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Re: no easy (mammal) answer



Jeffrey Martz wrote:
> 
>      Baby mammals and cold acclimated mammals have one edge over
> endothermic birds in this regard; Brown adipose tissue or "brown fat" is a
> tissue present in the above mentioned mammals but absent in birds, whose
> apparant only function is to provide heat.  I know it has been suggested
> that brown fat may have given mammals some kind of edge over birds after
> the K-T extinction.  I am surprised that it hasn't been brought up
> in these threads before.
> 

When a human is that tiny, being capable of very high metabolism
increases its chances of survival.  Molting in winter shows that a hen,
too, can maintain impressively high metabolism when necessary.

Whether the fuel is baby fat or chicken feed, it seems that the
requirement for oxygen must be proportional to metabolism.  Because
small endotherms require higher metabolism than large ones, their lungs
must maintain a higher gas-exchange rate, relative to their mass.

There were plenty of moose-sized dinosaurs but no mouse-sized ones. 
Could it be that dinosaurs could not evolve the high metabolism needed
for a tiny animal?  

Could respiration have been an evolutionary snag?  There seems to be
room to speculate.  Apparently, the nasal passages of dinosaur fossils
look different from those of birds and mammals.  Some dinosaurs had
hollow backbones, like part of a bird's respiratory system.  It's not
clear that dinosaurs had RTs, like birds and mammals.  

One might think that inhaling fine droplets of water would quickly clog
the microscopic air-sacs in mammal lungs.  Somehow, the lungs of mammals
and birds are so resistant to spray that some are even built to pursue
prey underwater.  

It seems nobody has ever found a dinosaur fossil built to do that.  If
no dinosaur ever found a niche as an aquatic predator, perhaps it was
because the dinosaur respiratory system was different in a way that made
it more vulnerable to aspirated spray.

If dinos breathed differently from mammals, the relative advantages may
have been like those of diesel and gasoline engines.  The existence of
gas engines prevents diesel vehicles from getting very small, and the
existence of diesel engines prevents gas vehicles from getting very
big.  Well, more or less.

- Stephen Throop