Re: Layperson question on endothermic dinosaurs

[Kelly Clowers <kelly.clowers@gmail.com>]

Rescued from truncation:

On Wed, May 15, 2013 at 12:17 PM, Jura <pristichampsus@yahoo.com> wrote:

The truth is that we are no closer to knowing the thermophysiology of
dinosaur now then we were in the 70's and 80's when this whole thing
was called into question. The biggest problem with metabolism is that
differences between groups are  often a question of grade rather than
of structure. For instance the cell membranes of crocodiles and cows
are almost exactly the same. However cows incorporate more
polyunsaturated fatty acids in their cell membranes than crocs do.
This makes the cell membrane less efficient at retaining certain ions
which forces the protein pumps in the membranes to work harder to keep
proper ionic concentrations, ultimately giving cow cells higher
metabolic rates than croc cells. Wu et al. (2004) actually "turned" a
croc cell into a cow cell by changing the unsaturated fatty acid ratio
in the membranes. All of this is soft-tissue related and differs only
in  ratios. None of it fossilizes which means we have no real way of
saying definitively anything about metabolic rate.

So we typically turn to proxies for metabolism, which is a problem.
Forty years of tackling thermophysiological questions and we are still
not sure what makes for a good proxy for automatic endothermy (i.e.,
mammal and bird-style endothermy in which the metabolism is always
revved up. Contrast this with the myriad of other endothermic critters
out there that use muscle power to become endothermic and only do so
when they need it). The presence of filamentous integument on some
dinosaurs (and full on feathers in others) has recently been cited as
a good proxy, but it is a just-so assumption with no real empirical
basis (i.e., endothermy requires insulation, but insulation hinders
ectothermy). The closest test of this came from a paper in 1958 by
Raymond Cowles. The paper briefly mentioned a student experiment
involving placing lizards in a crude covering of mink (spared no
expense there). The paper doesn't list much in the way of materials
and methods. It doesn't cover anything on acclimation period between
trials (how long were the lizards allowed to get used to their new
coats), or really how long the trial was performed. Hell, the paper
doesn't even deal with this study. It just mentions it in passing. The
other problem with the integument argument is that we only have two
vertebrate groups that have filamentous integument today. That's two
data points or a line. It might be a requirement or it might be a
coincidence. If we expand our search out to other animals with
integument we can incorporate arthropods. If we do that then the
support of integument and endothermy weakens (e.g., tarantulas aren't
endothermic, nor are fuzzy geometer moths). Seebacher (2003) used a
mathematical model to show how even an ectothermic, 3.8 kg,
_Sinosauropteryx_ could maintain thermal stability in a cool
environment. There also cases of observed ectothermy in roadrunners
and vultures. Both taxa are known to sun themselves in the morning to
get their metabolisms up and running. That filamentous integument is
dynamic and adjustable (rather than a flat mink coat) no doubt plays
an important role in how these animals are able to get past the
thermal barrier caused by their feathers.

Many other proxies (growth rate, activity levels, limb ratios) are
based on the aerobic capacity model for the origin of automatic
endothermy (Bennett and Ruben 1979). The argument goes that basal
metabolic rate (the minimum amount of energy necessary to survive) is
intrinsically linked to how active one can be. Thus the more active
one is the higher the standard/basal metabolic rate needs to be to
match these needs. Despite the popularity of this hypothesis (and it
is BY FAR the most cited origin for automatic endothermy) it does not
have a lot of empirical support. Not to get too off track about this
but in brief: the organs responsible for increasing endurance (heart,
lungs, skeletal muscle) are not the same organs that are responsible
for the majority of our metabolic rates (intestines, liver, kidneys,
brains). Studies that have bred mice with higher endurance capacities
have found no concomitant increase in basal metabolic rate (Gebczynski
and Konarzewski 2009) whereas mice bred for low basal metabolism were
actually found to have higher endurance capacities (Ksiazek et al.
2004).

Getting back on track, in the past forty years we have learned much
more about how dinosaurs lived and looked. We have gained a better
understanding of their potential behaviours, and as a side-effect of
wanting to know about their metabolisms, the field of comparative
physiology has actually learned a lot about how complicated and
variable metabolic rates (and thermophysiology) are. Unfortunately
none of this has brought us any closer to knowing what kind of
metabolism dinosaurs had. It is generally accepted that dinosaurs were
diverse enough that one size did not fit all. Thus there was likely a
spectrum of metabolic regimes employed throughout the Mesozoic. It's
also becoming increasingly more accepted that the biggest differences
between "cold-blooded" animals and "warm blooded" animals occur at the
small body sizes. Once one reaches the size of your average dinosaur
those differences become vanishingly small. So a "cold-blooded"
_T.rex_ probably acted near identically to a "warm-blooded" _T.rex_.

Lastly I think it's worth keeping in mind that the data also aren't
there for cold-blooded mosasaurs, or warm-blooded therapsids. This is
not a problem limited to dinosaurs. It's true for all prehistoric
life.

Jason


References

Bennett, A. and Ruben, J. 1979. Endothermy and Activity in
Vertebrates. Science. Vol.206:649-654.

Cowles, R.B. 1958. Possible Origin of Dermal Temperature Regulation.
Evolution Vol.12(3):347-357

Gebczynski, A.K., Konarzewski, M. 2009. Metabolic Correlates of
Selection on Aerobic Capacity in Laboratory Mice: A Test of the Model
for the Evolution of Endothermy. J. Exp. Biol. Vol. 212:2872-2878

Ksiazek, A., Konarzewski, M., Lapo, I.B. 2004. Anatomic and Energetic
Correlates of Divergent Selection for Basal Metabolic Rate in
Laboratory Mice. Physiol. Biochem. Zool. Vol. 77(6): 890-899

Seebacher, F. 2003. Dinosaur Body Temperatures: The Occurrence of
Endothermy and Ectothermy. Paleobiology. Vol.29(1):105-122

Wu, B.J., Hulbert, A.J., Storlien, L.H., Else, P.L. 2004. Membrane
Lipids and Sodium Pumps of Cattle and Crocodiles: An Experimental Test
of the Membrane Pacemaker Theory of Metabolism. Am. J. Physiol. Regul.
AIntegr. Comp. Physiol. Vol. 287:R633-R641

http://reptilis.net

"I am impressed by the fact that we know less about many modern
[reptile] types than we do of many fossil groups." - Alfred S. Romer