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RE: Bakker & whiptails - jog my memory please!
Yes, the tip of a bullwhip frays gradually - in fact the frayed part
increases the sound production. We speculate that the end of the tail was
dead skin that was renewed as it wore. This is what happens in modern
lizards (iquanas, monitors), although the fraying in those cases is simply
from tail dragging etc.
Okay, but during the breeding season the cracking must have become pretty
intense. What would happen to the skin after much repetitive cracking?
Could you amputate a tail from a big monitor lizard and crack it with due
effect? Sounds dopey, I'm sure, but just an idea. You might see how the
blood vessels were affected. Are there any living vertebrates that do crack
their tails awfully fast? Any info on that?
Note that leather is just cow hide, and it works very well for whips, so
nothing special is needed. The same is true for capilaries etc. A human
arm works fine in moving a whip, because the blood, tissue etc are not near
the fast moving tip. The speed of most of the tail is very slow - only the
very tip moves very fast, and only the tip experiences stress - and even
then it is not very high stress.
Yes, but there must have been capillarlies and blood vessels running to the
tip of the tail; otherwise necrosis of the tissue would result. Do you
think the ends of sauropod tails could snap off or break like in some
reptiles, with the resulting end growing a new but scar-tissue-like tail
tip? And if so, how would this affect the cracking? And, yes, a whip is
just cowhide, but that's my point, I guess. It's just skin, not skin and
bone and blood vessels, etc.
In fact, snapping a rolled up wet towel (i.e. as in high school locker
rooms) produces supersonic motion. No special materials or stresses are
needed for supersonic speed. You can even fold origami whips out of paper.
This is counterintuitive, so everybody asks about it - but the fact is
supersonic motion is only exotic in airplanes - whips are very simple.
paper calculates necessary forces, torques and energy to back this up.
Not questioning basic physics here, but the analogy between a living tail
with vertebrae, blood vessels, muscles, nerves, etc., and a wet paper towel
or origami whip seems a bit of a stretch. In humans, for instance, our big
toe can go numb when overstretching our sciatic nerve as we run, a very
common condition. Nerves must have stretched to that tail tip -- what is
the resiliance of a nerve under that sort of osciliation? Again,
capillaries in the tip or near the tip must have experienced some pretty
impressive stress. And as you say, whips are very simple -- not quite like
a living composite.
The issue of continous versus segmented is irrelevant. Conservation of
momentum applies either way. The momentum transfer mechanism is a basic
of physics and joints do not matter. There is a very extensive literature
in biomechanics documenting the same sort of phenomemon in jointed systems.
Some of this literature is human sports medicine - the same momentum
transfer is responsible for golf swings, karate chops, soccer kicks etc.
Again, not questioning the physics here. But, I am curious. What if you
took a bullwhip and segmented it. What would happen when you snapped it?
Sounds rather dumb, perhaps, but wouldn't the force traveling across each
segment be changed in some fashion? Would it be harder to get it to crack?
You assume that I am unaware of the vast literature on biomechanics
documenting momentum in jointed systems -- I am not. However, the momentum
of a golf club, karate chop, etc. involves far fewer joints than that
observed in a sauropod tail. Surely, the stiffened shaft of a golf club is
not an appropriate analogy for a sauropod tail, momentum or not. Does the
club head ever break the sound barrier, much less the arm of a karate
Bob Bakker wrote a number of years ago about the use of diplocodid sauropod
tails as weapons - his book is one place, but I don't think there were many
formal papers on the topic. This was prior to our paper. However, the
weapon idea is not new - it was suggested as early as 1901 by Hatcher.
In our paper we point out a number of reasons that the weapon hypothesis is
unlikely - the bones are too small, the joint surfaces are too vulnerable,
and there is no evidence of trauma in known specimens. Why does a 90 foot
long Apatasaurus need a weapon? Size is a great defense.
Okay, fine. My mistake, I mixed up your proposal with those of others, and
it has been a while since I read the tail paper. However, no evidence of
trauma in the tail could also mean that they are not cracking their tails
like whips, as well as tossing out the defensive strategy idea. Yes, size
is a great defense, but only for adult animals. Could juveniles have made
use of a tail weapon? I'm not suggesting they did. Rather, the
size/defense thing only works when you get big.
A much more important rival than a predator is a competing male of your own
species. The reason Elk, Moose, Deer etc have antlers is not defense
against predators (typically the females don't have them) - it is sexual
dimorphism brought on by selection and the need for combat/dominance
contests with other males.
I know you base part of this sexual dimorphism idea on the DISH (fusion of
verts) noted in the mid-portion of the tails of about half the known
diplodocid sauropods. Because our sample size for these giants is rather
small, sexual dimorphism may or may not be the culprit here. It could be,
sure, and I understand how this would stiffen up the tail to transfer energy
better to the "whip" end, but why couldn't DISH result from having to keep
the tail stiff and off of the ground? Many paleontologists seem to think
sauropods did not drag their tails, so could DISH have resulted from
muscular or even passive effort to keep the tails aloft?
The noise produced by the whiplash tails may have had some defensive
purpose, but the main suggestion of the paper is that it was used in
dominance contests between males, and perhaps for other means of
Beyond this I have to refer you to the paper.
Duely noted, I will re-read it when I get the chance.
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