[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Layperson question on endothermic dinosaurs

Thanks. I didn't even catch that it was truncated. Stupid Yahoo mail.


> From: Kelly Clowers <kelly.clowers@gmail.com>
>To: DML <dinosaur@usc.edu> 
>Sent: Wednesday, 15 May 2013 6:11 PM
>Subject: Re: Layperson question on endothermic dinosaurs
>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 empiric
lation, 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
es 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.
>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
>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):
i, 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
>"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