, the total metabolic requirements of vertebrates is proportional to the body mass to the 0.7 to 0.8 power (Fig. 3.2). However, as shown in Figure 3.3, the gut capacity of herbivorous mammals was proportional to body mass to the 1.04 power. This increase in gut capacity is due principally to expansion of the gut fermentation chamber and appears to apply to herbivorous lizards, as well (Troyer 1984b). Therefore, the ratio between metabolic requirements and gut capacity decreases with an increase in the body mass of herbivores, and the ratio is lower for ectotherms than endotherms (Fig. 3.4). This accounts for the facts that the largest terrestrial bird and mammals are herbivores, and the largest reptiles were the herbivorous dinosaurs. The contribution of microbial fermentation to the body heat of herbivores also increases with body mass. Farlow (1987) suggested that this increase in the relative size of the gut fermentation mass may have elevated and stabilized the metabolic rate of the dinosaur megaherbivores.
Figure 3.2. Log-log relationship between metabolic requirements and body mass of unicellular organisms, poikilothermic vertebrates and homeothermic (endothermic) vertebrates. Note that slope for all three groups is close to 0.75. (From Hemmingsen 1960).
Figure 3.3. Log-log relationship between the body mass and gut capacity of mammalian herbivores (r = 0.99). The log of gut contents was equal to 0.1Mg 1.04. Regression equation for all herbivores is log y = 1.032 log x - 0.936 and, when calculated separately, there was no significant difference between the slopes for the ruminants and nonruminants. (From Demment and Van Soest 1985).
Figure 3.4. Log-log relationships between the minimal metabolic requirements/gut capacity and the body mass of herbivorous lizards and mammals. (Adapted from data of Withers 1992, and Demment and Van Soest 1985).
Maintenance Energy Requirements:
Maintenance energy requirements, which include the additional energy needed for drinking, digestion, absorption, and metabolism of housed domestic or captive animals, were approximately twice the BMR in kangaroos and sheep on similar diets (Hume 1999). The maintenance energy of some endotherms is reduced by torpor (hibernation or aestivation) during periods of diminished food availability or a reduction in its nutrient value.
The field metabolic rate (FMR) for free-ranging domesticated and wild species is also usually a higher multiple of BMR. Nagy (1987) compared the FMR of 36 mammals and 25 birds to those of reptiles. The energy expenditures of the endotherms was about 17 times that of a reptile of similar size, due mainly to the costs of temperature regulation. During the night, the body temperature and energy metabolism of lizards declined with a drop in ambient temperature. However, the metabolic heat production of the endotherms increased, and their resting metabolic rate was 200 times that of the ectotherms of similar weight. Therefore, ectotherms can survive during much longer periods of food and water depravation, but the lack of temperature regulation has limited the distribution of present-day amphibians and reptiles to within 450 and their greatest diversity to within 200 of the equator (Fastovsky and Weishampel 1996)
Water, Protein and Nitrogen Requirements:
The water, protein, and nitrogen requirements are also related to the metabolic rate. Water turnover is much lower in reptiles than mammals and birds (Nagy and Peterson 1988), lower in marsupials than eutherians (McNab 1978) and lower in captive versus free-living animals. However, protein and nitrogen requirements are less affected by the additional costs of free existence and thermoregulation (Hume 1982).
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