Endothermy vs Ectothermy (was Re: dermal structures)

[twilliams@canr1.cag.uconn.edu]

> 
> << Just read Tom Hopp`s mention of "one time" evolution of dermal scutes,
> follicles etc., I myself have a strong feeling about a once only development
> of warm-bloodedness, and that being mainly for the incubation of the young in
> a colder enviorn. In fact, I`m looking at endothermy before the synapsid-
> diapsid split. (I don`t believe we should place diapsid beginnings way after
> synapsid development based upon the scant fossil record of early diapsid
> forms).>>

I've never thought of endothermy as an all-or-nothing condition, but 
as one end of a physiological spectrum.  The ectothermy exhibited by modern 
reptiles is at one end of the spectrum, mammalian and avian 
endothermy at the other end.  Among modern endotherms I believe it 
is the case that certain mammals are _more_ endothermic than others.  

Small mammals (such as mice and shrews) have a greater dependence on 
internally-generated heat than big mammals (such as elephants and 
hippos).  The higher surface area-to-volume ratios of small mammals 
mean that body heat is more easily lost through dissipation into the air.  
Hence the presence of insulation (fur - large mammals generally have 
less hair) and the voracious appetites of small mammals (a shrew eats 
more per unit body weight than an elephant does).  Big mammals can 
better exploit ambient heat to raise and maintain their body 
temperature.  

Big dinosaurs (such as sauropods and larger ornithopods) should
have been able to retain a moderately high and relatively constant body
temperature not through endothermic generation of heat but largely
because their sheer body mass (and low surface area-to-volume ratios) 
meant that heat was not easily lost from their bodies.  This is the principle of
mass homeothermy.  Small dinosaurs (like coelurosaurs and small
ornithopods) did not have this luxury - hence the presence of insulatory
integumentary structures in small theropods.

No way will I believe that endothermy only evolved once among 
amniotes.