rearing sauropods

[Nick Longrich <longrich@alumni.princeton.edu>]

> In a message dated 12/31/01 11:34:13 AM Pacific Standard Time,
> Buckaroobwana@aol.com writes:
>
>
> << Is there any advantage to horizontal feeding with such a long neck? >>
>
>        Yes. A much greater area could be foraged with a minimum of energy
> expended. An enormous swath could be cut by a sauropod with a thirty-foot
> horizontal neck. DV

	The idea looks pretty practical but a survey of modern 
biodiversity doesn't seem to support it. We don't see turkeys with 
three foot necks, or sheep with six-foot necks, or rhinos with 
fifteen foot necks ( we do see extant rearing browsers in the form of 
the gerenuk, admittedly several orders of magnitude smaller but its a 
viable strategy at least at one scale).
	Perhaps this is because an extra fifteen to twenty feet of 
neck is anything but free- it takes a large amount of protein and 
energy to build it in the first place. Once built, it takes a fair 
amount of energy to maintain all that muscle and bone and tissue. 
Every bit of protein  or energy spent on a longer neck means protein 
and energy that could go somewhere else- tail, limbs, overall size, 
eggs, tougher hide, food stores, etc, etc. Not to mention the added 
vulnerability- neck is one of the most vulnerable spots in an animal 
since the nervous, circulatory, and respiratory systems all run 
through in close proximity. In the end, the added cost of all this 
additional neck may just outweigh the cost of taking a few additional 
steps.
	Perhaps the greatest problem with the "vacuum cleaner" 
hypothesis is the predictions it makes when compared against 
observations of diplodocid neck structure. You would predict to see a 
great deal of lateral flexibility to sweep the neck back and forth 
across the feeding area to get as much vegetation as possible. 
Vertical flexibility, however, should be relatively poorly developed- 
the neck would only need to reach from ground level to a few meters 
above in search of low-growing vegetation and shrubbery. This is the 
precise opposite of what you see in Diplodocus according to Stevens 
et al. where vertical flexibility is considerable but there is 
relatively little horizontal flexibility. It fits a little better 
what you see in Apatosaurus, but Apatosaurus has a proportionately 
shorter neck than Diplodocus. This suggests that whatever the purpose 
of the really freakin' long neck seen in some sauropods was, at least 
in the diplodocids it wasn't to forage on the ground. My suspicion is 
that mamenchisaurus and the other chinese longnecks are going to look 
like Diplodocus but I guess we'll have to wait for that study to be 
done.

And another argument:
> > Simple, you DON"T rear up! Rearing sauropods IMHO is a myth.

	Regarding elephants, it seems to me that they do more to 
support Bakker's rearing hypothesis than refute it.  For one thing, 
the fact remains that elephants can and do rear up, if only often 
enough to perpetuate the species, despite the fact that they aren't 
particularly well-designed for rearing. The first problem is that the 
forelimbs, not the hindlimbs, usually bear most of the weight of the 
animal in elephants (as in most mammals). Second, the center of 
gravity isn't located particularly near the hips. This means that 
elephants have to bend their knees to rear, in order to put their 
feet below the CG and remain stable. This subjects the femora to 
bending stresses, where the breaking point will be lower than if the 
limbs are loaded axially and function as columns.
	Sauropods on the other hand, were much better suited to 
rearing (particularly diplodocids), as they suffered neither of these 
design problems:
	First, the hindlimbs bore much more of the weight of the 
animal, and so accordingly were much more robust than forelimbs, the 
reverse of your typical mammalian situation (check out d'Arcy 
Thompson's _On Growth and Form_ with respect to this observation. 
Check him out if you're in any way interested in biomechanics or 
morphometrics, too). You've also got to consider that sauropods had 
to walk, so the amount of load being placed on a single hindlimb with 
the other foot off the ground may well have exceeded 50% of the 
animals weight. So putting all the weight on the hindlimbs, or the 
hindlimbs with the tail, probably didn't even come close to the 
breaking stress of the femora. This varies from sauropod to sauropod 
of course. Another thing to consider is that the stresses involved 
are going to depend in part on the weight of the animals- some of 
these guys may have been a lot skinnier than typically reconstructed, 
and some, for all their length, are mostly air-filled neck and 
whiplash tail.
	Second, the center of gravity is much nearer the acetabulum 
than in elephants, because the front of the sauropod is filled with 
air, the forelimbs are pretty small, and the rear is pretty much 
solid, with a huge, muscular tail extending behind. Little if any 
bending at the knees would have been necessary in a bipedal/tripodal 
sauropod; instead the femora may have functioned as columns loaded 
largely in compression. This is to an extent true of all sauropods 
compared to elephants but especially so in Diplodocus, Barosaurus etc.
	As for high neural spines, having the highest spines over the 
hips implies again that most of the load of the body was suspended 
from the hips (sort of like the pier of a suspension bridge being the 
highest point) rather than the forelimbs (high neural spines are 
generally over the shoulders in mammals, e.g. bison). The unusual 
height of the neural spines in some sauropods may also suggest that 
there were some unusually high tensile loads being supported by the 
ligaments and muscles attaching to the neural spines, since the 
greater the height of a beam (in this case the spine) the greater its 
bending resistance. Again, if this doesn't make sense check out 
d'Arcy Thompson for a more extensive discussion of this stuff (he 
even refers to diplodocids and stegosaurs in particular) and a really 
readable intro to beam theory, tension, compression, and all that 
stuff. Basically, most of the body was already suspended from the 
hips, so suspending the whole darn thing is hardly a stretch.
	Finally, consider theropods. The elephant argument seems to 
be at odds with the mere existence of some of these creatures- you've 
got animals that may have ranged to ~4-5 tons (t. rex), and 
Spinosaurus (who knows how much but it must have been obscene), and 
finally a giant that weighed by some estimates as much as 8 tons 
(giganotosaurus, and they keep digging up bigger ones), and these 
creatures clearly must have spent substantial parts of their lives 
supported by just a *single* limb while moving. The maximum possible 
body size for an animal rearing onto two elephantine, columnar limbs 
logically must be quite a few tons higher yet- how much higher I 
wouldn't know but offhand I'm going to guess maybe even twice as much 
or more, perhaps near or even above 15-20 tons?

NL