[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: questions about the Odontochelys study



Hi Mike, the discussion is getting somewhat much locomotion-specific
for my knowledge, so I ask for some patience in my doubts on
locomotion, to be expressed below.

> Stride length can depend on many factors, including the those you mentioned
> and several others.

What should be these? restriction to further movement by the mere mass
of the muscles (as in bodybuilders)? limits to the extension capacity
of muscle and articular ligaments? simple will/behaviour of the animal
to move in a range lesser than the maximum possible?

> Because of the buoyant forces, simple walking in subaqueous environments is
> indeed quite slow.

Can it be that a more "crouched" position, where at a point of the
step you have a very flexed limb, and in all moments the foot is quite
close to the body midline (as opposes to what would happen in a
parasagittal animal, or a sprawler with less flexion) permit a
somewhat faster movement in that it reduces the friction with water
produced by the limb? I think I remember, but cannot ascertain, to
have seen a turtle move rapidly on the bottom this way.

> 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.

Do you say, something of the sort of turkeys, horses and kangaroos? I
suppose elastic storage can be extended to less fast
animals/"cursorial" animals, but shouldn't the tendons be somewhat
longer so as to be capable of producing more motion when releasing
their potential energy? As far as I know, turtles have short limb
segments, so I would think they would be less efficient at this than,
say, most digitigrade or long limbed animals.

> 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.

True. I think "long-limbedness" and digitigrady, and "cursoriality"
might be perhaps more related to energy saving than to faster gaits.
When the muscles are located closer to the fulcrum, you have to
contract the muscle less (in proportion of the lenght of the muscle)
to make the limb accomplish a similar retraction at the distal end,
and thus save energy of contraction. Most of the time, animals are
walking looking for food, instead of running after prey or running
from predators.

> It might also be worth noting that
> the more erect stance likely gives them better clearance of substrate
> obstacles in cluttered environments.

This seems to be reasonable, in algae beds, a less erect stance may
get more easily the body/limbs trapped in threads.

> 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.

But may we say that this mode of locomotion is more energy efficient
than one in which you do not recover energy, as there is no falling
phase, but that you did not put energy in elevating the body in the
first place? I suspect that most tetrapods that do not drag their
bellies on the ground raise and let fall the body in a greater or
lesser degree, but I think that it not necessarily follows that those
benefitting by greater relative free-energy "falling" expend less
energy in locomotion, as they have also a greater relative
energy-expensive "raising".

Sorry if I can sound somewhat defying in these doubts I raise, but I
am not trying to refute what you say, only to know if there is some
other reason for explaining the energy-saving.

> 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.

Well, then my suspicion would be wrong. However, the whole thing may
reduce to define with measurements what we call "weak" and "strong"
selection. I would say that I would this time prefer evidence from
simulations than from the field experiments, where there is the chance
further advantages went unnoticed (the problem of the possible
multiple possible advantages a trait can have).

> 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.

This may be, and is worth being tested by seeing the relative size of
muscular mass. As far as I was able to see in films, in the mid-side
of the legs, between them and the plastron, there protude flaps of
skin filled with fat, so it could also be argued that the space
permits to expand the volume required for fat storage. Considering
that in evolution, as an historical science, hypotheses are rather
possible to be falsified just at some point, and hardly completely
(this apllies to adaptationist hypotheses also), we should consider
relative possible falsations of the unadaptive theory for the plastron
reduction in the snapping turtle: finding that the muscles need so
much space, finding that these fat deposits are truly necessary, and
without them the turtle would suffer a hard time (and not that they
are just consequencies, for example, of great availability of food, as
in humans), or finding that the turtle really needs to move the limb
to the other side of the body for some yet unknown locomotory reason
(of course that such a mechanism should be supported by a study to
refute the theory).

> 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.

True that allometric correlations may be adaptive, but what I tryed to
state is that if you have an unadaptive hypothesis about allometric
relationships between two or more features, then you can
straightforwardly test that hypothesis by noting that such a
relationship does not exist, or is not so tight. With this I just
tryed to defend the testability of particular and definite unadaptive
hypotheses, to counter the belief that hypotheses of a character being
adaptive for a particular function are any more testable than
particular unadaptive hypotheses.
Many particular unadaptive hypotheses, are, for me, as testable as
most particular adaptive hypotheses, overall when they need of
correlations or lack of correlations that can be observed.

Other kind of unadaptive hypotheses is that of a direct change
produced by environment that does not need from fixation of new
adaptive alleles. For example, the much mentioned example of the
relationship between human height and availability of alimentary
resources (I mean, how height reduced at times of famine, for
example). Granted, the appearence of the correlation may be adaptive
(you will not lead so many resources to grow when you need them to
simply survive), but likely the particular increases or decreases in
height through history not so much, or not at least at the level of
adaptation by population through alelle substitution, but at the level
of individual adaptation to better deal with the now scarce resource.
I say that this unadaptive (in evolutionary terms) is quite testable,
for example if we note that some many times, the height decreased and
the food supply did not decreased. We should in such a case consider
the hypothesis refuted (at least partially) and can supplant it with
some other hypothesis of adaptive/unadaptive changes. For example that
the height decrease was correlated with spreading disease, which
should be in test relatively tested by seeing if there are more or
less registers of disease/s in the epoch in which the height
decreased.

Cheers,

Augusto