Oblivious sauropods being eaten

January 14, 2013

being eaten 600

My friend, colleague, and sometime coauthor Dave Hone sent the above cartoon, knowing about my more-than-passing interest in sauropod neurology. It was drawn by Ed McLachlan in the early 1980s for Punch! magazine in the UK (you can buy prints starting at £18.99 here).

I know that this isn’t the only image in the “oblivious sauropods getting eaten” genre. There’s a satirical drawing in Bakker’s The Dinosaur Heresies showing a sleeping brontosaur getting its tail gnawed on by some pesky mammals. I’ll scan that and post it when I get time (Update: I did). I’m sure there must be others in a similar vein–point me to them in the comments or email me and I’ll post as many as I can get my hands on.

I wouldn’t post stuff like this if I didn’t think it was funny. But if you want the real scoop on whether sauropods could have responded quickly to injuries to their distant extremities, here’s the deal:

First of all, sauropods really did have individual sensory nerve cells that ran from their extremities (tip of tail, soles of feet)–and from the rest of their skin–to their brainstems. In the longest sauropods, these cells were probably something like 150 feet long, and may have been the longest cells in the history of life. We haven’t found any fossils of these nerves and almost certainly never will, but we can be sure that sauropods had them because all vertebrates do, from hagfish on up. That’s just how we’re built. (This is all rehash for regular readers–see this post and the linked paper.)

Wedel RLN fig2 480

So how long does it take to send a nerve impulse 150 feet? The fastest nerve conduction velocities are in the neighborhood of 120 meters per second, so a signal from the very tip of the tail in a 150-foot sauropod would take about half a second to reach the brain.

Is it possible that sauropods had accelerated nerve conduction velocities, to bring in those distant signals faster? To the brain, probably not. The only ways to speed up a nerve impulse are to increase the diameter of the axon itself, which some invertebrates do, and to increase the thickness of the myelin sheath around the axon, which is what vertebrates tend to do (some invertebrates have myelin-like tissues that apparently help accelerate their nerve impulses, too). Fatter axons mean fatter nerves, and for at least half the trip to the brain, the axons in question are part of the spinal cord. And we know that sauropod spinal cords were pretty small, relative to their body size, because the neural canals of their vertebrae, through which their spinal cords passed, are themselves small–Hatcher wrote about this more than a century ago. So there’s a tradeoff–sauropods could have had very fast, very fat axons, but not very many of them, and therefore poor “coverage” at their extremities, with nerve endings widely spaced, or better coverage with more axons, but those axons would be skinnier and therefore slower. We don’t know which way they went.

Incidentally, you can experiment with the density of sensory nerve endings in your own body. Close your eyes or blindfold yourself, and have a friend poke you in various places with chopsticks. Seriously–start with the two chopsticks right together, and gradually spread them out until you can feel two distinct points (or, if you want to get really tricky, have your friend mix up the close and widely spread touches so there’s no direction for you to anticipate). The least sensitive part of your body is your back–over your back and shoulders, you’ll probably have a hard time distinguishing points of touch that are less than an inch apart. On your hands and face, you’ll probably be able to distinguish points only a few millimeters apart; in fact, for fingertips you’ll probably need finer instruments than chopsticks–maybe toothpicks or pins, but I take no responsibility for any accidental acupuncture!

Back to sauropods. Could predators have taken advantage of the comparatively long nerve conduction velocities in sauropods? I seriously doubt it, for several reasons:

