Reptile Activity 
191 
(White, 1970). Hence, there may be distinct 
advantages of hearts that traditionally have 
been regarded as incomplete stages in the 
evolution of the four-chambered heart. 
The temporal duality in function of the 
CV, the apparently critical alignment of 
septa within the pulsating heart muscle, 
and the delicate balance of pressures appar- 
ently necessary to maintain laminar flow 
within the heart — all lead to the prediction 
that effectiveness in separating the blood 
streams will vary as function of heart rate 
and possibly also of stroke volume. Thus, 
while we do not know with certainty the 
precise effects of any of these factors upon 
the efficiency of separation, it is probable 
that the squamate heart does not function 
both at rest and at work or at high pressures 
with the same efficiency as the mammalian 
heart consisting of two separate pumps. This 
conclusion is based on a study of the heart’s 
structure and its flow patterns in restrained 
and anesthetized animals. Direct experi- 
mental evidence is limited at this time, con- 
sisting only of the observation that oxygen 
content is not always equal in the two sys- 
temic arches. There is an increase in arterial- 
venous differences in oxygen saturation at 
higher metabolic rates, but this is caused by 
increased oxygen use by the tissues, not an 
improvement in the separation of the two 
flows (Tucker, 1966). 
The squamate heart is probably too highly 
specialized to evolve into a complete double 
pump through a series of small morphologi- 
cal changes. Because of this and the advan- 
tages of the squamate system for ectothermy 
lizards are locked into a relatively inefficient 
cardiovascular system (from the standpoint 
of a complete separation of bloodstreams at 
high pressures and heart rates [though see 
Holmes, 1975.].) 
Hearts in Competition 
Characteristic activity patterns and be- 
haviors of mammals and of reptiles reflect 
their foraging strategies. Hence, an evolu- 
tionary and comparative perspective must 
consider the adaptive zones {sensu Simpson, 
1953) occupied by each group and the struc- 
tural, physiological, and energetic constraints 
that have determined the evolution of their 
present situation. 
All other things being equal, a squamate 
should not be able to compete effectively with 
a mammal in situations requiring sustained, 
intense muscular effort. If two hypothetical 
species make equivalent evolutionary ad- 
vances in lung structure, oxygen transport 
systems, and cellular metabolic machinery, 
a species with a five-chambered heart would 
still be at a disadvantage relative to a species 
with a double pump for reasons discussed in 
the next section. 
Hence, due to competition we might have 
predicted the general evolutionary bias 
among lizards, snakes, and turtles toward 
more passive feeding strategies : sit-and-wait 
feeders, slow cruisers, and sluggish browsers. 
Animals with four-chambered hearts could, 
on the other hand, effectively radiate into 
niches open to actively hunting carnivores, 
browsers, grazers, and omnivores. 
Aerobic and Anaerobic Metabolic 
Capacities of Reptiles 
Aside from the newborn and hibernating 
species, mammals die after about 5 minutes 
in pure nitrogen. Most reptiles are, however, 
quite tolerant of anoxia. Belkin (1963) re- 
ported that 43 species of lizards, snakes, and 
crocodilians would live for 20 to 118 minutes 
breathing pure nitrogen. The turtles in 
Belkin’s study were all remarkably tolerant 
and survived 114 to 1,980 minutes. Belkin 
(1968) argued that, in turtles at least, the 
prolonged maintenance of CNS functions is 
related to anaerobic glycolysis. Also, he 
showed that circulation of the blood is essen- 
tial to prolonged anoxic brain function, at 
least in turtles. 
Recent studies show a considerable depend- 
ence in reptiles on glycolysis for strenuous 
activity, and most species rapidly accumu- 
late lactic acid and quickly become fatigued. 
Subsequent repayment of the oxygen debt 
