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Re: Bipedality



On Oct. 22, George Olshevsky wrote:

>No, it's the transition to an erect stance that decouples the rapid
>sprawling slither from interfering with the action of the lungs.

        Actually, the evolution of erect posture probably had little to do
with improving lung ventilation during locomotion.  Semi-erect alligators
can effectively ventilate their lungs during exercise (Van et al 1995), and
even sprawling lizards do not seem to incur a posture-related mechanical
constraint on lung ventilation during locomotion (Wang et al 1995).
        Interestingly, ventilation frequency in nearly all terrestrial
mammals, including fully erect ones such as horses, dogs and kangaroos, are
tightly phase-locked to stride (or hopping) frequency, with a breath/stride
ratio of 1:1.  This synchronization does not vary even during substantial
changes in metabolic demand, such as running uphill, and compensation occurs
by increases in tidal volume, arterial oxygen content or oxygen extraction.
Humans are the only terrestrial mammals that can disassociate breathing
from stride frequency.
        It's difficult to think of any clear advantage bipedalism would
have.  For example, bipedalism does not enhance running speed, endurance or
agility (Djawdan, 1993; Djawdan & Garland, 1988) in bipedal vs. quadrupedal
rodents, and the cost of locomotion between bipeds and quadrupeds is not
different. Some authors (Christian, Horn & Preuschoft, 1994) have suggested
biomechanical constraints have played a role in the development of 
bipedalism, though (in my view) without a great deal of evidence.

REFERENCES
        Van, H., Kostamaa, H., Wang, T. & Hicks, J. W. (1995).  Alligators
ventilate during exercise.  American Zoologist, 35 (5), p. 34A.
        Wang, T., Carrier, D. R. & Hicks, J. W. (1995).  Ventilation and gas
exchange in exercising lizards.  Physiological Zoology, 68 (4), p. 91.
        Christian, A., Horn, H. G. & Preuschoft, H. (1994).  Biomechanical
reasons for bipedalism in reptiles.  Amphibia-Reptilia, 15, pp. 275-284.
        Djawdan, M. (1993).  Locomotor performance of bipedal and quadrupedal
rodents. Functional Ecology, 7, pp. 195-202.
        Djawdan, M. & Garland, T. Jr. (1988). Maximal running speeds of
bipedal and quadrupedal rodents.  Journal of Mammalogy, 69 (4), pp.
765-772.
        Alexander, R.McN. (1993). Breathing while trotting. Science, 262,
pp. 196-197.
        Bramble, D.M. & Jenkins, F. A. Jr. (1993). Mammalian locomotor-
respiratory integration: implications for diaphragmatic and pulmonary
design.  Science, 262, 235-240.

Guy Leahy
n9435712@henson.cc.wwu.edu
Western Washington University
Dept. of PEHR
Bellingham, WA 98225