Why did sauropods have such long necks?

May 11, 2011

Why did sauropods have such long necks?

Mamenchisaus hochuanensis skeletal reconstruction (Young and Zhao 1972:fig. 4), based on the holotype

It’s the single most obvious and important question about sauropods, so it’s a bit surprising to think that we’ve never really addressed this question directly.

Maybe sauropod necks are so obvious and familiar that we just take them for granted, and move straight on to questions of how they were able to grow so long and remain workable.

Well, let’s fix that.  Let’s think about why they had such long necks.  What were they for?  What were sauropods doing with their necks that was valuable enough to justify all that investment?

Back in the good old days, everyone assumed that sauropod necks were all about high browsing.  If you have a 9.5m neck, then of course you will use it to browse high up in trees — it’s intuitively obvious.  But of course “intuitively obvious” is not the same thing as “true”.

Then John Martin (1987) proposed that the long necks were used for low browsing — not raised above shoulder level, but swept back and forth to allow food to be gathered across a wide area without all that tedious mucking about with locomotion.  This interpretation was of course endorsed by Stevens and Parrish (1999) in their DinoMorph work.

There has been plenty written about habitual sauropod posture — including by us (Taylor et al.2009).  But actually the high-browsing and low-browsing explanations of sauropod neck elongation have much in common.  Most crucially, they both relate to enlarging the feeding envelope; more broadly they are both explanations that rely on the neck having a survival benefit.  But Senter (2006) proposed a completely different explanation — that sauropod necks were sexual signals, selected not for survival advantage but for reproductive success.  The idea is that female sauropods, being very shallow, would go for the males with the biggest protuberances.

Are there other candidate explanations that I’ve missed?

Or is it between high browsing, low browsing and sexual selection?

Comments are open!

References

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29 Responses to “Why did sauropods have such long necks?”

  1. kattato garu Says:

    High + low browsing and sexual selection do not have to be mutually exclusive. I have 3 more possibilities (not desperately original ones): 1) predator spotting – especially herd animals where one can be spotting (alert posture, head waaaaay up) whilst the rest are eating. If your head’s 40 feet above the ground that’s quite some advance warning for approaching theropods. 2) Vocal communication (bellowing) a vast throat could be a highly resonant instrument for distant calling (could be sexually selected as in Red Deer + many others: honest signal of size and fitness) – alarm calls, herd cohesion, territorial calls etc 3) intraspecific competition (neck-wrestling a la giraffe). Less convincing especially since none seem to have horns or knobs for head-to head purchase.
    How am I doing so far?

  2. kattato garu Says:

    And 4) knocking predators off their feet. We’re always seeing sauropods ‘whipping’ theropods with their tails (or kicking them into the next field, hey!), and I doubt very much whther this was a prime driver of neck growth, but as long as their windpipe was well protected, a few meters of solid neck could have swept a pretty wide arc and made for a useful defence against smaller theropods.

  3. kattato garu Says:

    Predictably, Tet Zoo has got ahead of me with a critique of the nex for secks hypothesis in giraffes:

    http://scienceblogs.com/tetrapodzoology/2007/02/dammit_and_i_sooo_loved_the_ne.php

    The paper he’s looked at is available in full with lots of good data…

    http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.2010.00711.x/full

  4. Heinrich Mallison Says:

    feeding envelop enlargement. I guess that’s the ticker. High, low, intermediate, all of them combined – it don’t matter. As long as you get at a lot of food cheap.

  5. John the Hutch Says:

    I’ll play devil’s advocate and suggest that non-adaptive ‘spandrel’ explanations are considered, e.g. developmental constraints and so forth. Those still tend not to get their due, even if (as in this case probably) there is ample reason to suspect an adaptive explanation.

    And a phylogenetic explanation, emphasizing the origin of long necks and subsequent increases/decreases thereof. There seems to be more than one issue here than “long necks good.”

  6. Nima Says:

    Some very interesting reasons. I’m not 100% convinced about the sexual display theory. For one thing it doesn’t explain why with sauropods you get crazy long necks like Euhelopus and short necks like Dicraeosaurus. Surely they’re not all selecting for the longest necks.

