Re: Warm or Cold ~ My View

[carey@lerc.nasa.gov (Charlie Carey)]

Stang1996@aol.com (Peter Buchholz)said in an earlier post:

>{SNIP}

>    Another thing I like to point out about mass-homeothermy is its 
>assertation that sauropods (and big dinosaurs in general) can't be 
>warm-blooded because they would have overheated.  What!?  Being 
>warm-blooded would have been _absolutely_ what they wanted.  
>Think of it theis way.  It's 90 degrees F outside, nice and warm; but 
>Mr. Diplodocus' temperature is 99 degrees F.  What is heating up 
>what?  Think about it for a minute, according to thermodynamics, Mr. 
>Diplodocus is heating up the air, not the reverse.  
Not necessarily so. Both the air AND Mr Diplodocus may be heating up.

I usually keep quiet and lurk because paleo is not my expertise, just an 
interest and I use the list as a way to learn. However, this one did step on 
my turf: thermodynamics.

The example above would be correct if Mr. D. was a lump of metal or possibly 
a bag of water, but he was not. 
        A. Heat is generated primarily in the muscle mass and must flow 
generally outward to the cooler environment. This requires a continuous 
temperature gradient from warm to cool which is dependant on the amount of 
heat flowing, the material in which it flows and (in living creatures) the 
rate of blood flow. The result of this is that a non-sweating human can 
easily overheat at 70F (brain temperature over 106F).

>    Well then, say it's 106 degrees F now.  Would Mr. Diplodocus get 
>overheated now?  
>
        B. Endotherms have at least two mechanisms for body temperature 
control: one to warm up and one to cool down. In humans muscle activity and 
digestion warm us up and heat transport from the body interior to skin and 
lungs and sweating cool us down. By controlling the activity rate, blood 
flow rate and transpiration rate our bodies maintain a reasonably constant 
internal temperature.
        C. Mr. or Ms. D., if they were endotherms, had to have these 
characteristics: 
                1. the ability to increase their temperature by muscle 
activity, a vigorous life style or something like shivering.
                2. The ability to cool down by decreased activity, sweating, 
panting or large internal/external surfaces with good blood circulation to 
act as heat exchangers such as air sacs.
                3. A control system that permits temperature control by 
changing the rate of blood flow through the skin, mouth, tongue, lungs or 
air sacs. Or, an adjustable insulation like feathers or hair.
        D. Large bodied animals have a harder time using their skin as a 
heat exchanger. As the body size increases the volume to surface ratio 
decreases linearly with the body dimension. (1/r) This means a sweating 
elephant is not as efficient in cooling itself as a sweating pig because the 
area to sweat from is relatively smaller. This also somewhat true of panting 
animals because the size of the lung is related to body size, but the 
relationship is weaker because the respirarion rate and exposed lung/air sac 
surface can be increased proportionally to body volume.

Think of it this way.  Get a one pint saucepan and 
>fill it with water and put it on the stove; then get a 10 gallon spaghetti 
>pot and fill it with water and put it on the stove on high.  Who is going 
>to boil first?  Exactly.

Not a good analogy to animal metabolism. A better one would be to mix baking 
soda and vinager in the pot and saucepan (in the same ratio). Since this is 
an endothermic reaction the heat is generated throughout the volume of the 
containers, as it is in animals. In this case I think you'll see the bigger 
pot gets hot in the center faster. 
        The mass of the animal does damp the rate of temperature change but 
when the heat is generated volumetrically it can increase internal 
temperature faster in a large body faster for _a comparable level of 
activity_. This is one of the problems seen in acutely obese people who try 
to keep up with people of normal weight.

I hope this helps. The problem of temperature variability  in endotherms is 
not as simple as comparing thermal capactances (mass x specific heat).

Thanks,

Charlie Carey

SMTP mail => carey@lerc.nasa.gov
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