194 
Regal 
The Evolution of Endothermy 
Endothermy among reptiles is known only 
in the large Indian rock python during incu- 
bation and in the leatherback turtle (Hutch- 
inson et al., 1966; Friar et al., 1972). Per- 
haps it is rare among reptiles because, for 
an animal with a five-chambered heart, a 3- 
to 8-fold increase in the standard metabolic 
rate would result in an animal with little if 
any metabolic scope. When modern reptiles 
are forced to vigorous “maximal” activity, 
58 percent to 90 percent of the ATP produc- 
tion is anaerobic (Bennett and Dawson, 
1972 ; Bennett and Licht, 1972). A struggling 
lizard quickly builds up an oxygen debt and 
lactic acid burden and fatigues, then rests 
and pays back the debt. An endothermic 
lizard, with a constant high resting meta- 
bolic rate would retain little or no metabolic 
scope (or would have very great food require- 
ments). 
Endothermy may be relatively more eco- 
nomical for animals with complete double 
pumps and advanced lungs, but why should 
it have evolved at the expense of absolute 
increases of 3- to 8-fold over reptilian 
metabolic rates ? 
One obvious possibility, often argued, is 
that endothermy is an adaptation for cold 
at high latitudes, altitudes, in a cooling 
climate, or at night. There is, however, an- 
other set of selection pressures that is inde- 
pendent of paleoclimatic arguments. For 
example, as birds became more active in the 
air, increased convective heat loss gave ad- 
vantages to the conservation of heat gener- 
ated in activity and to an increase in basal 
metabolic rate, insofar as they damped out 
body temperature fiuctuations (Jerison, 
1971; Regal, 1975). Processes such as the 
repayment of an oxygen debt take place most 
efficiently at particular temperature optima 
(Moberly, 1968a; 19686; see also Hochachka 
and Somero, 1973) ; and in such facts may 
be the advantage of temperature stability for 
very active animals. 
Similarly, a small terrestrial ectothermic 
protomammal actively “poking” about in 
search of food and moving in and out of 
shade would either suffer a decrement in 
efficiency (e.g. fluctuations in sensitivity to 
fatigue) by allowing its body temperature to 
fluctuate or might need to budget time for 
basking if diurnal. 
The same problem would have faced a 
semiaquatic animal with active habits. This 
is significant because some authors have 
argued aquatic associations for the early 
mammals (Olson, 1966; Smith, 1972). 
Basking is a thermoregulatory strategy 
that is particularly compatible with “sit- 
and-wait” predation. In many lizard species, 
basking orientations can take place in the 
sunny but cool desert mornings and after- 
noons or in cool clear climates while the liz- 
ard sits and waits for prey to approach. 
Perhaps this consideration helps to explain 
why many teiids actively forage for a re- 
stricted time period during the hot hours of 
the day when high body temperatures can 
be maintained with a minimal time invest- 
ment in basking. The ecological conditions 
that allow such behavior may be limited and 
are in need of elucidation. 
Active ways of life could have provided 
the behavioral substrate for more efficient 
locomotor systems and erect and bipedal 
postures in mammals, birds, and extinct rep- 
tiles. One possible factor in the abandon- 
ment of the primitive sinusoidal undulations 
of the trunk is that the efficiency of sinu- 
soidal locomotion may be constrained by the 
natural resonant frequency of the trunk. 
Some modern lizards running at top speed 
on level ground may become bipedal with the 
front legs held off the ground. 
These and other advantages of erect pos- 
tures would apply to either endothermic or 
ectothermic animals. It is not correct to rea- 
son that an erect posture necessarily indi- 
cates endothermy (or even necessarily active 
foraging) as has been argued for dinosaurs. 
Similarly, not all fusiform-shaped fish are 
warmblooded. 
Yet, once endothermy based on a high 
metabolic rate is established in a lineage, it 
becomes increasingly probable that selection 
will tend to perfect mechanisms for rapid 
energy assimilation because tachymetabolism 