  • Simple reflex arcs are governed by interneurons in the spinal cord. The tail-tip-to-spinal-cord distance was a lot shorter than the tail-tip-to-brain route. Even over the round trip of “sensory impulse in, motor impulse out”, it would have been at worst equal, and that’s assuming the nerve impulse had to go all the way to the base of the tail.* Call it half a second, max.
  • It gets worse: the peripheral nerves outside the spinal cord are not limited by the size of the neural canal, so they can be more heavily myelinated, with faster conduction times. For example, each of the sciatic nerves running down the backs of your thighs is much larger in cross-section than your entire spinal cord. If sauropod peripheral nerves were selected for fast conduction, they might have been bigger and faster than anything around today.
  • Half a second is not much time for a theropod to formulate a plan, especially if Step 1 of the plan is “grab 150-foot sauropod by the tail”.
  • This assumes that said theropod was able to sneak right up to the sauropod without being detected. You go try that with a big wild herbivore and let me know how it works out. (Also, a big animal tolerating your presence, because you are pathetically small and weak, is not the same as it being unaware of your presence.)
  • All of this assumes the theropod only went for the bony whip-lash at the tip of the tail–the fastest-moving extremity, and the least-nourishing single bite anywhere on the target. If the theropod went for a meatier bite closer to the base of the tail, it would have to sneak closer to the sauropod’s head (better chance of being spotted), and the nerve conduction delay would be shortened.
  • A 150-foot sauropod would probably mass somewhere between 50 and 100 tons, and would be capable of dealing incredible damage to even the largest theropods, which maxed out around 15 tons. There’s a good reason predators go after the young, sick, and weak. Smaller sauropods would be less dangerous, but they’d also have faster tail-to-central-nervous-system-and-back reaction times.
  • A theropod big enough to go after a 150-foot sauropod would also be subject to fairly long nerve-conduction delays, which would limit whatever trifling advantage it might have gotten from such delays in the sauropod.

So, although I have no doubt that in their long history together, giant theropods did occasionally tackle full-grown giant sauropods–because real animals do all kinds of weird things if you watch them long enough, and lions will take on elephants when they get desperate–I am extremely skeptical that the theropods enjoyed any advantage based on the “slow” nervous systems of those sauropods.

* Some relevant hard-core anatomy for the curious: sauropods have neural canals in their tail vertebrae, and usually far down their tails, too. But that doesn’t mean much–you have neural canals to the bottom half of your sacrum, but your spinal cord stops around your first or second lumbar vertebra. From there on down, you just have nerve roots. So the shortest reflex arc from your big toe has to go up to your lower back and return. Why is your spinal cord so short? Basically because your central nervous system stops growing when you’re still a child–it will add new connections after that, and a few new cells in your olfactory bulbs and hippocampus, but it won’t get appreciably bigger or longer. After mid-childhood, your body keeps growing but your spinal cord stays the same length, so you end up with this freaky little-kid spinal cord tucked up inside your grown-up vertebral column. Weird, huh?

So did sauropod spinal cords stop at mid-back or go all the way into the tail? We have several conflicting lines of evidence. In extant reptiles, the spinal cord does extend into the tail in at least some taxa (I haven’t done anything like a complete survey, just read a couple of papers). Birds are no help because their tails are extremely short, but their spinal cords do extend into the synsacrum (and expand there, thanks to the glycogen body, which was probably also present in sauropods and responsible for the inaccurate “second brain” meme). But then birds grow up very fast, with even the largest reaching full size in a year or two, so they don’t share our problem of the body outgrowing the nervous system. We know that sauropods grew pretty quickly, but they also took a while to mature–somewhere between one and three decades, probably. Did that protracted growth period give their vertebral columns the time to outgrow their spinal cords? I have no idea, because the division of the spinal cord into roots happens inside the dura mater and doesn’t leave any skeletal traces that I know of. Someone should go figure it out–or at least figure out if it can be figured out!

19 Responses to “Oblivious sauropods being eaten”

  1. excellent summary of this issue, Matt!

  2. Mark Robinson Says:

    Great read.

  3. Crown House Says:

    Great post, i’ve been wondering for years, if this “second-brain” did exist or not. Also good luck for academic spring!

  4. Allen Hazen Says:

    The pesky facts RUIN a joke I heard in the 1970s: “Q: What did dinosaurs use for alarm clocks? A: They bit themselves on the tip of the tail when they lay down to sleep, and 8 hours later the pain would wake them up.” (Hey, it’s as funny as the “Punch” cartoon!)

    And I second the motion of Heinrich and Mark: that’s a beautifully done post! It would make a marvellous article for, say, a science magazine for school children

  5. Al Says:

    On the topic of sauropods being eaten, do you think its possible that a large theropod could kill a sauropod by attacking the sauropod’s neck and head? I’ve always thought that these parts of the sauropod seemed vulnerable.

