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Re: why insulation = endothermy
It's certainly true some endotherms bask (echidnas and sloths come
to mind), but having insulation does not represent an optimal situation for
uptake of external heat. Roadrunners, for example, bask by holding the wings
away from the body and erecting their cervical plumage. This exposes areas
of black skin (as opposed to pink skin elsewhere), for increased uptake
of solar radiation. Even in endotherms specialized for basking, naked skin
is preferable to insulation-covered areas.
Elsewhere, on Jan. 31, Mickey Rowe wrote:
>Endothermy (as practiced by modern mammals and birds) results from suites
>of characters. It's not just one thing. So you can't a) give one of those
>characters to a modern ectotherm to demonstrate the impossibility of that
>trait's existance without the rest of the suite.
Of course you can...removal or addition of anatomical features
or hormones is one of the most effective methods in experimental biology
to determine cause & effect relationships. Because birds become hypothermic
below thermoneutrality when their feathers are removed (Brush, 1965), it's
entirely reasonable to conclude insulation is functionally coupled to high
resting metabolic rates.
The experiments of Cowles (1958), demonstrating that furry lizards
cannot utilize external heat as well as naked-skinned lizards, strongly
suggests furry ectotherms are unlikely. The fact that insulation also
serves as a heat shield in mammals and birds (Schmidt-Nielsen et al;
Maloney & Dawson, 1995), independently confirms the results of Cowles that
animals who depend on environmental heat for most of their metabolism are
not going to be growing fur coats. The observation that insulation (a)
evolved independently in birds & mammals, (b) is functionally coupled to
endothermy, (c) is absent from all living endotherms, and (d) interferes
with ectotherm thermoregulation, indicates to me that any fossil taxon
for which insulation can be demonstrated was probably a tachymetabolic
endotherm (but perhaps only metabolically comparable to monotremes and
Mickey also wrote:
>Note that this is different from the RT argument. In that case, the
>argument is that no animal can have the chronically high metabolic rates
>(resulting from a suite of characters) of modern endotherms without some
>mechanism for retaining respiratory water. IMHO, the only thing left to
>argue on this point is whether or not there is any evidence that
>dinosaurs had such a mechanism.
The *only* thing? Ruben and others have (a) noted a structure
(RT) which is absent from living ectotherms, though present in living
endotherms (b), hypothesized a function for RT which could link RT
to high resting metabolic rates, and (c) attempted to determine the
presence or absence of RT in fossil taxa. What they have *not* done
is to experimentally demonstrate that the nasal cavity actively regulates
water loss. As I have noted in a previous posting, there's no direct
evidence of this, and what little experimental data exists suggests the
Maloney, S.K. & Dawson, T.J. (1995). The heat load from solar
radiation on a large, diurnally active bird, the Emu (_Dromaius novaeholl-
andiae_). Journal of Thermal Biology, 20 (5), 381-387.
Cowles, R.B. (1958). Possible origin of dermal temperature
regulation. Evolution, 12, 347-357.
Schmidt-Nielsen, K., Schmidt-Nielsen, B., Jarnum, S.A. & Houpt, T.R.
(1957). Body temperature of the camel and its relation to water economy.
American Journal of Physiology, 188 (1), 103-112.
Brush, A.H. (1965). Energetics, temperature regulation and
circulation in resting, active and defeathered California Quail, _Lophortyx
californicus_. Comparative Biochemistry and Physiology, 15, 399-421.
Ohmart, R.D. & Lasiewski, R.C. (1971). Roadrunners: energy
conservation by hypothermia and absorption of sunlight. Science, 172,
Dept. of PEHR
Western Washington University
Bellingham, WA 98225