It has long been known that the rate of oxidative metabolism (the process
that uses oxygen to convert food into energy) in any animal has a profound
effect on its living patterns. The high metabolic rate of small animals, for
example, gives them sustained power and activity per unit of weight, but at the cost of requiring constant consumption of food and water.
Very large animals, with their relatively low metabolic rates, can survive well
on a sporadic food supply, but can generate little metabolic energy per gram of
body weight. If only oxidative metabolic rate is considered, therefore, one
might assume that smaller, more active, animals could prey on larger ones, at
least if they attacked in groups. Perhaps they could if it were not for
anaerobic glycolysis, the great equalizer.
Anaerobic glycolysis is a process in which energy is produced, without
oxygen, through the breakdown of muscle glycogen into lactic acid and adenosine
triphosphate (ATP), the energy provider. The amount of energy that can be produced
anaerobically is a function of the amount of glycogen present—in all
vertebrates about 0.5 percent of their muscles’ wet weight. Thus the anaerobic
energy reserves of a vertebrate are proportional to the size of the animal. If,
for example, some predators had attacked a 100-ton dinosaur, normally torpid,
the dinosaur would have been able to generate almost instantaneously, via
anaerobic glycolysis, the energy of 3,000 humans at maximum oxidative metabolic
energy production. This explains how many large species have managed to compete
with their more active neighbors: the compensation for a low oxidative
metabolic rate is glycolysis.
There are limitations, however, to this compensation. The glycogen reserves
of any animal are good, at most, for only about two minutes at maximum effort,
after which only the normal oxidative metabolic source of energy remains. With
the conclusion of a burst of activity, the lactic acid level is high in the
body fluids, leaving the large animal vulnerable to attack until the acid is
reconverted, via oxidative metabolism, by the liver into glucose, which is then
sent (in part) back to the muscles for glycogen resynthesis. During this
process the enormous energy debt that the animal has run
up through anaerobic glycolysis must be repaid, a
debt that is proportionally much greater for the larger vertebrates than for
the smaller ones. Whereas the tiny shrew can replace in minutes the glycogen
used for maximum effort, for example, the gigantic dinosaur would have required
more than three weeks. It might seem that this interminably long recovery time
in a large vertebrate would prove a grave disadvantage for survival.
Fortunately, muscle glycogen is used only when needed and even then only in
whatever quantity is necessary. Only in times of panic or during mortal combat
would the entire reserves be consumed.
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