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Re: Metabolic rate



Adam,

     Temperature does have an effect on the rate at which most metabolic 
reactions take place.  Indeed, one of the largest advantages of elevated body 
temperature is that everything "goes faster" (to a point; many organic 
molecules are sensitive to overheating in regards to efficiency).  Elevated 
body temperatures therefore are necessary, but not sufficient, to make a high 
RMR endotherm.
     There are a bunch of factors at work here.  Although low RMR animals can 
do a surprisingly good job of maintaining a constant temperature, they cannot 
"count" on being able to regulate their temperatures at all times.  Also, they 
usually do not keep their core temperatures as high as high RMR animals.  These 
two facts mean that their enzymes, (etc.) are generally adapted for a wider 
(and lower) thermal range.  The trade off is efficiency.  
     So to begin, low RMR TTs (terrestrial tetrapodsâthis may apply to other 
organisms as well, but I canât say for sure), low RMR TTs have metabolic 
pathways that are generally less efficient.  Also, a low RMR bars you from 
having high aerobic exercise capacity.  Low AEC means smaller foraging ranges, 
so in low RMR TTs natural selection does not âfine tuneâ their molecular 
machinery for quick metabolic turnover and fast growth; instead their 
metabolisms must cope with a wide array of thermal conditions and caloric 
intake rates.  Like most everything in evolution, there are tradeoffs, and 
maximizing one goal (wide tolerance) usually compromises others (quickest 
turnover).
     Artificially raising and maintaining (the last is very important) a low 
RMR TTs body temperature and providing it with abundant calories will increase 
growth rates, but still not to the levels seen in most birds and mammals.  
Forcing the same animals to also exercise regularly will make the bone 
histological sections look even more like high RMR TTs, because many bone 
processes (e.g. secondary osteon production) play a role in recovering from the 
micro-fractures in bones that are characteristic of active, high RMR high AEC 
TTs.
     One thing I would point out here.  According to recent calculations on 
dinosaur growth, many smaller dinos grew at moderate speeds compared to 
placentals and ratites.  So it may be that most dinosaurs were in the 
mid-mammalian range (e.g. 30-50 kcal / kg^.75), which also coincides with the 
low avian range (kiwis).  Big dinosaurs grew at higher rates, so ironically I 
think the growth data supports the idea that the biggest dinosaurs may have 
been the best candidates for placental-level metabolic rates.  Why would this 
be?  Mammals were very small for a long time during the Mesozoic, so it may be 
that the mammalian emphasis on strictly maintained temperature ranges may be an 
artifact of their evolutionary past.  Itâs entirely possible (though I admit 
speculative) that because dinosaurs were consistently medium to large (though 
not always!), that inertial homeothermy relaxed the selective pressure to 
develop as strict of temperature controls.  In this case, they would ha!
ve had high RMRs, high AEC, but alowed their body temperatures to vary more 
(though still less than extant ectotherms).

Cheers,

Scott

P.S.  I'll try to get you a list of good references in the next day or so.  I'm 
taking 18 credit hours, teaching, and freelancing at the moment, so I 
appreciate everyones patience with my day-late responses.