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Re: questions about the Odontochelys study

Augusto Haro wrote:

If both are fast, then it may be that velocity did not need the
punting. Alternatively, perhaps punting was a further refinement in an
already fast clade

Punting is unlikely to be an adaptation to increase maximum speed. Instead, it is an efficient and reasonably rapid way to move along the substrate of a water body without free swimming. It appears that punting occurs in turtles as a means of transport for large, bottom-living, ambush predators that are poor open-water swimmers. There has not, to my knowledge, been any good work to demonstrate the conditions under which punting is most advantageous, but I suspect cluttered environments and shallow water would be prime cases. It may be that walking on the bottom incurs a lower cost of transport than free swimming for an animal whose hunting strategy involves sitting on the substrate to begin with.

Ok., punting beneficiates from limb lenght, granted. But what I tryed
to mean is that stride lenght is not correlated with use of
gravitational force for advance, but with excursion ranges at limb
articulations and limb lenght.

Stride length can depend on many factors, including the those you mentioned and several others. Whether or not an increased stride distance is important depends on the gait and particular benefit in question. Cost of transport and maximum speed, for example, rely on different factors.

I think that the advantages of employing gravitational potential
energy for advance should be diminished when submerged in water,
because of the greater resistance to advance (and the involved
deceleration would counter aceleration produced by gravity); the
contribution of gravity seems to me to be too slow in that medium.

Because of the buoyant forces, simple walking in subaqueous environments is indeed quite slow. This is why punting animals "bounce" along the bottom with small hops - the gait ends up as a combination of a biomechanical walk and run. The walking component benefits from a greater effective hip/shoulder height. Running components will benefit more from elastic storage. To the best of my knowledge, no one has yet quantified which type of gait bottom-walking turtles more closely approximate. The more pendulum-based the motion is, the more important gravitational potential will be.

I think the pendular-like advance/fall of the center of gravity can be
left to gravity at low speeds.

This is more or less correct, with the caveat that it applies not to absolute speed, but relative speed. That is, walking is effective at the lower end of a given animal's speed range (high efficiency), but when a gait shift occurs to a trot or run, then elastic storage potential tends to take over. Slow-speed gaits can be very important to the evolution of an animal, however, if they spend a lot of time walking, rather than running, especially if low cost of transport is important to their ecology.

Oops, I though we were talking about the more erected gait of the
matamata in relation to its velocity, when compared with that of
Hydromedusa. I think that except for really low locomotion, in the
water gravity-assisted advance would be of little contribution to
energy saving.

My hypothesis is that a more erect gait will increase the maximum walking velocity; that is, it will raise the speed limit where the turtle has to transition to a less efficient gait. Thus, while it involves velocity, the hypothesis is more related to efficiency than top speed. I suspect that matamatas enjoy a low cost of transport. It might also be worth noting that the more erect stance likely gives them better clearance of substrate obstacles in cluttered environments. It could be, in the end, that they stance change has no effect on locomotor efficiency or speed at all, but I rather doubt it.

Now a question: ok., so gravity helps advance while the body is
descending, but is not then more energy wasted in elevating the body
again, in relation to animals that move less pendularly?

Energy is spent in raising the body, but then most is reclaimed during the "falling" phase that follows. This makes pendular-type gaits (that is, walking) very energy-efficient. Again, this is based on terrestrial walking. I don't believe that subaqueous "walking" has been well-studied in turtles, so I am merely suggested some testable hypotheses, rather than making assertions.

But I think precisely that when selection is weak (that is because the
selective advantage is not great) is when it is less likely to fix
something because drift tends to distort its "achievements" in allele
frequency changes.

In both simulation and field tests, small selection coefficients have resulted in substantial adaptive changes. It does not take much. That said, the selection coefficient on some traits is probably more or less zero, in which case drift will take over.

In the case of the snapping turtle, the plastron seems to be reduced
to a cross, and if that plastron reduction was considered as related
to increase ventral movility range , we would have to hypothesize the
femur can be adducted so as to point ventromedially, towards the other
side of the body! This may thus indicate that the plastron reduction
was at least not so necessary linked to limb mobility, or if it was,
was while the reduction can really permit the leg to be more adducted,
but once the femur cannot be adducted anymore, or is no necessary for
it to do so, the further reduction is possibly non-adaptive.

Good points - so perhaps there are both adaptive and non-adaptive components (probably very common). Your comments brought to mind another potential advantage, which is that a reduced plastron may leave room to expand some muscle groups or other soft-tissue components. This model would predict differences in relative limb muscle mass between groups with large and small plastron areas.

However, non-adaptive reasons for the presence of
features can also be testable, for example, considering a character as
responding to a simple allometric transformation associated with size

To some extent, non-adaptive models can be tested. It's often tough, though. In the case of simple allometry, for example, a particular allometric exponent could be the result of adaptive trends.

At the end, we can test alternative hypotheses of adaptation, but we
cannot test if a character was adaptive or not, because there will
always be the possibility that the character is an adaptation to
something yet unknown or unimagined. Thus, you can not test if the
character was an adaptation, you can only test if it was an adaptation
to more efficiently use a resource in particular (and you can thus not
test whether or not there was selection, but if ther was a particular
kind of selection).

Absolutely. In the case at hand, I only mean to propose some potential adaptive models, not to argue that shell reduction must be adaptive. However, my suspicion is that certain forms of shell reduction do have an adaptive component.



Michael Habib, M.S. PhD. Candidate Center for Functional Anatomy and Evolution Johns Hopkins School of Medicine 1830 E. Monument Street Baltimore, MD 21205 (443) 280 0181 habib@jhmi.edu