Chapter 7 Summary

Summary

Fundamentals of Animal Energetics

  • Forms of energy vary in their capacity to do physiological work. Chemical-bond energy is totipotent for animals. Electrical and mechanical energy can do certain types of physiological work but are not totipotent. Heat cannot perform physiological work of any kind.
  • Animals use their absorbed chemical energy for three major functions: biosynthesis, maintenance, and generation of external work. Biosynthesis, which preserves some of the absorbed energy in the form of chemical energy, includes both growth and the synthesis of organic materials that are exported from the body during an individual’s life.
  • Some energy is degraded to heat (low-grade energy) whenever one high-grade form of energy is transformed to another. Energy transformations are always inefficient.
  • Animals take in chemical-bond energy and put out heat, chemical-bond energy, and external work.

Metabolic Rate: Meaning and Measurement

  • An animal’s metabolic rate is the rate at which it converts chemical energy into heat and external work.
  • Metabolic rate is important because it helps determine the amount of food an animal needs, and therefore the food energy that the animal removes from its ecosystem. An animal’s metabolic rate also provides a quantitative measure of the total activity of all its physiological mechanisms.
  • An animal’s rate of O2 consumption is the most common measure of metabolic rate. Metabolic rates can also be measured by direct calorimetry or studies of material balance.

Metabolic Scaling: The Relation between Metabolic Rate and Body Size

  • BMR, SMR, and other measures of resting metabolic rate are allometric functions of body weight within phylogenetically related groups of animals (M = aWb, where b is usually in the vicinity of 0.7). Small-bodied species tend to have higher weight-specific metabolic rates than do related large-bodied species, an effect so great that the weight-specific BMR is 20 times higher in mice than in elephants.
  • Maximum aerobic metabolic rate also tends to be an allometric function of body weight in sets of related species. In many cases studied thus far, the allometric exponent for maximum metabolic rate differs from that for resting metabolic rate.
  • The allometric relation between metabolic rate and weight exerts important effects on the organization and structure of both individual animals and ecosystems. Heart rates, breathing rates, mitochondrial densities, and dozens of other features of individual animals are allometric functions of body weight within sets of phylogenetically related species. In ecosystems, population biomasses and other features of community organization may vary allometrically with individual body size.
  • Physiologists are not agreed on the explanation for the allometric relations between metabolic rate and body weight. Rubner’s surface “law,” based on heat loss from homeothermic animals, does not provide a satisfactory explanation.
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