Reptile Activity 
189 
PHYSIOLOGICAL AND ECOLOGICAL 
CONSTRAINTS ON REPTILE ACTIVITY 
Tetrapod Hearts 
When tetrapod vertebrates invaded the 
land, a variety of systems for the separation 
of oxygenated and deoxygenated blood 
evolved in the hearts of animals in the major 
phyletic lines. Contemporary lizards, snakes, 
and turtles possess complex hearts that 
apparently lack the potential to evolve a 
complete double pump for the pulmonary and 
greater circulation. Birds and mammals 
evolved from groups in -which the interven- 
tricular septum was apparently aligned so 
as to allow the eventual evolution of the 
double pump. A complete double pump “en- 
couraged and allowed” the full development 
of energetically advantageous aerobic meta- 
bolic pathways, and I will argue that this 
and other considerations in turn allowed 
mammals to develop fully, active-foraging 
food-gathering strategies and endothermy. 
Functional Anatomy of the Lizard Heart 
An understanding of the function and 
evolution of the heart in vertebrates is only 
now developing (Foxon et. al., 1956; Johan- 
sen, 1959; Tucker, 1966; White, 1956, 1959, 
1968, 1970; Webb, 1972; Baker, 1974, Baker 
and White 1970). 
The “three-chambered heart,” consisting 
of two auricles and a ventricle, is, in lizards, 
snakes, and turtles, actually a five-chambered 
heart because the ventricle is functionally 
divided into three compartments (Fig. 2). 
Recent studies show that this five-cham- 
bered heart is capable of maintaining a 
relatively complete separation between oxy- 
genated and nonoxygenated blood (Foxen et. 
al., 1956; White, 1956, 1959, 1968, 1970; 
Tucker, 1966 ; Steggerda and Essex, 1957 ; 
Baker and White, 1970; Johansen, 1959; 
Baker, 1974), but these as well as other 
studies (Praskash, 1952; Khalil and Zaki, 
1964) indicate that separation of the blood- 
streams is not always present. Separation is 
possible largely because of (1) the “septa” 
which divide the ventricle into “pockets” 
(trabeculation), (2) pressure gradients with- 
in the chambers that facilitate laminar (non- 
turbulent) flow of the blood, and (3) a time- 
sharing of certain spaces within the ventricle 
by the two bloodstreams. (In particular, dur- 
ing ventricular filling, the CV is in part a 
pathway for blood between the right atrium 
and the CP. Then, during late ventricular 
systole, the CV becomes a pathway for the 
oxygenated blood flowing between the CA 
and the two aortic arches). 
It should be noted that there is relatively 
good separation of the blood streams even 
in some lungfish, amphibia, and the tuatara 
where division of the ventricle into separate 
chambers is not so advanced as in the lizards, 
snakes, and turtles (Johansen and Hoi, 1968; 
Johansen and Hanson, 1968; Grigg and 
Simons, 1972). [Separation within the heart 
is complete in the crocodilians. However, the 
left systemic arch originates in the right 
ventricle along with the pulmonary arch. 
Deoxygenated blood is pumped back into the 
systemic circulation except when oxygenated 
blood from the left ventricle flushes the left 
systemic arch through the foramen of 
Panizzae which connects the left and right 
arches. This separation of the pulmonary 
and systemic circulations is documented in 
anesthetized Caiman (White, 1956, 1968, 
1970). The complicated dynamics of this 
heart probably relates to diving.] 
The vascular pump of lizards appears awk- 
ward when compared with the double pump 
in birds and mammals, yet it may have cer- 
tain advantages. The pulmonary circulation 
can be bypassed, and this may have advan- 
tages because the flow of blood to the skin 
can be increased to pick up heat during 
basking without the energetic expense of 
forcing this blood through the lungs (Tucker, 
1966; Baker and White, 1970; Baker, 1974; 
Webb, 1972). Bypass of the lungs also has 
been shown to take place when turtles (and 
alligators using a different system) dive, and 
preferential distribution of blood to the lungs 
can occur with emergence from the water 
