Re: Reptiles NOT anaerobic

["King, Norm" <nking.ucs@smtp.usi.edu>]

Sorry, folks, I deleted this message from Douglas Orr before saving any 
of it electronically for a response, so I can't regurgitate much of the 
original text.

Orr is right in asserting that living reptiles would rather be aerobic 
than anaerobic.  And the problem is, indeed, that their cardiopulmonary 
system is inadequate to supply oxygen at a high enough rate to fuel high 
levels of muscular contraction.

However, that is just the problem.  Reptiles can do fine using aerobic 
metabolism when lying around on their bellies.  When they have to do 
something that requires exertion, the need for ATP quickly exceeds their 
ability to produce it, and they switch to anaerobic metabolism to make up 
the shortfall.  Anaerobic metabolism produces lactic acid as a 
by-product.  The rapid build-up of lactic acid results in the feeling of 
fatigue (I presume reptiles would feel it--humans can) and loss of 
ability for muscular contraction.  That's why reptiles can't run very 
far.  That's also why very large animals like dinosaurs, which must have 
expended large amounts of energy just walking around, probably had 
cardiopulmonary systems adequate to supply enough oxygen to fuel elevated 
rates of aerobic metabolism.  Whether those elevated rates reached 
anything near to what mammals and birds do is another matter.

Even endotherms switch to anaerobic metabolism when they have a sudden 
need for a large amount of energy.  Joggers can run all day when they 
stay "within themselves," but sprinters cannot sprint very far because 
they exceed their aerobic limit, switching to anaerobic metabolism and 
becoming fatigued the same way lizards may when they are merely walking 
around. 

The statement "The generation of the majority of body heat in
endotherms is not a result of aerobic metabolism (which also occurs in
ectotherms) but is, instead, a result of muscular contraction" (that
is an accurate quote from Orr's posting) may be misleading.  Aerobic
metabolism is only about 40% efficient.  The other 60% of energy
generated is given off as waste heat.
 Thus, even when animals are totally inactive, they are producing body 
heat.  My point is to emphasize that aerobic metabolism DOES produce 
excess heat that contributes to the elevated temperatures of endotherms.  
However, Orr may be right in asserting that this is a minority of the 
heat that endotherms produce, and that ectotherms produce body heat the 
same way--he seems to know the numbers better than I.

But it is not just a rumor that living reptiles rely primarily on 
anaerobic metabolism for muscular activity.  As noted above, they RELY on 
anaerobic metabolism for doing anything except just sitting there, or 
perhaps leisurely feeding or casually slithering down to a water hole.


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Norman R. King                                       tel:  (812) 464-1794
Department of Geosciences                            fax:  (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712                      e-mail:  nking.ucs@smtp.usi.edu