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Re: Dinosaurs vs. mammals: a hypothetical scenario



> 2011/4/8 Raptorial Talon <raptorialtalon@gmail.com>:
>
>> I can't pin down a source right now, but consider: at the same limb
>> lengths and the same range of flexion, a quadruped will always have a
>> slight stride advantage purely because one pair of limbs is situated a
>> distance in front of/behind the other set. A biped, with only one pair
>> of limbs, has to rely entirely on the reach of those limbs; it cannot
>> use a pair of forelimbs to reach beyond the flexion of the hindlimbs.

Now I get your point and get there is already a further quadrupedal
advantage in addition to spine/ scapular flexing, namely that if the
strides of both limbs occur at different moments (as in
near-symmetrical fast gaits in mammals), the total stride increases at
equal limb lenght.
However, this may mean a proportionally smaller number of cycles per
limb, because a limb pair would have to be kept inactive for more than
just the suspended phase. And, a greater number of cycles also
increases velocity.
In addition, if the fore and hindlimbs propel the body without help of
the other pair, they have to be as proportionally powerful and massive
as the single pair of propulsive limbs in the ostrich, and this seems
to imply a greater inertia to be dealt with in the quadruped while
accelerating.
Thus, I am not sure if we can say in principle the maximal velocity of
quadrupeds have to be greater than in bipeds. I think there is no much
empirical proof for this in practice either.
We are biased towards comparing the fastest biped and quadruped,
namely cheetah and ostrich, but we have to take into account that the
ostrich is not the same size as the cheetah, but larger, and mass can
have something to do.
A better comparison would be with a running bird in the size range of
the cheetah, or a quadruped in the size range of the ostrich. It may
well be that at smaller size the quadruped has and edge and at bigger
sizes the biped does? (After all, bipedal dinosaurs in the size range
of Recent hippos and rhinos seem to be faster than these because of
the tendence to relative limb shortening in large cursorial mammals).