I do not normally go in for speculative pieces on the blog and when I have ideas about Mesozoic biology I tend to try and get an excuse to write a paper about them or consult with some colleagues and see what merit the ideas may have. But something popped into my head the other day and it’s been rattling around and I thought it would be fun to put it out there into internet land.
First off, I’ll preface what follows with the important point I’m no real expert on the details of sauropod physiology and digestive biology. So it’s quite possible that I’ve missed some major discussions on this in the literature (or online) be it that the idea is already out there and this isn’t new, or it’s already been discussed and dismissed. I’d also add that while I’ll discuss sauropods here, the central issue may also apply to sauropodomorphs, various other ornithischians and potentially even the bigger herbivorous theropods. I’ll try and boil down the argument as simply as possible, though of course I’m deliberately skipping a lot of nuance.
In short:
Big sauropods would need to eat a lot but allowing for thermal inertia, long digestion times with higher efficiency, and reduced metabolism at large size they have the potential to function without eating 24 hours a day.
For juveniles though, they lack some of these benefits and especially would not have the benefits of long digestion times to break down tough plants. They’d have (proportionally) higher metabolisms and would be getting less return from what they ate.
One solution to this would be coprophagy. And yes, that is what you think it is.
Elephants are a good example here (well without the XXXXeating bit) since they eat a lot of rough material like dried grasses and tree bark. They are bulk feeders cramming everything in, stripping out the nutrition they can and moving on. I was warned years ago when working at a zoo that if offered an apple when visiting the elephant house not to take it. Apparently these occasionally passed through untouched and then would be handed out to unknowing guests. The point is, elephant dung contains a lot of undigested material. If you are a young sauropod, something like that which has already passed through your system and is starting to be broken down could, second time round me a lot more nutritious. And you don’t have to go anywhere to find it, it’s a ready source of calories right there.
That really is the limit of my suggestion. As I say, I suspect I’ve missed something important but I can see an obvious few benefits from this and there’s a good few animals that go in for this practice so it has plenty of precedent. I recognise that reptile and bird waste is often very different from mammals, but then we don’t have many 5 ton lizards that eat ferns around for a comparison and the waste of large tortoises certain can contain plenty of grass shards.
Thoughts below, and if I’ve stumbled across a good idea here I’d be happy to try and expand on it.
Dave Hone's Archosaur Musings 
Just some thoughts: if adult sauropods indeed had “long digestion times with higher efficiency”, wouldn’t their waste also be less nutricuous? Also it has been proposed young sauropods did not live in a herd with big ones? Thirdly, wouldn’t diseases be a problem?
I’m not suggesting the juveniles ate those of adults but their own, sorry if that’s not clear enough. The adults have the body size and digestion time to digest efficiently, the juveniles dont. But they could eat their own and thus digest twice.
Have you seen Gill et al. (2018) in Palaeontology? They model sauropod digestion rates and the nutritional value of Mesozoic plants, but do not model for potential coprophagy. Follow up investigation perhaps?
I don’t know that specific paper but I used to be part of the giant sauropod research group that they were in too which is where my (limited) knowledge of their work comes from. But yes, I think the focus there has always been working out how sauropods could get so large and get sufficient nutirtion and that digestion seems to be the answer. But then my logic is that this might work for 30-80 ton sauropods but what about the little ones ‘trying’ to be good sauropods but are a fraction of that size. They don’t get the diegestion time bonus while still having a bauplan that won’t let them move around a lot. Does this help fill that gap (assuming the gap is real)?
Dave’s suggestion that juveniles “could eat their own and thus digest twice” is exactly the hack that lagomorphs use. Ecologically lagomorphs and artiodactyls do very similar things and the max lagomorph size and minimum artiodactyl size in a fauna typically approach each other but do not overlap (this was more true in extinct faunas with more lagos and more artios, modern faunas are more gappy). But digestive efficiency is roughly particle size x retention time, and lagos aren’t big enough to benefit from the same retention times as larger-bodied artios, so they cheat and eat their poop to get double the retention time.
So there’s no reason to think it wouldn’t work for juvenile sauropods, but I also wonder whether they would really need it. Baby geese eat lots of horsetails, which are surprisingly nutritious and easy to digest if you can deal with all the silica, which is what keeps orally-processing mammals away. If that works for juvenile extant dinos that need to grow fast, it should have worked for juvenile extinct dinos that also needed to grow fast. Possibly supplemented with a little poop now and then?
One thing about the rabbits, it’s not like they’re eating any old poop. Their digestive system produces two kinds of feces, a soft pellet after the first pass that is intended for re-consumption, and then the harder “rabbit turds” that we’re all familiar with after the second pass. So to really do the coprophagy right involves some physiological adaptation to go with the behavioral.
Thanks for that Matt. While I didn’t discuss it, it was rabbits that gave me the thought on this coupled with some info on digestive patterns of mid-sized vs big herbivores like eland vs hippos and rhinos. So the points you raise were largely the ones I was thinking of, so we’re largely on the same page there. And yes, there’s specialised pellets and behaviour going on in bunnies, but I didn’t cover that as I was running that under the ‘well they’d obviously evolve that’ balnket of vague arm waving for things like this. 🙂
The central issue you raise though is would it be needed. I guess that comes down to local landscapes and densities of highly nutritious material (and issues like seasonality) and populations densities and competition for food. I’m not suggesting this was universal of course, but I can see how it would be valuable at times for some or generally for others based on the right scenarios. Still nice that there’s no reason to think it’s obviously incorrect.
Mark Hallett and Matthew J. Wedel suggest exactly the same in their book The Sauropod Dinosaurs
Ah! Matt fails to mention that in his comment above, and it also shows I’ve still not finished reading my copy of their book.
According to this 2002 doc in which Green Iguanas do that, “some scientists believe that baby dinosaurs once did the same” (See 13:00-14:30). What I’d like to know is, who discussed that pre-2002?: https://www.youtube.com/watch?v=1CPf8wm8ins