192 
Regal 
and removal of the lactic acid burden are 
slow. The rate at which energy is aerobically 
generated seems to be independent of body 
temperature in Iguana but not in Sauro- 
malus. However, in those species studied, 
there is certainly an optimum temperature 
at which each can repay its oxygen debt and 
and at which the “aerobic scope” is greatest. 
Consequently, a lizard may become fatigued 
more quickly when its body temperature is 
below this optimum (Berkson, 1966; Moberly, 
1968a, 19686; Bennett, 1972a, 19726; Ben- 
nett and Dawson, 1972; Bennett and Licht, 
1972; Bennett et al., 1975; Gatten, 1975; 
Ruben, 1976a, 19766). 
An active predatory life is correlated with 
a relatively increased aerobic capacity and 
the ability to pay back quickly an oxygen 
debt in some species. Bennett (19726) found 
that “Varanus recovers from activity more 
than three times as rapidly as Sauromalus. 
Its total oxygen debt is only two thirds that 
of the iguanid.” Depending on how one cal- 
culates the data, the resting, largely aerobic, 
metabolic rates of reptiles are about 13 to 
20 percent of those for a resting mammal. 
Yet the oxygen consumption of a vigorously 
active Varanus may exceed that of a resting 
mammal. Varanids have highly developed 
lungs with a large surface area, unusual for 
lizards. Ruben (1976a, 19766) reported rela- 
tively high aerobic scope correlated with 
specializations in lung structure and high 
axial muscle myoglobin content in the species 
of active snakes he studied. 
In short, reptiles generally have limited 
aerobic capacities probably correlated with 
their simple lungs and an incomplete double 
pump for the blood. Accordingly their de- 
pendence on anaerobic glycolysis during ac- 
tivity is great. The circumstances under 
which oxygen debts are accumulated and re- 
payed will be seen to have important conse- 
quences for their biology. Complex lungs 
have evolved in some snakes and turtles, 
varanid lizards, and crocodilians. These and 
advances in cellular physiology may result 
in an improved capacity for oxygen consump- 
tion and oxygen debt repayment in a rela- 
tively few species. 
Metabolic Costs and Foraging Strategies 
in Reptiles and Mammals 
Air and the Ecological Cost of Energy 
Air is rich in oxygen, and therefore meta- 
bolic energy is ecologically cheap for terres- 
trial and surface-water animals. In aerobic 
metabolism of glucose, some 36 high energy 
phosphates (ATP) are formed as compared 
with only 2 in anaerobic glycolysis. This 
means that an anaerobic organism must find 
and eat 18 times as much food to do the 
same job as a comparable aerobic individual. 
In an oxygen-rich habitat an aerobic species 
would always win in direct ecological com- 
petition with an anaerobe since the former 
would require only one-eighteenth as much 
food or it would spend one-eighteenth the 
time searching for food, and could theoreti- 
cally divert the surplus time and energy to 
rapid growth, shortened generation time, 
care of young, hiding from predators, terri- 
torial defense, etc. Likewise any degree of 
anaerobic metabolism where lactic acid is 
not reconverted to glucose or glycogen would 
be expensive to an individual in terms of the 
time required simply to obtain food even in 
a rich habitat. 
During activity most terrestrial verte- 
brates build up lactic acid, as part of the 
oxygen debt, which they later reconvert to 
glucose or glycogen when oxygen is avail- 
able. The “toxic” products of glycolysis are 
handled by storage, tolerance of lactic acid, 
and reconversion. Hence, the price paid for 
less than maximal efficiency of aerobiosis is 
usually not a loss of energy, but one of time 
and efficiency as fatigue sets in and the in- 
dividual slows or stops ongoing activity to 
repay the oxygen debt. 
Evolutionary Options 
Primitive terrestrial vertebrates may have 
had simple lungs, incomplete double circula- 
tion, and a high dependence on glycolysis, 
as do most modern amphibians and reptiles 
