[HarHarVeryFunny]

I've got to wonder how realistic this pose is.

Even if it could rear up and balance like that, the energy expenditure vs calorific gain seems like a losing proposition. You're talking about raising the center of gravity of it's 40-ton body mass by 10-20 feet just to grab a very small mouthful of low calorific leaves.

I'd guess the reason the sauropods had an extra long neck was rather so they could AVOID moving as much as possible - stand in one place and just swivel neck around to graze a large area.

[pivot_root]

I thought the same thing, which sent me down a bit of an unexpected rabbit hole in the topic. Greg Paul argued that thr chevron shape of the bones in the bottom of the tail point to sauropods rearing and using their tails as support . Heinrich Mallison did some biomechanical modeling and found that some of the anatomical features previously thought to support rearing might actually hinder it. And last year, a study on larger sauropods (Dreadnoughtus and Giraffatitan) showed that their femurs most likely couldn’t handle sustained stress of resting.

So it looks like this pose is based on anatomy, not biomechanics, and the one rigorous biomechanical sauropod-rearing study that exists didn’t even test this genus - which means the rearing question Mamenchisaurus is unresolved.

https://en.wikipedia.org/wiki/Mamenchisaurus

https://reptilis.net/DML/2009Apr/msg00036.html

https://onlinelibrary.wiley.com/doi/10.1111/pala.70019

ALSO, consider how stiff their neck was, it could very well have spent most of the time grazing on the ground, like you said!

[throwup238]

That’s an ongoing debate within paleontology that often takes place on a species by species basis. The argument goes that since diplodocids (which this species is not) had heavy muscular tails, their center of gravity was near their hips, making it easy to rear up since they were effectively already a balanced seesaw and could use the tail as a third point of contact to balance. Species in Mamenchisaurus share similar pelvic and tail features and M. youngi was show to have a stiff neck that couldn’t lift very easily, so it’s inferred that these may have reared as well. There are center of mass and skeletal models and stuff to determine whether rearing is possible but one hasn’t been made for this species specifically.

Sidenote: you underestimate the cardiovascular cost of pumping blood up a 5-15 meter neck. It’s not at all clear that a rearing strategy is more expensive energy wise. In their case it’s less spending energy to standup than just leaning back to let their skeletal structure and center of mass do the work.

Mostly I think this pose is a matter of logistics. They probably just had more vertical space than horizontal to work with for this exhibit. Even though they’re fiberglass, the casts for these guys run well into the tons per skeleton so it can be challenging to mount the armatures in an existing structure and it turns into a game of fossil tetris balanced by the cost of structural support modifications needed (there almost always are for a fossil of this size).

[HarHarVeryFunny]

I suppose these animals all must have had some ability to use this type of hip pivot to get their front legs off the ground, if for no other reason than mating!

It'd be interesting to see an accurate energy analysis of the calories needed to do this. Even if the animal can position itself into a teeter-totter position with center of mass over the pivot/legs, it would still be using muscular energy to straighten up and extend, and then coming back down can hardly have been lossless - it'd be a combination of again using muscles to come down in a controlled manner (and not destroy it's front joints!), and then a final plop down which would transfer kinetic energy into compressing the landing spot... all for a mouthful of leaves.

[mncharity]

Apparently there's a population of African elephants which rear up and balance to feed higher.[1]

[1] nature video starts with example: https://www.youtube.com/watch?v=3XzQ4BQe4fM short clip: https://www.reddit.com/r/NatureIsFuckingLit/comments/19bge4y... longer clip with two: https://www.youtube.com/watch?v=Jpxgqu_Cfkg

[HarHarVeryFunny]

Cool videos, but just because you can doesn't mean you should!

At least an elephant, having a trunk, can pull down a whole branch and make the effort worthwhile as that first video shows. It seems that a sauropod with only its tiny mouth for grabbing wouldn't be able to do that, so the outcome would be more like in that last video where the elephant was only able to grab a couple of leaves, which I assume can't have been a calorific win!

[mncharity]

Hmm. Wouldn't a long neck permit amortizing the body lift across a lot of foliage?

[HarHarVeryFunny]

I suppose in theory they could stay reared up browsing for a while, but every mouthful that needed to be chewed and swallowed in that position would be expending at least some muscular energy in stabilizing itself and maintaining leg extension.

If it was swinging it's neck from side to side while upright, then it would also need to be expending energy not only to do that but also to shift it's weight to counterbalance.

My intuition says it wouldn't be worth it, and the size some of these dinosaurs grew to suggests that easy (e.g. ground level) food was plentiful back then. I've always supposed that evolution bred gigantism out of most DNA lineages as a hard won lesson that food won't always be plentiful, and that in times of shortage being smaller is an advantage.

[w10-1]

If they could eat from higher branches than others, they could avoid competition for resources.

[HarHarVeryFunny]

Sure, but they wouldn't routinely be doing it unless a) it was possible, and b) there was an actual benefit - they gained more calories per rear than they expended doing it.