For the aerobic capacity hypothesis to work, both the maximal and the resting metabolic rate of mammals and birds need to be substantially higher than those of lizards. This is well known to be the case.2 Lizards become exhausted quickly and have a low capacity for aerobic exercise. While they can move very fleetly (when warmed up) their muscles are mostly powered by anaerobic respiration to produce lactate (see Part 2). They can sustain a burst of speed for little more than 30 seconds, enabling them to dart for the nearest hole and hide, whereupon they often need several hours to recover. In contrast, the aerobic performance of similarly sized mammals and birds is at least six to tenfold greater. While not quicker off the mark or fleeter of foot, they can sustain the pace for far longer. As Bennett and Ruben put it in their original Science paper: 'The selective advantages of increased activity are not subtle but rather are central to survival and reproduction. An animal with greater stamina has an advantage that is readily comprehensible in selective terms. It can sustain greater levels of pursuit or flight in gathering food or avoiding becoming food. It will be superior in territorial defense or invasion. It will be more successful in courtship or mating.'
What must an animal do to improve its stamina and speed? Above all else, it has to augment the aerobic power of its skeletal muscles. To do so requires more mitochondria, more capillaries and more muscle fibres. We immediately run into a difficulty with space allocation. If the entire tissue is taken up with muscle fibres, there is no room left over for mitochondria to power muscle contraction, or for capillaries to deliver oxygen. There must be an optimal tissue distribution. To a point, aerobic power can be improved by a tighter packing of these components, but beyond that improvements can only be made by greater efficiency. This is indeed what happens. According to Australian researchers Tony Hulbert and Paul Else, at the University of Wollongong, New South Wales, mammalian skeletal muscles have twice as many mitochondria as the equivalent lizard muscles, and these are in turn more densely packed with membranes and respiratory complexes. The activity of respiratory enzymes in rat skeletal muscle is also about twice that of the lizard. In total, the aerobic performance of rat muscle is nearly eight times that of the lizard—a difference that wholly accounts for its greater maximum metabolic rate and aerobic capacity.
This deals with the first part of the aerobic capacity hypothesis: selection for
2 The scaling equation is given as metabolic rate = aMb in which a is a species-specific constant, M is the mass, and b is the scaling exponent. The constant a is fivefold greater in mammals than in reptiles, but both groups still scale with size (the lines are parallel). The fractal model can't explain why different species should have different a constants, in other words why the resting metabolic rate and capillary density in various organs is different in mammals and reptiles; nor can it explain the rise of endothermy. The explanation again lies in the tissue demand for more oxygen to power greater aerobic performance: this is the driving force that leads to the remodelling of muscle and organ architecture, and with it the fractal supply network.
endurance raises the mitochondrial power of muscle cells, leading to a faster maximum metabolic rate; but what about the second part? Why is there a link between maximal and resting metabolic rate? The reason is not clear, insofar as none of the possible explanations has been proved. Even so, there is a good intuitive reason to expect a connection. I mentioned that lizards may often take several hours to recover from exhaustion, even after a few minutes of vigorous exertion. Such a slow recovery is less dependent on muscles than on organs, such as the liver and kidneys, which process the metabolic waste and other breakdown products of vigorous exercise. The rate at which these organs operate depends on their own metabolic power, which in turn depends on their mitochondrial power—the more mitochondria, the faster the recovery. Presumably the advantages of endurance also apply to recovery time: given the eightfold rise in aerobic power of mammalian muscles, if there were no compensating changes in organ function it would take a whole day, rather than a few hours to recover from exercise.
Unlike muscles, organs are not faced with a dilemma of space allocation— while the density of mitochondria doesn't change with size in muscles, it does in the organs. As animals get bigger, the power laws that we discussed in the last chapter mean that their organs become more sparsely populated with mitochondria. This is an opportunity in waiting. For the organs of a large animal to gain power, the tissue architecture doesn't need to be restructured as it does in muscle: it can simply be repopulated with mitochondria. This opportunity seems to have given rise to endothermy. In their classic comparative studies, Hulbert and Else showed that the organs of mammals contain five times as many mitochondria as an equivalent lizard, but in all other respects the mitochondria are no different. For example, the efficiency of their respiratory enzymes is exactly the same. In other words, for every hard-won increment in muscle power, it's relatively simple to counterbalance the new power by filling up the half-empty organs with a few more mitochondria, so as to ensure speedy recovery from aerobic exertion. The important point is that the function of organs like the liver is coupled to muscle demand, and not with the need to keep warm.
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