Re: ERECT LEGS, WALKING SPEEDS & METABOLICS (was naked mole

[ccp@okway.okstate.edu]

GS Paul wrote:

>To clarify some points concerning the link between limb posture and 
energetics. 
     
     {several links in the chain of reasoning snipped}
     
     Only animals with high aerobic scope (these days birds and mammals) 
     can sustain walking speeds over 1-2km/h. This is true regardless of 
     limb form or body mass. No reptile has been shown to do so (even the 
     most aerobically capable monitors). The teiid lizards that stand and 
     walk for many hours move at only a fraction a kilometer per hour. 
     
     ...Long erect legs work under a strong pendulum effect. They are 
     therefore ill suited for slow walking, and tend to force walking 
     speeds to be above 3 km/h. 

..Because long erect legs probably force land walking speeds to exceed 2 
km/h, and reptilian aerobiosis cannot sustain such high speeds, the evolution 
of erect legs probably forces aerobic scopes to be elevated above the 
reptilian level. 
     
Because dinosaurs had long erect legs, and because trackways show that they 
almost always walked faster than 3km/h, they should have had an aerobic 
capacity above that observed in reptiles.<

My response:
Thanks for this very clear and logical presentation of the argument. I 
certainly agree that selection for enhanced locomotory performance could 
plausibly lead to the evolution of increased aerobic capacity, and that it 
could also lead to the evolution of erect posture. I am still skeptical about 
a direct link between the two ("forcing"), and here's why. Missing from these 
arguments is the effect of scaling. How can you use an absolute velocity 
(e.g. 3 km/h) as a comparative for animals of such widely differing body 
sizes as extant teiids and sauropod dinosaurs? Stride length obviously 
increases with increasing body size; stride frequency decreases but not as 
much. Therefore their product (velocity) increases with body size. Normal 
realized walking speed scales positively with body mass, and maximal aerobic 
speed almost certainly does too (though I can't find any documentation at the 
moment). Therefore a really large animal can maintain a given absolute 
velocity with a relatively smaller increase in energy expenditure. So it 
seems to me that a REALLY big animal, with very long legs, ought to be able 
to move rapidly (e.g. 3 km/h) by virtue of its size alone, without requiring 
a high aerobic scope. Have I missed something?
        Also, as I have previously argued, erect posture and columnar legs 
are a likely prerequisite for really large body mass, for engineering support 
reasons regardless of locomotory considerations. In other words, large body 
size "forces" erect posture no matter how fast the thing moves or can move, 
and regardless of its metabolic rate. 
        Two reasons why, when the all-important variable of body mass is 
factored in, the posture/metabolic rate link is not persuasive. To me.
        And, by the way, I agree that chameleons are so specialized as to be 
a red herring; they do, however, represent a counterexample to any strong, 
necessary link between limb posture and metabolic rate even in small animals.
OK,
CC Peterson