Re: Why are mammals warm blooded?

From: G. Stewart (galenstewart_at_yahoo.com)
Date: 09/28/04


Date: Tue, 28 Sep 2004 05:26:38 +0000 (UTC)

Your confusion arises from using the terms and associated conceptual
dichotomy of "cold-blooded" vs. "warm-blooded".

While firmly established in biology curricula around the world, the
termininology is a loose and misleading pedagogical simplification.
The "either/or" dichotomy implied by the terminology is too simplistic
to capture the full range of thermo-physiologies out there, and thus
is, in many cases, rendered false by physiologies that cannot be
characterized by either term. In fact, many animals are neither
strictly "warm-blooded" or "cold-blooded", and I wish people would
stop using these misleading terms to falsely pigeon-hole life forms! A
better way of characterizing thermoregulation is with the context of
the following terms: homeothermy vs. poiklothermy, and endothermy vs.
ectothermy.

Animals can be either homeothermic (maintaining a fairly constant body
temperature) or poiklothermic (a body temperature that varies,
typically with that of the ambient or environmental temperature).

They can also be either endothermic (relying on internally-generated,
metabolic heat to maintain body temperature) or ectothermic (relying
on environmental or ambient heat to maintain body temperature).

A "poiklothermic ectotherm" (or "ectothermic poiklotherm") is
generally what is meant by "cold-blooded", and a "homeothermic
endotherm" (or "endothermic homeotherm") is generally what is meant by
"warm-blodded". For example, we, and many other mammals (but not all!)
are obligate endothermic homeotherms. We use internally generated
metabolic heat to maintain a stable body temperature whether we like
it or not. Many frogs, on the other hand, are obligate ecothermic
poiklotherms - they body temperature rises and falls with the
environmental temperature, and ambient heat is the only source of heat
that they use to regulate their body temperature. So, as far as humans
and frogs are concerned, you can use the terms "warm-blooded" and
"cold-blooded" safely enough without a problem. But you get a lot of
animals (mammals, reptiles, fishes, etc.) that are neither
"warm-blooded" nor "cold-blooded" by this definition, and this is way
this dichotomy falls apart.

For example, many lizards maintain a very, very stable body
temperature. In this regard they are homeotherms. However, they do not
rely on metabolic heat to do this. Instead they employ behaviorial
thermoregulation - moving to warm spots when they start to cool down
too much and to cools spots when they start to heat up too much. These
guys are "ectothermic homeotherms", and the
"warm-blooded"/"cold-blooded" dichotomy entirely fails to adequately
characterize them. Conversely, many bats regularly enter a state of
torpor during which their bodies undergo a considerable drop in body
temperature to generally meet that of their environment. So we can
quite correctly characterize these mammals as facultative endothermic
poiklotherms.

As I hope should be clear by the above examples, not only are the
terms "warm-blooded" vs. "cold-blooded" misleading, but blanket
statements of mammals being "warm-blooded" and reptiles (or fish,
etc.) being "cold-blooded" do not really work beyond high-school
biology textbooks.

The case of sharks that you raise is good example. Many sharks have a
lot of blood vessel-rich muscle tissue. By vibrating and/or using
these muscles they can generate heat, which is then circulated through
core parts of the body. These guys are neither really "cold-blooded"
nor "warm-blooded", but can be considered facultative
endothermic/ectothermic homeotherms.

Other examples (or, rather, counter-examples to this "warm-blooded"
vs. "cold-blooded" issue) abound, as any close reading of the natural
histories or ecologies of many species shall reveal.

Even the notion that "cold-blooded" critters are poorly adapted for
cold conditions is false. You got poiklothermic ectotherms (frogs) at
extremely high-elevations in subzero conditions in the himalayas. No
"warm-blooded" animals can survive there, simply because there is not
enough resources (food) to support the metabolically expensive
endothermic body machinery. You also get many, many poiklotermic
ecothermic species (fishes, shrimp, etc.) doing quite well in the
polar and subpolar seas, whereas relatively much fewer endotherms can
cut it there, and even then it is not only with a whole suite of
extreme phsyiological and morphological specializations (much more so
than their ectothermic poiklotherm counterparts), but also at a very,
very high energy cost. Obligate endothermic hometherms like us have a
huge and expensive "housekeeping" energy budget that cannot be ignored
or turned off. Even if we were to do absolutely nothing, engage in no
activity whatsoever and just lie down all the time, we would use up
something like 1200-1600 calories a day. A big ol' crocodile, in
contrast, can go for up to two years without eating.