  6. Mike Taylor Says:

    I agree that the long neck and small head look like the most vulnerable parts of a sauropod — a theropod would certainly be justified in expecting more success by attacking there than going for the legs. But as we pointed out in our sexual selection paper:

    The costs of long necks seem to have been overstated. The only example given by Senter (2006, p. 47) was his claim that a horizontal sauropod neck would leave the animal vulnerable to attack from large predatory theropods, as ‘a single bite that severed carotid arteries, jugular veins or vagus nerves would have been sufficient to dispatch a sauropod’. However, this assertion is problematic. Extant predators rarely attack adult animals (especially those many times larger than themselves) when juvenile prey is much more vulnerable, and there is evidence from the size distribution of bones in the fossil record that this was also true of Mesozoic theropods (Hone & Rauhut, 2009). Injuring or killing a sauropod with a single bite to the neck would be more difficult than the phrase ‘a single bite that severed carotid arteries’ suggests. The neck was not simply a mass of external blood vessels and nerves, but was constructed from tough elements including the often robust cervical ribs, bony laminae, ligaments and tendons. A theropod could hardly dispatch a moving apatosaur with one swift bite, and a raised neck would further reduce vulnerability.

  7. Mike, you’re missing part of Al’s point: even in a juvenile the neck would be a prime target!

  8. Mike Taylor Says:

    I don’t understand how that is different from what I said.

  9. your answer seems to address sauropods that are large compared to the predator. Other way round – an adult Allosaurus can grab a 75 kg sauropod by the neck and simply bite down hard enough to kill it outright quite easily!

  10. sublunary Says:

    For some reason, old pulp fiction tended to assume that all dinosaurs, when shot, would take a really long time to ‘realize that they were dead’. I wonder if that has something to do with this?

  11. […] to the vertebrae the farther we are from the head because of our child-sized spinal cords (see this post for […]

  12. aiden Says:

    a sauropod was never “predator immune” even though it had a large size it still was not agile and fast like a carnivore and it might not have been able to react to hit and run situations. a sauropod was still pretty safe though. but if a sauropod is hit enough times to pierce skin and tissue (depending on the attackers bite force) bleeding is inevitable. and when your 80 tons , blood loss isn’t good. the carnivores then might have waited for the sauropod to weaken due to blood loss or infection. theropods are highly underestimated. even though a typical hunt would result in injury or death, not all of them would. even lone theropods would hunt sauropods (even fully mature ones) direct evidence in many places show that this was true . footprints of an acrocanthosaurus were found very close to that of a sauroposeidon . one footprint was missing. that missing footprint might have been a strike on the sauropod. the acrocanthosaurus had shin bones larger than thigh bones and that meant it was likely a slow runner , meaning the only prey it might have been able to catch were sauropods. also, many mapusaurus fossils were found on a hill side near an argentinosaurus fossil (not direct evidence with the mapusarus fossils though). and don’t think something as massive as a sauropod can just easily blow a hit on an agile theropod. yeah sure the tail could send a theropod flying a few meters, but i think theropods would stay away from the tails of sauropods. theropods have far better endurance than a sauropod. the only advantage the sauropods had were strength and size. the fastest sauropod was probably no faster than the slowest (medium-large) theropod, nor as agile. but without a doubt a theropod would not just pick a fight with a sauropod any day.

  13. Mike Taylor Says:

    Neither is an elephant “agile and fast like a carnivore”. Yet they are essentially immune to predation. If you want a way to think about why this is, consider even the most agile and fast three-year-old trying to take down an adult human. It’s just not gonna happen. Size matters a lot. It’s why boxing has so many finely graduated weight classes, and why a boxer who’s able to move up a weight-class and still compete is considered big news.

    I’m not saying it never happened that a pack of big theropods in desperate straits somehow managed to bring down an adult sauropod. A lot of weird things happen in a hundred and fifty million years — and we do know of aberrant cases where pack-hunting lions bring down an elephant. But this would most surely not have been anything like a common scenario.

    You are making a lot of confident assertions in your comments. Much more confident than professional palaeontologists would make. You might want to think about that.

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