Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere by Peter Ward

Author:Peter Ward [Ward, Peter]
Language: eng
Format: mobi, pdf
Tags: Biology and Life Sciences. Earth Sciences. Explore Science. Biology and Life Sciences : Animals, Plants and Other Organisms
Publisher: National Academies Press
Published: 2006-09-25T16:00:00+00:00

METABOLISM AND THE EVOLUTION OF ENDOTHERMY

Here let’s return to a subject raised in the last chapter, but one whose history is even more important in this chapter’s interval of time than last’s. Metabolism is the term used to characterize the acquisition and use of energy by organisms, and metabolic rate is the pace at which acquired energy is utilized. It dictates the amount of fuel consumed and the amount of heat generated. It takes work to conduct the activities of life, which include physical activity, food processing, and tissue synthesis. But even in the absence of these activities, energy is expended in just staying alive—a significant amount of energy, it turns out. The baseline activities of life include ion pumping, protein turnover (they wear out!), blood circulation, and respiration. The minimal level of metabolism that proceeds even in the absence of activity (needed for feeding, defense, and reproduction) and growth is termed basal, or maintenance, metabolism. Much work has gone into figuring out what percentage of total metabolism or energy expenditure is needed for this. Just as a car idling is using up gasoline, so too is just living, with about one-third to one-half of all energy expenditure going to baseline metabolism-burning energy. Living is clearly expensive. Reducing that cost has been a major driver of natural selection: the more efficient an organism is in conserving energy, the more energy there is available for actual activity. Metabolic rate is dictated by only three factors: body temperature, the organism’s mass, and the phylogeny (evolutionary history) of the animal in question, and it is of first importance in dictating growth rates and reproductive strategy or pattern.

The highest metabolic rates are observed in animals that maintain a constant body temperature that is independent of ambient, environmental temperature—animals that are described in the last chapter as warm-blooded or, more properly, endothermic. Endothermy is a characteristic of birds and mammals and is considered an advanced trait. Endothermic animals maintain the same internal temperature irregardless of ambient temperature, and thus on cold mornings, when ectoderms are still sluggish from the evening, endoderms are already moving and moving fast—to prey on animals, to avoid predation, to find shelter or mates. There are even more basic advantages of maintaining constant temperature. Metabolism is the work of every cell necessary to stay alive. Each cell needs energy to stay in disequilibrium with the environment, for equilibrium is the state of matter that is not alive. To remain alive, each cell must have a constant supply of energy to run the chemical reactions that are called life. The chemical reactions that keep any cell alive run at different rates at different tempera tures, and thus there is a huge advantage to keeping the cell (or cells in a multicellular organism such as an animal) within a fairly narrow chemical range. Thus, endothermy provides the maintenance of relatively constant temperature and thus accomplishes nearly optimal chemical conditions for the various reactions required for growth, reproduction, energy acquisition, and the other aspects of life.

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