Dinosaur Conundrums - How did Dinosaurs sleep?

One of the best aspects of palaeontology is that no matter what we think we know there is always so much that we don’t. Dinosaur palaeontology is often dominated by cladistics and phylogenetic analysis and this is quite understandable but, for me, how these animals lived and behaved is much more interesting.

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How did dinosaurs sleep? Come to think of it, did they actually sleep at all? The first place to start is with the extant phylogenetic bracket (EPB) which, as most of you are aware, places dinosaurs between crocodiles and birds. Crocodiles do sleep, not in the traditional mammalian sense, but in a rather disjointed way by which they tend to grab short sessions of sleep but without falling into a deep slumber. They sleep with one eye open, so to speak, and are able to be instantly awake at the slightest disturbance.

These short sleeping sessions usually occur after bouts of activity and especially after eating although, being poikilothermic, this may not actually be that often although crocodiles also use these short sessions to shut down non-essential biological functions to aid digestion. The other thing to take into account with crocodiles is that, if they were to fall into a deep sleep, they would lose control of their body temperature which the animal regulates by moving in and out of the sun and water to adjust its body temperature. Deep sleep could be potentially fatal to any poikilotherm.

We all know that birds sleep as anybody who has kept a budgerigar or parrot will testify. And yet birds too sleep somewhat lightly but for different reasons. Crocodiles are big apex predators which are able to rest comfortably in the knowledge that it is extremely unlikely that anything will bother them. For birds, the issue of sleep is much more life threatening and for a bird to be caught off guard and in a deep sleep could be disastrous.

Birds employ a number of strategies when it comes to sleeping which are employed to negate the possibility of being predated when at rest. Some birds sleep on the ground using a combination of camouflage and ground cover for protection whilst some water birds will find refuge on predator free islands or, indeed, sleep on the water itself. By far the most common strategy, which includes birds that spend the majority of their time on the ground, is to take cover in trees and shrubs. This prevents the vast majority of ground predators from reaching them and, even if they could climb, chances are that vibration and noise would warn the bird of impending danger.

Safety in numbers is another solid technique for avoiding predation and some roosts may number hundreds, if not thousands, of birds. Interestingly though, despite all of these anti-predator measures, birds, like their crocodilian cousins tend not to sleep very long with an average length of two and a half minutes in some phases while other sessions will be counted in the seconds. Also of note is that birds, particularly those that sleep or perch standing up, manage to keep their muscles taught when asleep and flexor tendons ensure that the toes lock around the perch thus preventing the bird from falling.

So did dinosaurs sleep and, if they did, how? By way of inference, utilising the EPB, then it is almost a certainty that they did sleep but just how this occurred must remain a matter of conjecture but we can make a few educated guesses. Theropods ranged in size from very small to very large and would have employed various resting and sleeping techniques. Since nearly everyone agrees that most dinosaurs were much more than ectothermic, and probably nearer endothermic, then it follows that theropods spent a lot of their time resting or sleeping since hunting and feeding would take up only a small proportion of their time.

Small theropods may have slept in a squatting position, similar to ground dwelling birds of today, perhaps with the head tucked in against the body. Indeed, the high profile discovery Mei long, from Early Cretaceous sediments in Liaoning Province in China, was found in just such a position and the name translates as “soundly sleeping dragon”. Although Mei represents the best evidence of an avian rest posture to date, it is not the only example and a specimen of Saurornithoides recovered in the early nineties from Mongolia was found preserved in a similar posture. We must remind ourselves, however, that this is still an untestable assumption of an avian sleep posture and that these animals may have simply died in this position.

Mei long - Image by Bruce McAdam

It is much easier to imagine that small theropods would have been able to rest and sleep very comfortably and would have been able to regain their footing with some ease. But what do you do if you a big multi-tonned predator such as a spinosaur or tyrannosaur? In theory, there is no reason to suggest that a big theropod could not sleep or rest in the same way that Mei may have done. But it is harder for us to imagine animals approaching 40 feet long (and more) being tucked up in this position and yet – why not?

Could they sleep standing up? It’s hard to comprehend that a big tyrannosaur weighing a few tons would risk falling over because it fell asleep whilst standing up! But, as we have seen, extant birds do possess the mechanisms that prevent just such an accident from occurring so it is not beyond the realms of possibility that perhaps theropods had similar devices that prevented this.

Tyrannosaurus, of course, has been studied intensely since it was first discovered and has had its possible resting positions looked at too. The focus of study centres on the fact that the pubic boot could be utilised to take the weight of the animal as it rested which still left the legs able to assume multiple positions including kneeling, crouching and, of course, standing. Worth popping over to Dinomorph for a fuller explanation.

Whether all large theropods may have utilised the pubic boot is unclear but seems unlikely to me. You would imagine that pubic boots would show signs of stress if being rested on for any amount of time by tons of flesh and bone and yet I am unaware of any study that may have looked into this. Maybe we are complicating things and large theropods were quite capable of lying down to rest and sleep like most extant animals of today. Perhaps it’s time similar studies were performed on other large theropods, apart from Tyrannosaurus, and comparisons made.

Of course, if you have four legs, you are much more stable and are less vulnerable to falling down anyway but what happens if you fall asleep. Taking sauropods, by way of example, they obviously needed copious amounts of food throughout their lives and probably ate almost continually. They are the most likely candidates for sleeping whilst standing up and, again, using birds as the basis here, maybe they had a similar arrangement of tendons that kept everything rigid whilst they slept although there is not a shred of evidence to suggest they did. It’s worth pointing out that today’s mammals that sleep upright also possess a similar mechanism but, although this bears no relevance to sauropods or other dinosaurs, is indicative that any creature that slept upright would almost certainly require counter-measures to prevent a fall.

Perhaps, then, sauropods were able to kneel  or lay down and this is where the human mind can struggle to comprehend such actions since an animal approaching a hundred feet long, weighing upwards of seventy tons, being able to lay down, sleep and get up again almost defies imagination. And yet there is no reason that this should be the case and we have to be constantly aware that Nature is extraordinarily adept at providing animals with the ways and means to exist – and this would include both rest and sleep.

Ultimately, understanding how dinosaurs rested and slept will always be an inferance based on speculation. It’s worth noting that both birds and crocodiles do not spend a lot of their time asleep anyway and it’s entirely possible that dinosaurs only required similar limited amounts of both rest and sleep. As usual we are left with more questions than answers, more dichotomies than you shake a fist at and haven’t even thought about how pterosaurs may have slept either!

From Milner et al 2009

References

Milner, ARC, Harris, JD, Lockley, MG, Kirkland, JI, Matthews, NA (2009) Bird-like anatomy, posture, and behaviour revealed by an Early Jurassic theropod dinosaur resting trace. PLoS One 4 (3): e4591. doi:10.1371/journal.pone. 0004591

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