Text 259, 110 rader
Skriven 2004-10-01 17:29:00 av Brett Aubrey (1:278/230)
Ärende: Re: Why are mammals warm
================================


"G. Stewart" <galenstewart@yahoo.com> wrote in message
news:cjasme$m2p$1@darwin.ediacara.org...
> 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.

Thanks for an enlightening post.  After reading it, I looked up these terms
in Websters online ( http://www.m-w.com/dictionary.htm ) for my own
interest.  While they make an attempt to clarify poiklotherm(ic), their
explanation for the other three terms are basically restricted to:
'xxxx-blooded'.

I only mention this due to your Paragraph 2 "wish", which is less likely to
come true with this type of resource providing the high-school definition.
A quick note to them via their online form at (
http://www.m-w.com/info/contact.htm ) might start a ball rolling to at least
help your wish along (it seems you'd only need to edit to post above.)
FWIW.  Regards, Brett.
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