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WHAT ECTO&ENDOTHERMY REALLY MEAN
While the discussion about dinosaur energetics is on, this is a good time to
look at a semantic problem that is a source of endless confusion and
misunderstanding. Endothermy unfortunately has come to mean two things. Its
strict meaning is nothing more than that an animal generates the majority of
its heat internally, and therefore is not dependent upon environmental heat
sources. Conversely, ectothermy simply means that an animal is largely
dependent upon external, environmental sources for its body heat. These
definitions are used by those who research insect energetics.
Alas, matters have not remained so straightforward and correct among the
vertebrate set. Endothermy has also come to mean that a vertebrate has a high
resting metabolic rate, and a nonexercise means for generating heat in
response to cold. This not only corrupts the true meaning of endo & thermy,
it screws up the terminology. Say an animal produces enough heat through
exercise that it is not dependent upon environmental heat, but lacks a system
for defending its body temperature against cold. De facto it is a true
endotherm, But it is not an endotherm according to the broader definition.
Duh? Consider further. If the creature is not an endotherm according to the
broad definition, is it an ectotherm? But the latter gathers most of its heat
from the environment, which our beast does not do. So what is the vertebrate?
This problem is very real. For example, because some primitive birds have
cortical bone rings, it has been argued that they were not "endothermic". Do
the researchers mean that these birds were ectothermic? Ruben does, but
others who think these birds' metabolics were nonavian may not. After all,
these primitive birds have body feathers, which strongly suggests that they
were not dependent upon the environment for their body heat and that they
were therefore endothermic.
That such a sloppy use of terminology exists is appalling. How to solve it?
Let's all be well behaved scientists and define the terms strictly. If you
want to say that an animal has a low resting metabolic rate say it is
bradymetabolic. If it has high aerobic capacity it is tachyaerobic. Example,
a creature that has a low resting metabolic rate, a high aerobic capacity,
and generates large amounts of body heat internally is a bradymetabolic
tachyaerobic endotherm (this applies to large flying insects). For things
that straddle the boundaries between the two levels we can use the prefix
(Note: brady simply means low, meso intermediate, tachy high. For example, if
you are rushed to the hospital with severe chest pains and hear through the
haze of dwindling consciousness the doc say you are bradycardiac [like they
do on ER], then be afraid. Very afraid.)