Ok Boadicea seeing as none of the cognoscenti on this forum saw fit to answer your question I will do my best.
I did a bit of research on the internet and came up with this interesting backgound article on endothermy and ectomthermy
http://www.redorbit.com/news/science/15 ... ndothermy/
Here are a few excerpts;
By measuring an American alligator's oxygen consumption on a treadmill, Colleen Farmer and colleagues found that alligators' basal metabolic rate was low, while their maximal oxygen consumption during exercise was moderately high.
Bennett and Ruben, along with Hicks, continued their work by again looking at metabolic rates in resting savannah lizards and found that even when they were fed excessively, increasing their visceral metabolic rates, the animals became neither endothermic nor homeothermic (maintaining a constant body temperature). Bennett and Ruben referred to the aerobic scope as the absolute increment of the maximal rate of oxygen transport above the resting rate of oxygen transport. This, they said, should be higher in endotherms.
Pawel Koteja, of Jagiellonian University in Krakw, Poland, has in recent years also come to the conclusion that the origin of endothermy is related to parental care. He summarizes the two types of hypotheses on its evolution. "The first assumed that thermoregulatory advantages were sufficient to explain the evolution [of] high basal metabolic rates and endothermy in birds and mammals," he notes. "The second assumed that the evolution of high BMR was (at least initially) a side effect of a selection for other traits." He adds, "My hypotheses that endothermy evolved as a side effect of intensive parental care associated with feeding the young [published in Proceedings: Biological Sciences in 2000] belong to the second group." Koteja sees the parental care model as an "alternative to all the 'thermoregulatory' hypotheses as well as an alternative to the 'aerobic capacity model.'" He notes, "The keystone of the 'parental care' models is that the cost of an increased metabolic rate of adults is rewarded by several advantages to embryos and/or juveniles." Farmer, however, hypothesizes that parental care came first. "I'm saying there's independent selection for metabolic rate-it's about rapid rates of reproduction." In Farmer's view, the gist of endothermy can be distilled simply to a better chance of survival.
There is also this little gem found in a paper on ventricular haemodynamics in Python molurus at
http://jeb.biologists.org/cgi/reprint/206/23/4241.pdf
Alternatively, it is possible that ventricular pressure separation
in Python molurus is related to its use of shivering
thermogenesis during egg incubation, which results in large
and prolonged metabolic increments (Hutchison et al., 1966;
There is another interesting titbit
herehttp://www.sebiology.org/meetings/2 ... dfs/a7.pdf
notably Python molurus, and some other species
of pythons, are unique by being able to increase
body temperature through shivering thermogenesis while
incubating their eggs. This associated with long-lasting
and profound increases in metabolic rate. The link
between shivering thermogenesis and ventricular pressure
separation is, however, not obvious. Thus, while
Python reticulates has been reported to incubate their
eggs, Python regius does not. It is possible that shivering
thermogenesis and ventricular pressure separation coevolved
in the stem group of pythons, but that some
species subsequently lost incubation of eggs, while
retaining a functionally divided heart.
Vinegar et al., 1970).