    My view is that there were both high-browsing and low-browsing reasons for sauropod neck length. Most big macronarians were high browsers, most diplodocids (at least in North America) were low browsing “vacuum cleaner sauropods”, which explains their generally short necks relative to tail length. However Tendaguru diplodocids appear to have been mostly high browsers based on tooth wear and mouth shape (Whitlock, 2011). Barosaurines were the diplodocids best adapted to high browsing (Tornieria has a pretty rounded “high browser” mouth), and their necks, I’m guessing, were much longer than in square-mouthed Diplodocus for a reason other than simple sexual advertising.

    Of course even low-browsing diplodocids could become high browsers by rearing up.

    Feeding envelopes are very important, a high browser has potentially twice the feeding envelope of a low browser since it can move the neck both above and below shoulder level. This IMO goes a long way toward explaining why brachiosaurs were much larger and heavier than diplodocids of the same length.

    Nevertheless some sauropod necks do appear to have been heavily influenced by sexual selection. The extreme proportions of mamenchisaurs necks may be a case of this, but since they were similar in height to brachiosaurs, just with a much smaller body, this may be just for high browsing reasons – a more obvious case of likely sexual selection is the deep-necked Isisaurus, or perhaps other deep-necked titanosaurs like Futalognkosaurus. There’s really no clear mechanical reason for their neck neural spines to be so tall – they seem to be purely a billboard display device.

  7. Andrea Cau Says:

    Perhaps, it’s merely an allometric effect of large size: small sauropods : “eusaurischian standard” length necks; mid-sized sauropods : long necks; giant sauropods : very long necks.

    At least, this hypothesis allows falsification… :-)


  8. I suggest that the long necks evolved in sauropods for feeding and sexual selection.
    However, a secondary purpose for evolving could have been to balance the body, or keep even with their very long tails.
    Mamanchisaurus has a long tail, so it has a long neck.
    Although, my point may be contradictory in the taxon Brachytrachelopan, which is more iguanodont-like in proportions (longer tail than neck).

  9. David Says:

    Pure conjecture: visual symmetry between neck and tail as a defense mechanism. Impure conjecture: “eye” spots on the tail!

  10. kattato garu Says:

    Maybe it was simply that neckties were getting longer throughout the Jurassic and that Sauropods were sartorially sensitive?


  11. Why to reach air while standing at the bottom of sheer-walled lakes of convenient depth, of course. ;)

  12. Nima Says:

    @ Taylor Reints

    A few points I have to disagree with here:

    Mamenchisaurus actually has a pretty short tail compared to its neck, as the drawing above shows… its neck was far longer than its rather modest-sized tail. That drawing is not accurate in posture of skull shape, but in terms of raw length it’s pretty accurate. Sauropod necks and tails were not for balance; that’s a myth that’s been repeated millions of times in popular books, but it’s dead wrong since the neck and tail aren’t that massive compared to the torso and limbs (though most books claim that long tails evolved to balance long necks – you’re basically reversing the cause and effect of that theory, which is even odder IMO because the necks were so light and pneumatic that they were nearly always lighter than the tails). Diplodocus had a much longer tail than Giraffatitan, but Giraffatitan had a much longer neck. If it needed the neck and tail for balance, it should have had a long tail like Diplodocus, but it doesn’t.

    Heavy quadrupedal animals don’t need long tails for balance, nor do they need long necks for balancing long tails (don’t take this the wrong way, but you come across as almost implying that sauropods just randomly evolved big tails for no apparent reason). The neck and tail make up only a small fraction of the animals total weight – most of the mass is in the torso. Elephants had huge heads, giraffes have long necks, but both have dinky little tails that make even the shortest sauropod tail look huge – they balance just fine without big tails. Furthermore, why did some sauropod tails get so long in the first place? In diplodocids they are very long, probably to be used as defensive weapons and also as a strong prop for rearing. In brachiosaurs they are much shorter and lack the whip-like end, so they probably didn’t use their tails as weapons – brachiosaurs had a huge mass advantage over diplodocids, not to mention longer legs and a better defensive “stomping reach”.

