Ectothermy?

[Chris Lavers <chris.lavers@nottingham.ac.uk>]

Dear all

any views?

There seems to be a fashion these days for down-playing the difference
between endothermy and ectothermy (or endothermic homeothermy and
ectothermy) by pointing to interesting departures from what is a very clear
norm. I think the fashion is mostly a "hurrah" response to the ignorance of
earlier workers compared with our present state of smug enlightenment. The
fashion seems to be particularly strong among dinosuarologists, I guess
because it was their work which made physiologists think again about the
warm-blooded/cold-blooded divide (hurrah to that). 

The interesting departures (like naked mole-rats, sloths, insectivorous
bats, small warm-blooded hibernators, brooding pythons,  functionally
endothermic moths etc) are even sometimes referred to as occupying
"intermediate states". Yuck. They are not intermediates at all but just
modifications of the ancestral metabolic system (interesting, but that's
all). Brooding pythons are always put forward to show that reptiles can be
endothermic but why not running Komodo Dragons? Ectotherms produce heat by
muscular activity which they can retain if they are large enough (small
ectotherms produce heat too but their large SA/V ratio and lack of
insulation means that they cannot retain it). It seems that pythons are a
popular example because they do it while sitting around rather than running
about. No physiologist would confuse the metabolic systems of sloths,
brooding pythons, hibernating mice and large moths just because they
produce variable amounts of heat.

Cold-blooded vertebrates can elevate their aerobic metabolic rates by about
a factor of ten above resting rates to a level similar to that of an
unmodified mammal (for example) at rest. Mammals can also elevate their
aerobic rates by roughly a factor of ten above resting so the maximum power
output of unmodified mammals (the vast majority) is an order of magnitude
higher than that of any reptile. The metabolic engines of mammals and birds
are *different* from those of other vertebrates in a very fundamental way
(no value judgements, just different). Thermal homeostasis is only one
aspect of what, IMHO, should still be considered The Great Metabolic
Divide. When all aspects are considered and "intermediates" classified
properly as modifications then there is clear blue water between mammals
and birds and everything else.

The endo- ecto-  poikilo- homeo- etc. terminology remains confusing but it
shouldn't confuse our perception of real and valid divisions. Vive la
difference!

Chris