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Re: _Discover_ article



On Wed, 4 Dec 1996, guy leahy wrote:

> evolved to reduce rates of respiratory water loss because of high ventilation
> rates in endotherms sounds logical on the surface.  However, there is no 
> direct experimental evidence which demonstrates that the nasal cavity 
> actively regulates water loss.  A reduced exhaled air temperature 
> certainly conserves water, but it also conserves heat, and it could be that
> a reduction in respiratory water loss represents an entirely passive and
> unavoidable consequence of active heat conservation (Hill, 1978).  
> Lizards can conserve respiratory water, even though they lack RT, as do 
> whales, who exhibit extremely low rates of respiratory water loss
> (Kasting et al 1989). For example, if the nasal cavity actively controlled
> water loss, one would expect that heat-stressed, dehydrated mammals would
> have reduced respiratory water loss relative to hydrated ones, yet this 
> does not appear to be the case (Schroter et al 1987).

        Now that's a thought... the thing to do would be to look at 
animals with various lifestyles- say, polar bears, saiga and musk-oxen on 
the one hand, and animals that live in dry conditions on the other- 
camels, elephants, kangaroos- and see who has bigger RT structures. I 
know the saiga has a truly elaborate nasal setup which is supposed to 
warm up incoming air (the look sort of like they've got tapir-style 
trunks, I believe). 
        And perhaps yet another thing to do would be to look at animals 
like river otters, hippos, desmans and see how they compare. Water loss 
presumably would be of minimal importance to freshwater animals.
        You know, that might make more sense, the heat loss bit. I mean, 
if it were there to conserve water, shouldn't it be in things like 
tortoises and desert lizards- some of which don't even drink, but take in 
their water from food. It would be predicted in this case to be much more 
elaborate in small creatures (hummingbirds, shrews) than large ones which 
do not need to conserve heat (elephants, rhinos). 

        nick L.