    Brachytrachelopan pretty much proves that long necks were not necessary to maintain balance in a quadruped, though it’s an unusual case. Its very short neck was a derived adaptation to eat very low-growing vegetation to feed a very small body. Its tail stayed long most likely as a defensive measure similar to earlier diplodocoids.

  13. Anonymous Says:

    There’s another factor that might play into why sauropods developed such long necks: thermoregulatory purposes. While long necks were present in smaller sauropods and prosauropods, it has been suggested that the large amount of surface area on the neck and tail of a sauropod would have helped them cool themselves. This would be important regardless if sauropods were endotherms, quasi-endotherms, inertial homeotherms, what have you. Though it seems doubtful that thermoregulation was the only reason for the long necks.

  14. Jura Says:

    This reminds me of a study that a friend of Howard Snell did with the saddlebacked Galapagos tortoises. She wanted to see if the extreme neck elongation in this species was due to sexual selection (since males often try to look larger than each other) or to reach food. Her results were inconclusive. Most of her behavioural notes showed that neck stretching was done during male fights, but all the male fights were about food. Even extant example can be far from clear cut.

  15. Anon Says:

    To reach things in very high cupboards.

  16. Mark Robinson Says:

    I have to say that I generally don’t agree with these black-or-white statements about the natural world, where it must be one thing and not another.

    There are numerous selective pressures that shape both the form and behaviour of an animal but the two strongest are usually energy efficiency and mating opportunities which, of course, determine the only important thing – reproductive success. (If you live long and are ‘sexy’, you’ll tend to have more offspring).

    Whilst the initial trend toward longer necks may have started due to efficiencies gained by being able to eat more without moving around, there’s no reason why these then wouldn’t have also being sexually selected for very early on in their development as an indicator of fitness.

    So, I see these two selective pressures acting in concert to mutually favour longer necks. Then, as longer necks enabled them to harness more energy, they were able to grow larger bodies, which could support larger necks, and so on up to some point of diminishing return which would depend on the environment and behaviour of the animal concerned (presumably different for browsers at different heights and whether they were broad croppers or selective feeders).

    Along the way, this wonderful new thing could easily be exapted to provide advantages in other areas. As their bodies started to become more and more massive, getting rid of heat would have become more of an issue. The higher surface area-to-volume ratio of the neck would have been able to assist with this. Perhaps they were highly vascularised and allowed sauropods to dump heat as required.

    I don’t think bull sauropods would necessarily have whacked each other with their necks but they certainly could be used to make a statement about the size and fitness of an individual. Who knows, they may have been brightly coloured or adorned with some soft-tissue display structure to enhance their visibility?

    However, since Mickey is a real palaeontologist and because I have a picture of it in my How and Why Book of Dinosaurs, I’m going to have to go with the snorkel theory. ;) Incidentally, the H&W book is the same one that has hadrosaurs using their cranial crests as aqualungs.

  17. Dave Godfrey Says:

    To keep their skulls away from their body. Given how powerful a detonation of a skull like that of Graffatitan could be, you wouldn’t want it anywhere near delicate organs like the heart.


  18. I kinda agree with Mark Robinson. An either-or argument seems ineffectual. We can have it both ways. Or three ways. Or an “orgy” of ways — no, I don’t have anything on my mind…. Stop looking at me!


  19. Sound generation.

    Mamenchisaurus, Barosaurus, and Sauroposeidon had the organ pipes of the gods…

  20. Maija Karala Says:

    A good discussion with many very good points. As an evolutionary biologist I’m not too familiar with the finer points of dinosaur anatomy, but if there’s something we can learn from the modern world, it’s that things are never simple.

    Sauropod necks may well have been used for both low browsing in some species and high browsing in other (there often were many contemporarous large sauropods, weren’t there?), as sexual signal on some species, as a weapon occasionally, and for predator watching all the time.

    Even if sauropods lived today, we would have one hell of a problem trying to determine what was the leading selective pressure for the long necks. And, indeed, there necessarily wasn’t one.

    One more point for Nima:
    “I’m not 100% convinced about the sexual display theory. For one thing it doesn’t explain why with sauropods you get crazy long necks like Euhelopus and short necks like Dicraeosaurus. Surely they’re not all selecting for the longest necks.”

