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Reply: Scaling problems in Hutchinson 2004

I notice that in Hutchinson's new studies emus have well over a quarter of 
their body mass as leg extensors, an extraordinary amount. Since tyrannosaurs 
had extensive weight reduction in order to concentrate mass in the legs they 
certainly could have similarly oversized locomotary extensors. That would be 
plus tonnes of leg muscles, able to generate immense power far beyond that 
seen in elephants which have much smaller extensors. 

Interestingly the femora of Tyrannosaurus were not consistently longer than 
those of elephants and sauropods of similar mass. This means that the knee 
extensors were not longer than those of quadrupeds of the same size, so there 
no reason to conclude that the thigh muscles would be less efficient in the 
giant biped due to being longer fibered. Nor would it necessarily be a problem 
for the ankle extensors because the muscles could be concentrated in a short, 
massive drumstick bundle. 

H notes the importance of muscle force in determining speed potential in 
animals, the evolutionary robotics simulation of course takes that into 

H notes that I have not provided measurements on the masses of muscles etc in 
horses and other living animals. Not having ready access to thoroughbreds I 
tried to talk one of the fellows who sells fruit from a horse drawn court to 
sell his beast but for some reason he abjectly refused. Seriously it is good 
that H is cutting up animals to make the badly needed measurements. There are 
some cautions. Sample animals need to be healthy within their lifestyle 
parameters. Domestic animals should not be aged or have experienced a long 
prior to death, an accident or very brief infection is preferable. Nondomestic 
animals are even more difficult. Kept under artifical conditions their physical 
condition upon death even if nominally healthy immediately prior is suspect. 
For example Alexander et al. did not publish the muscle masses from a captive 
rhino they dissected because it was clearly in such bad condition. There is 
currently a movement to remove elephants from zoos and circuses because they 
invariably in poor physical condition being barred from the extensive walking 
exercise they normally get in the wild. They suffer arthritis, foot problems 
and other conditions, and die at a younger age, so muscle mass data may be 
suspect. Superior data would be gathered by dissecting wild elephants shot 
culling operations in Africa (one reason I take the Robertson-Bullock data 
seriously, limited as it is, is because it was taken from wild bulls). That 
be a rather grim task, and logistically difficult. 

Also, the trunk muscles utilized during galloping in mammals should be 
included in the total of locomotary muscles, since the trunk acts as a fifth 
during the highest speed gait. This would not apply to elephants since they 
gallop. Also potentially important the muscles that operate the scapula in 
those mammals in which this element is mobile. Of course in bipeds these 
are not pertinent, not including them in quadrupeds understates their 
locomotary muscles vis-a-vis bipeds. 

G Paul