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D. Orr argues that the muscular contraction associated with erect limb
posture forces the presence of endothermy. This argument is dubious. There
are a few sprawling reptiles - a predatory teiid comes to mind - that spend
many hours per day walking hither and yon, without frequently laying on their
bellies to rest. Clearly they are using constant muscular contractions as
they stand and walk, yet their metabolic rates are very low. Also remember
that the cost of locomotion is independent of limb posture, so erect posture  
does not force high metabolic rates for this reason. 

As I have explained earlier, the link between limb posture and metabolism is
probably a result of walking speed. The constantly moving reptiles mentioned
above move VERY SLOWLY, at well under 1 km/h. This is because their typically
pathetic aerobic exercise metabolism cannot sustain higher walking speeds.
The stable gait associated with sprawling legs is well suited for walking
slowly. The pendulem action and narrow trackways associated with erect limbs
tend to force walking speeds to be above 2 km/h. Such speeds are well beyond
those that can be powered by reptilian aerobics. Ergo, because animals with
erect legs tend to walk fast, they need to high aerobic exercise capacities
high enough to move as fast as their pretty little legs tend to carry them.

Something I am not clear on is this. Anaerobic power is inefficient in that it
costs about ten times as much food as aerobic power to do the same amount of
work. Does this mean that an animal that uses anaerobic power to do a given
amount of work has to yet ten times as much as an animal that uses aerobic
power to do the same work, or does the energy dept work out the same after the
entire metabolic process is complete? Please answer this only if you are
certain of the correct answer.