Reptile Activity 
193 
that we know of. The early reptiles might 
have had in glycolysis a “reserve capacity” 
(or metabolic scope) for bursts of activity 
but would be expected to have had a strategy 
of usually slow behaviors in order to main- 
tain this reserve (as do many modern rep- 
tiles). 
Conservation of reserve capacity could ex- 
plain why many reptiles are relatively 
motionless sit-and-wait predators. This feed- 
ing strategy has long been an efficient pat- 
tern in the context of primitive oxygen sup- 
port systems. Note, too, that with such for- 
aging strategies as sitting and waiting and 
cruising there are relatively small demands 
on the oxygen support systems and so their 
evolution would tend to be (and has been) 
conservative. 
Still other primitive reptiles with the same 
physiological limitations might have aban- 
doned their metabolic reserve and foraged 
intensively near the limits of exhaustion. 
Such foraging patterns would have provided 
the selective basis for the subsequent evolu- 
tion of improved oxygen delivery systems 
since the maintenance of some reserve would 
usually be of advantage in escaping preda- 
tors, winning mates, or dashing after fast 
prey. The evolution of more efficient lungs, 
cardiovascular and cellular metabolic sys- 
tems would each improve the cost/benefit 
ratio of active foraging: By increasing the 
amount of intensive foraging vs. time recup- 
erating, the food benefits accruing from a 
given length of exposure would be increased. 
Physiological advances would result in in- 
creased net benefits in terms of the ecological 
effectiveness of an active forager. Advance- 
ment to this next level of organization would 
only be possible in the absence of more 
efficient competitors. 
The Synapsida, mammal-like reptiles, were 
present in the Pennsylvanian and this 
makes them among the earliest of reptile 
groups. Their mammal-like posture and den- 
tition suggest that in the absence of compe- 
tition some of these early forms were active 
animals. This radiation, and subsequently 
that of the true mammals, may have filled 
the niches for small active animals for the 
last 300 million years. This would have re- 
duced lepidosaurian evolutionary options so 
that lizards retain many primitive aspects in 
their locomotor and oxygen support systems. 
It is interesting to speculate as to why a 
few lizards such as some teiids and varanids 
have evolved relatively advanced active-for- 
aging abilities. Several Cnemidophonts, 
Ameiva, and Varanus forage during the day 
in desert or dry habitats, whereas a mammal 
with similar habits would expose itself to 
dehydration because of its higher metabolic 
rate. Also, seasonal scarcity of food would 
present many types of small mammalian 
carnivores or insectivores with energy-bud- 
get problems. Water monitors occupy special- 
ized niches, and possibly the ability to make 
long dives in part allows them to minimize 
competition with mammals. Interestingly, 
varanids are most diverse in Australia which 
has been essentially free of larger-brained, 
advanced, placental carnivores. 
Studies on niche separation and competi- 
tion (particularly in humid forests) between 
such active reptiles and mammals, which take 
into account their respective metabolic de- 
mands and capabilities, are needed and would 
be of considerable interest not only for the 
evolution of brain functions. 
Strategies in Reptiles and Mammals 
Metabolic Costs and Thermoregulatory 
The resting metabolic rate and food re- 
quirements of mammals are several times 
those of reptiles. Basking reptiles may obvi- 
ously have advantages in ecological situa- 
tions where solar energy is abundant and 
food is scarce, while mammals can be active 
in habitats where adequate food can be 
found but where the climate would be un- 
favorable for ectothermy. Other considera- 
tions may have also been involved in the 
evolution of endothermy among mammals 
and in the retention of ectothermy among 
reptiles. 
