Warm/Cool/Cold blood

[tonyc@foe.co.uk (Tony Canning)]

>       One of the things that bothers me about the warm/cool/cold blood
> arguments relates to the theory that nature does not like to invent the
> same idea twice. 
> 
>       Baker's argument that dinos and birds are related, and that warm
> bloodedness was in their common ancestor leaves one gaping hole to me. No
> "family tree" I have even seen left any doubt that early mammals existed
> before the dino/bird branch broke off the reptile. 
>       Does this mean that warm bloodedness arose at least twice, Once
> for the dino/bird and once for mammals. Or does it mean the theory is
> correct and that dino/bird/mammals broke off of the reptile branch in
> some, as yet, unknown way?
>       Personally I think that some ancestor of the dinosaurs, birds and
> early mammals evolved "cool" bloodedness. Then the various branches
> evolved this in various ways, and to various degrees of "warm". This means
> that the dino/bird/mammal ancestor broke off earlier then we normally
> believe. But considering how "holey" the fossil record is this is very
> possible. 
> 
> Ralph Lindberg email=>dragonsl@hebron.connected.com
> more hobbies then time
> 
Endothermy (a large amount of body heat generated from internal metabolism)
has certainly involved many times independently - in several separate groups
of large insects, 4 or 5 times in fish and sharks, in at least 1 living 
turtle, in mammals, pterosaurs, birds and maybe dinosaurs.  It has been
achieved by various physiological means in different groups.

It is confusing to think of "warm" "cool" or "blooded" animals, as there 
are many different ways of achieving warm and/or constant temperatures in
both endotherms and ectotherms.  Ectotherms (heat largely obtained from the
environment) may maintain high, constant temperatures for long periods of
time. 

It is also possible for an animal to be both and endotherm and an ectotherm
- pythons are normally ectotherms, but females incubating eggs switch to
endothermy until the eggs hatch.  Pythons can also raise their metabolic
rate by a factor of at least 7 to digest large meals, which brings them
close to the mammalian metabolic level.  Some fish in the tuna family are
selectively endothermic, keeping their brains at a constant warm 
temperature but not the rest of their bodies.

Whatever we discover about dinosaur metabolism, it is unlikely to change
our view of mammal/reptile phylogeny.

Tony Canning  tonyc@foe.co.uk