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Why insulation = endothermy

Mickey argued that a single character cannot be used to diagnose the entire
metabolic system of an animal. I agree. However, there are characters that on
there own can tell us what is going on with a certain aspects of an animal's
thermodynamics. Insulation is one of them. 

In recent years Chinsamy et al and Ruben have argued that even some of the
fully insulated Mesozoic birds were not endotherms, and in some cases have
further argued that they were ectotherms. 

Back to the trip to the zoo. The physiologist I talked to there said he could
conceive of a arid-area ectotherm that used some form of fur or feathers as a
solar screen, as long as it had enough bare area to use to absorb lots of
environmental heat in the morning. He agreed that a complete insulation coat
means that the animal has isolated itself from the thermal environment, and
that this makes sense only if the creature is trying to retain body heat when
it is cool, and does not need to  take in the large amounts of external heat
needed to raise body temperature a large degree. This would work only if the
animal's energy level was high enough to make body heat worth retaining (not
true of reptiles), and such an animal would be an endotherm because it is not
dependent upon environmental heat. Ergo, a well insulated animal is almost
certainly not a reptile in energetic terms. 

That is all that the presence of complete insulation tells us. It does not
tell us what kind of endotherm the animal was, exactly how high its resting
metabolic rate was (some insulated mammals have metabolic rates just above
the reptile level), what its body temperature was (some furry mammals have
low body temps), how high its aerobic capacity was, etc, etc. Maybe an animal
with complete insulation could have been some sort of thing between reptiles
on the one hand and birds on the other (although this stage is may be more
likely to be characterized by partial insulation). But it must have been an
endotherm with an energy budget above the reptile maximum. Of course, this
conservative conclusion is supported by the total correlation between the
presence of insulation only on endotherms among living tetrapods and even
insects over 1 gram. (I have read the work on insect energetics, and all
flying bugs over 1 g are endotherms. Below 1 g it is not possible to be
endothermic no matter what the metabolic rate or insulation are like.)

Check out Nature in a couple of weeks to see why complete insulation is
important to understanding dinosaur energetics.