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Re: questions about the Odontochelys study



 Augusto Haro wrote:


It was the one with two "barbels" in the chin, I think the name is
Hydromedusa tectifera.

Ah, gotcha. Very interesting species. Don't know of any velocity measurements for them, but they are pursuit predators and pretty strong. Interestingly enough, it may be the best living outgroup to the matamata - with makes ancestral state reconstruction for the punting stuff I was talking about before a real pain. There are a few fossil Chelidae that might be able to help (can't recall how much material is there).



Are there many velocities measured for clades of aquatic turtles?

There are a handful of decent measurements, mostly for terrestrial locomotion. Swimming velocities are known with some precision for a few trionychids, some sliders, and possible pig-nosed turtles. Cruising velocities are also known for sea turtles, at least roughly.


I do not understand why punting will increase the effective stride
lenght. I mean, I can see the correlation with limb lenght (of which I
cannot say anything with respest to the turtles we are comparing), but
at equal proportional limb lenght, and expansion of the arc of
movement in the antero-posterior sense, I can not see differences.

It's the other way around - greater limb length will tend to improve punting, to an extent. Punting is an odd form of locomotion, in that it incorporates aspects of both walking and running, from a biomechanical standpoint. To the extent that it acts like walking, there is gravitational potential utilization, and this improves with limb length. It's the same reason that maximum walking speed increases in humans with longer lower limb lengths - individuals with shorter limbs transition to jogging gaits as lower velocities than those individuals with longer limbs. Of course, part of what makes the whole thing odd is that there is also buoyancy involved, which is also a form of gravitational potential, so the calculations get a bit messy...


On a side note: if there is anyone out there interested in collaborating on some work on punting turtle kinematics, I've got a hand-raised matamata that should be willing to run in a flow tank.


For example, I am not sure that if we put a mouse and lizard of relatively
the same size, limb lenght, and excursion range, we should see the
mouse winning in velocity by being more erect.

Different sort of problem, in part because you seem to be referencing top speed rather than walking efficiency. It's also a tough analogy because the lizard introduces the effects of anguilliform motion. In the case of punting turtles, I am only suggesting that the erect stance helps because it increases the effective limb length for gravitational potential transfer, not that erect gaits are uniformly faster.



Or not, perhaps the more erect posture is a result of the prior reduction of the plastron. After all, most river turtles are predaceous and bottom dwelling (I mean, they stay most of their time in the bottom while on water than swimming, they do not pursue prey), as far as I know.

Quite a few river turtles are pursuit predators or swim in the water column while foraging. My hypothesis only refers to those few taxa that are adapted specifically to bottom walking and "running". Still, you could be correct - we don't know with certainty that the plastron was not reduced first.

I suppose the reduction is achieved in turtles by a delay in ossification instead of resorption (somebody who knows may clarify this), and this may simply imply developmental arrest for these skeletal parts.

Seems reasonable enough, but it doesn't actually speak to an adaptive versus drift-based paradigm - the aforementioned mechanism works equally well in either.


Now on selection vs. drift, I think selection must be
strong to overcome drift, if not, it is easy for stochastic processes
such as drift to dismantle the slowly-to-achieve change in allele
frequencies produced by selection. I have some difficulty in seeing
the usefulness of parasagital walking on the bottom, so as to give it
much advantage over other turtles that also dwell into the bottom with
a less reduced plastron.

Whereas I see the possibility for a distinct advantage in mobility. Just as you are correct in asserting that even modest drift can prevail when selection is weak, even a small selective advantage can fix a trait - the performance benefit doesn't have to be huge, especially if costs are low.


In any case, both hypotheses are reasonable. Testing for drift is fiendishly hard in this sort of circumstance, but a functional advantage might be tested for with some simple comparative studies of locomotor efficiency and/or velocity. The tricky bit would be finding bottom-running/punting species with full plastron development - I'm not sure there are any.


Yes, that may be. It is said that it is hydrodynamically important for avoiding turbulence in water to have a skin deformable by water (in cetaceans) or scales in fishes. I do not know if this has to be with velocity (I should think yes) or if turbulence can be reduced regardless of velocity, you may know better.

The flow regime will depend on velocity, but most turtles probably swim in a roughly similar Reynolds Number range, so the actual impact of the shell on flow will probably depend more on specific gait, limb kinematics, and agility of the animal involved (as well as substrate effects for bottom-runners). For example, trionychids are relatively fast swimmers, especially in a sprint, but they are still drag-based swimmers with substantial surge losses during swimming. A sea turtle, by contrast (as well as pig-nosed turtles, which happen to be the sister group of trionychids, interestingly enough) is an aquaflyer, and experiences substantially less surge-associated acceleration as a result.


In the first case, are
sea turtles fast?

They are relatively quick, yes. I have some papers with specific speeds listed, but not handy at the moment. I do know at least one paper indicates that trionychids, while among the fastest freshwater turtles, are apparently slower than aquaflying species (sea turtles, in particular), but this seems to be mostly a reference to cruising speed. I'm not sure if there are any good records of sea turtle sprinting rates.


But as the reduction occurs in turtles from very different habitats
and locomotion styles, including terrestrial ones, I do not think a
selective/adaptationist explanation will be useful if applied to all
the cases.

I agree; but there are a few cases where plausible selective hypotheses are present and worth testing. Pure drift, by contrast, is difficult to get a good handle on (but that doesn't make it untrue).





Cheers,

--Mike



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