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Re: Avian endothermy: birds do it differently from mammals

--- On Mon, 7/20/09, GUY LEAHY <xrciseguy@q.com> wrote:

> From: GUY LEAHY <xrciseguy@q.com>
> Subject: Avian endothermy: birds do it differently from mammals
> To: "Dinosaur Mailing List" <dinosaur@usc.edu>
> Date: Monday, July 20, 2009, 1:07 AM
> DMLers,
> Important new work:
> http://jeb.biologists.org/cgi/content/abstract/212/15/2328
> This research suggests a couple of interesting items. The
> molecular
> basis for endothermy in birds appears to differ from
> mammals, thus the selective pressure which drove endothermy
> in both groups may not have 
> been the same.  The coupling of avian metabolic heat
> production with
> oxidative metabolism would, at least in birds, support the
> "aerobic 
> capacity" hypothesis of Bennett and Rubin (1979) in
> Science, where they 
> proposed that selection for increased stamina resulted in a
> corresponding
> increase in resting metabolic rates.  


The paper is definitely interesting (a tad acronym heavy for my tastes, but 
then what molecular paper isn't?), but it doesn't really support the aerobic 
capacity model. 

The researchers found a high degree of mitochondrial oxidative capacity, but 
they also found a concomitant increase in mitochondrial decoupling (which would 
make that ATP unavailable for other activities). 

Of the 3 main hypotheses for the evolution of endothermy - McNab's "hot is 
good" hypothesis, Farmer's "parental care" hypothesis, and Bennett & Ruben's 
"aerobic capacity" hypothesis - Water and Seebacher found their data to agree 
most with McNab's often underrepresented hypothesis. 

The author's wrote:

"It [aerobic capacity] posits that greater capacity for oxidative ATP 
production led to selective advantages by allowing more sustained levels of 
activity. Heat production would have been a corollary of increased metabolic 
capacity without being directly selected for. In chickens however, the 
increases in oxidative capacity (state 3 respiration rate) are accompanied by 
increased unc
eased cellular ATP demand, both of which reduce the ATP available for activity. 
It is unlikely, therefore, that the advantages of increased activity or any 
other single initial selection pressure led to modern endothermic phenotypes 
(Kemp,2006). " 

The big thing that birds do differently, is that they don't use uncoupling 
proteins to uncouple Na+/K+ pumps and mitochondria (they use adenine nucleotide 
translocase instead).