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Re: Jobaria and the Elephant Commit Suicide
Richard Travsky writes (in response to my which elephant? question):
The jobaria web page specifies Sereno had access to a 20 year old African
elephant named Angus.
Yes, I am aware of this. I don't think I stated my point clearly. The
point is, which elephant behavior are you going to use, and why use that one
particular elephant over another. While I am no elephant expert, do we know
how often certain elephants rear, how their behaviors differ, etc.? We are
only using an analog for potential sauropod rearing, and we must be careful
not to extend these behaviors to sauropods.
I am well aware they weighed this elephant, front and back, and found a
"reversed" condition of weight bearing in Jobaria. While it was great that
we have the live weight of an elephant on record (most elephant weights come
from carcasses), the observation that Jobaria has smaller hands than feet,
the reverse of an elephant, is nothing new. I'm sure they included it to
emphasize their point, but this in and of itself does not tell us that these
guys were rearing up. If sauropods are descendants of bipedal relatives,
this difference in hand vs. foot size could merely be a relict of their
former bipedal natures.
I wonder how they calculated the femur's weight-bearing capacity? Sure, you
have circumference, cross-sectional measures, etc., but how they arrived at
3 times the weight of the animal would be interesting to know. But let's
say this is true. Is this "overbuilt" as has been suggested? Well, all
animals have bones with safety factors, in some the safety factor can get up
to 5 times the weight. 3 times falls within the range we see reported for
elephants (Knut-Schmidt Nielsen, 1984, Size) so I'm not too sure the
condition in Jobaria would be overbuilt ... unless they meant three times as
much as an elephant -- that's a little different.
Still, let's go with the 3 times idea. When animals move their limbs, they
increase the loads on them dramatically, even when just walking. From
memory without a reference in front of my face, a human walking moderately
fast can nearly double the normal weight going through their limbs.
Something as large and heavy as Jobaria certainly would have greatly
increased the force through its limb bones just walking to be sure. Now
take that weight and spread it out on only the hindlimbs ... not saying this
was impossible, but just something to consider.
Here's something else to think about. Maybe the bones can support 3 times
the weight in COMPRESSION (i.e., when they are vertical), but less (I would
expect) if they are bent. Sereno et al. give us restorations of the
elephant and sauropod with BENT knees while rearing (and actually, all the
restorations I have seen of Jobaria and other sauropods restored by Sereno
et al have bent knees while walking). What effect does such a position have
on the femur? What torques are produced? Should the femur bend, as it does
in an elephant, for a sauropod to rear or is this a case of the analog
leading the functional interpretation? See what I mean about the analogs
being a slippery slope?
I again emphasize the Caudofemoralis longus tail muscle. Elephants have no
real tail to speak of, yet the tail of sauropods had muscles which were
intimately tied with locomotion. How are these tail/femur muscles being
accounted for? If you bend your knees so that the femur swings forward, how
far can that muscle stretch? Would such a muscle restrict how far you can
rear up? At this point, I am not satisfied with current sauropod rearing
Richard further adds:
If a threat display deters a predator(s),then this is a cheap means of
survival. A lot cheaper than incurring wounds.
Seems like an awful waste of energy. Allosaurus and Ceratosaurs aren't all
that big -- a sauropod is still a pretty huge animal to them. Do elephants
rear to scare away predators? Are we sure that no rearing incurs wounds
Which would make it effective in defense. (Think of the noise alone!)
Yikes! When I said the crashing down of a sauropod gives me the willies, I
meant that it stands an awful risk of breaking its bones, not the noise. =)
Remember those weight safety factors? Imagine you are a gigantic animal
that is pretty heavy and relies on all fours to get around. Would such an
animal engage in activies that would push its limb bones to the extremes of
their safety factors? Break a bone in your forelimb, now what?
If you rear up, how do you look down at your attacker? No matter how the
neck goes together, that little head is way up there, maybe trying to look
down with one or the other eye in its skull. But let's say you can. So now
you're up on your hindlimbs stuck in place. You can't pivot because your
femora have cylindrical heads and you have stiff-action ankles. And you
can't use your tail because its on the ground as a tripodal support.
I insist we have to be careful and do more rigorous work on sauropods.
Analogs are EXTREMELY tricky and we must use them with caution.
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