Re[2]: pectoral muscles

[ccp@okway.okstate.edu]

In my self-appointed role of Comparative Physiology Cop, allow me to correct a 
few mistatements and misconceptions in the posts repeated below.
1. White meat in chicken is pale in color not because of lower hemoglobin 
content, and not because of a reduced blood supply, but because of lower 
myoglobin content (and also lower mitochondrial density). Myoglobin is an 
oxygen-binding protein similar to hemoglobin, but it is found inside muscle 
cells themselves, where it helps to unload oxygen from blood to muscle cell, and
also stores some oxygen within the muscle for later use.
2. Chicken pectorals are low in myoglobin (and mitochondria) not because of 
disuse atrophy, but because they are composed of specialized muscle cells called
white glycolytic fibers. This type of muscle tissue is specialized for rapid, 
short-term bursts of power output that is metabolically supported by anaerobic 
pathways (don't require oxygen; therefore don't require myoglobin or 
mitochondria). Chickens have white pectorals not because they don't use them 
much, but as an adaptation to what they use them for: quick bursts of flight for
escape purposes.
3. Such quick bursts actually require MORE power (mechanical energy per unit 
time) than sustained forward flapping fight. The tension and strain on the 
muscle attachments are likely to be HIGHER for such burst flight than for 
sustained forward flight. White muscle generates more power than red muscle 
(because anaerobic pathways supply energy faster, and because without 
mitochodria taking up space, contractile muscle fibrils are more densely 
packed). Thus white muscles are not merely "sufficient," they are _better_ 
suited to certain functions than red muscles. However, anaerobically supported 
power output cannot be sustained for more than a couple of minutes; there's the 
trade-off.
4. According to the most recent attempt to collate the data, Galliform birds as 
a group do not have notably low resting metabolic rates. Even if they did, the 
metabolic rate could not be _caused_ by the anatomy of the ventilatory apparatus
(metabolic rate is caused by cellular activities that use energy and generate 
heat). Ventilation must be sufficient to support the maximum sustained rate of 
oxygen consumption which, in contrast to resting metabolic rate, _is_ relatively
low in Galliformes. 

OK,
CC Peterson


______________________________ Reply Separator _________________________________
Subject: Re: pectoral muscles 
Author:  <m_troutman@hotmail.com> at SMTP
Date:    6/12/98 2:46 PM


<<At first glance, it is a logical conclusion. But then I thought about 
chickens. Galliform birds like chickens have large pectoral muscles like 
most other birds but are generally weak flyers (except for migratory 
quail) as they spend much of their time on the ground and fly in short, 
quick bursts. Anyone who savors chicken breasts can appreciate the white 
meat of the pectorals, which is the result of a lower hemoglobin content 
than, say the legs.>>
     
According to Terry Jones, it is possible that the galliform resting 
metabolism is lower than most other birds, presumeably because of the 
incised sternum which would mean that the airsacs are not being 
ventilated to the same as other birds.  The same is maintained for 
tinamous.  The deeply incised sternum may also preclude very strong 
forces being exerted by the pectoralis and supracoracoideus.  The low 
hemogobin count is probably due to many factors, the most obvious is 
that the "disuse" of the forelimbs (when a muscle is not used the blood 
supply to it diminishes, hence making it "white").  
     
<<So now the question is raised. Are large muscle attachments on bones 
sufficient evidence to say that the associated muscles could be used 
strenously over a long period of time? Or is this the wrong way to 
approach it? Titanis may have had large pectoral muscles, but perhaps 
they weren't high in hemoglobin. Would such muscles be sufficient to 
allow Titanis to swipe at its prey and pin it down? If we envision a 
scenario between Titanis and a pronghorn antelope (the scene depicted in 
the article), Titanis would be swiping at the antelope, taking stabs with 
its large dagger-like main claws and trying to grasp unto its victim by 
pinning it between the large and small claw. Such a death struggle would 
hardly have been a long, protracted event, so perhaps Titanis didn't need 
"hemoglobin rich" breast muscles. Maybe "white meat" muscles were 
sufficient for the job.>>
     
The large muscle attachments are indicative of strong muscles, in 
galliforms the muscle attachments are comparatively weaker than other 
birds (with the exception of Megapodidae and Quercymegapodidae).  As I 
have noted above the low hemoglobin count is due to the "disuse" of a 
body part (this counts only for bird muscles as far as I know).  What is 
seen in phorusrhacids is a rearrangements of the flight muscles that they 
inherited to a musclature that would be advantageous to a predatory 
animal.  
     
>PS. Where can I get a detailed article about Robert Chandler's work >on 
Titanis?
     
He has something coming out soon in JVP.  And he published some thing in 
a Florida museum journal (I lost the ref).
     
Matt Troutman
     
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