MRs OF SLOTHS etc

[darren.naish@port.ac.uk]

Some quick responses to David's recent terramegathermy opus (though 
I regret that I don't have time to read through the whole thing)...

> ...Migrators must be tachymetabolic. 

As a generalisation this is nonsense, seeing as gecarcinid crabs, some 
rattlesnakes, monarch butterflies and a million other bradymetabolic 
animals migrate. 

> while populations of megamammals are
> small and unstable and can't easily be refilled by reproduction.

Unfortunately I don't have the literature with me, but some historical 
elephant populations have been huge. 'Can't easily be refilled by 
reproduction'? - you MUST read Clive Spinage's _Elephants_ (Poyser 
Natural History) for detailed discussion on this. Even megamammals 
can breed fast and young and produce populations that cannot then be 
supported by their environment (hence the proposed 'boom and bust' 
model for elephant populations).

> Desert elephants never overheat, so HiMR sauropods shouldn't have
> either. Therizinosaurs, and maybe early dinosaurs with small ilia,
> probably had InMRs like giant sloths. 

I wasn't aware David Oren and colleagues had finally captured a live 
mapinguary - have they published? Sarcasm aside, how do you know 
anything about the MR of ground sloths? Extrapolation from extant 
sloths might be reasonable but is obviously speculative. And BTW 
therizinosaurs hardly have 'small ilia' (nor, sensu Colbert, are they 
brachiliac).

Don't get me wrong, I have no opinion on dinosaur metabolism either 
way. It's just that we're on very dodgy ground if extrapolations on the 
MRs of fossil animals are based on equally dodgy assertions or 
extrapolations about living ones.

DARREN NAISH 
PALAEOBIOLOGY RESEARCH GROUP
School of Earth & Environmental Sciences
UNIVERSITY OF PORTSMOUTH
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