    Actually, wild variation between species is exactly what you’ll get with sexual selection. It’s because there’s no point in it whatsoever. :) Look at galliform birds, for example. There are peacocks with astonishing, completely out-of-proportion sexual signals. There are capercailles and pheasants with modest signals – and guineafowls, in which genders are identically colorless. All in one order, losing and re-evolving their sexual signals all the time.

    But I agree other pressures probably were more important than sexual selection. If not for other reasons, then because animals tend to lose their costly sexual signals more often than gaining them (though the speciation rate for ornamented ones is higher).

  21. Darren Naish Says:

    Yeah, those who haven’t done so might like to read this article. I’m not saying “this is why sauropods have long necks”, but it’s definitely worth keeping in mind.

  22. heteromeles Says:

    Can I ask a super-dumb question: what environment were sauropods using their long necks in?

    Here’s the issue: if long-necked sauropods are the majority of herbivores in their environment, then the answer to “what they use their necks for” probably involves feeding at all levels, because no other species was around filling those niches.

    If they are among the rarest animals in their environment, then they are probably specialists, and the default suggestion is that they are high-feeding specialists, because absent arboreal herbivorous dinosaurs, they have no competition for the highest browse.

    This diverts the discussion to something that might be measureable, at least where there is a large assemblage of fossils that give us hints about what dinosaur herbivore communities looked like.

    I’d also point out that we may see gigantism in sequoias precisely because of sauropod feeding. Redwoods are interesting in that they stump-sprout like crazy, almost as if they’re adapted to being eaten. they also, as mature adults, tower high enough to be out of the feeding range of any sauropod (first branch at 25 meters, that sort of thing). We often talk about how redwoods are adapted to coastal California, but as a clade they’ve been around for a long time. Some of their extant traits may have evolved under browsing pressure from sauropods.

  23. Zach Miller Says:

    Long necks, long tails…I think they were compensating for something.

  24. Vertebrat Says:

    John Whitlock’s tooth wear analysis, which Nima mentioned, is interesting because it’s evidence not just of how necks could have been used, but of how they actually were used – and the results are diverse and consistent with the snout shapes, so they might even be right.

    Necking puts a lot of lateral force on the neck, which makes more sense for giraffes’ thick necks than for sauropods’ laterally compressed ones. Is it possible to tell from sauropod vertebrae whether there was much lateral musculature?

  25. scidog Says:

    digestion.to come up with enough energy to power such a huge body the sauropod’s would have to start processing food immediately.the esophagus was not so much for swallowing as it was part of the digestive system.with the body space filled with the organs needed to support all that bulk,lungs,heart,spleen and so on the length of the neck added needed room for the break down of food and was an auxiliary intestine.so your “pods” had gut before and after the stomach.

  26. Nima Says:

    Some titanosaur necks (mainly lognkosaurs and more derived groups), and also the neck of Apatosaurus, were very robust and had huge massive cervical ribs. These animals are probably the best candidates for “necking” behavior in sauropods.

    I’d be surprised if mamenchisaurs or even brachiosaurs regularly did it. Of course it’s next to impossible to prove or disprove the theory. Necking doesn’t exactly leave behind much evidence of its occurrence.


  27. […] to everyone who joined in the discussion last time on why sauropods had such long necks.  I’ve discussed this a little with Matt, and we are both amazed that so many different […]

  28. Nathan Myers Says:

    Wow, 24 responses, and nobody got the obviously correct answer: They grew long necks (and, yes, tails) to supply us with more stupendous sauropod vertebrae to study. Here we are, and there they are. Q.E.D.

    And H. evolved to provide a species that would dig up and analyze them.


  29. […] Set against this background has more recently been the controversy over sauropod neck postures and quite how much (or even if) they could raise their heads and necks. But it gets more complicated still (yay!). Back in the mid 1990s it was suggested that actually giraffe necks hadn’t evolved for feeding high in trees, but instead were sexually selected structures. Perhaps not surprisingly, the same hypothesis was then extended onto sauropods too! If they couldn’t raise their necks up, and the only obvious living example (in giraffes) was only an opportunistically high browser, then maybe sauropod necks were the result of sexual selection too? (Mike Taylor has a preview of these ideas here). […]


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