Why the long necks? Probably not sexual selection

May 16, 2011

Thanks 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 hypotheses have been advanced (even if some of them are tongue-in-cheek).  We’ll probably come back to all these ideas later.

But today, we want to draw your attention to a new contribution to this discussion — a paper in the Journal of Zoology, with the tell-it-like-it-is title “The long necks of sauropods did not evolve primarily through sexual selection”, written by the three of us SV-POW!er rangers together with our buddy Dave “Archosaur Musings” Hone (Taylor et al. 2011).

Taylor et al. (2011), fig. 1: Sauropod necks, showing relationships for a selection of species, and the range of necks lengths and morphologies that they encompass. Phylogeny based on that of Upchurch et al. (2004: fig. 13.18). Mamenchisaurus hochuanensis (neck 9.5 m long) modified from Young & Zhao (1972: fig. 4); Dicraeosaurus hansemanni (2.7 m) modified from Janensch (1936: plate XVI); Diplodocus carnegii (6.5 m) modified from Hatcher (1903: plate VI); Apatosaurus louisae (6 m) modified from Lovelace, Hartman & Wahl (2008: fig. 7); Camarasaurus supremus (5.25 m) modified from Osborn & Mook (1921: plate 84); Giraffatitan brancai (8.75 m) modified from Janensch (1950: plate VIII); giraffe (1.8 m) modified from Lydekker (1894:332). Alternating grey and white vertical bars mark 1 m increments.

This is one of those papers that has been literally years in the making, which is why it’s a rather belated response to the paper that we were responding to — Phil Senter’s (2006) argument that sexual selection was the primary driver of neck elongation in sauropods.

Senter supported his hypothesis by laying out six predictions which he argued should be true for sexually selected necks; then showing that, while the first two could not be assessed, the last four all supported sexual selection.  In our paper, we do three things.  First, we make the point that sexual selection and feeding advantage are not mutually exclusive.  Second, we revisit all six predictions and show that they do not in fact support sexual selection — in fact, most of them provide support for feeding advantage.  Finally, we show that no tetrapod clade comparable with Sauropoda has consistently selected for a single sexual signal.

My email records show that Darren, Matt and I were discussing this as early as 22 September 2006, just six weeks after Senter’s paper was published, and that we started working on a response only a couple of days later.  But as so often happens, it got crowded out by a hundred other things.  Then in November 2007 Dave Hone mentioned that he was independently thinking of writing a response, and we decided to join forces.  And then … we all went back to working on other things again, touching on the necks-for-sex issue every now and then.  It’s mostly due to Dave’s repeated prods that this project wasn’t allowed to wither away, and has now, finally, made it across the finish line.

Like the neck-posture paper (Taylor et al. 2009), this was a true collaboration — one of those where, for many parts of the text, none of us is sure which of us originally wrote it.  It went through the wringer many times before reaching its final form, and most of the text must have been rewritten two or three times along the way.  We hope all the shuffling and polishing has resulted in a paper that reads straightforwardly and even seems obvious.  “When something can be read without effort, great effort has gone into its writing” — Enrique Jardiel Poncela.  That’s the goal, anyway.

The paper itself is available at the link below, so take a look and see whether you find our argument convincing.  As always, comments are open!

Update (the next morning)

Co-author Dave Hone discusses this paper on his own blog.

References

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25 Responses to “Why the long necks? Probably not sexual selection”

  1. Taylor Reints Says:

    If they were not used for sexual selection why was their such a degree in variance among sauropod neck length?
    For at least some of them had to have been used for sexual selection because some necks were not very long. This variation can be seen in Mamenchisaurus and Erketu – which have very long necks – to Brachytrachelopan and Dicraeosaurus – which have tremendously short necks.

  2. Mark Robinson Says:

    I sometimes find scientific papers to be a little dry and bit of a struggle to get through, but I am less than half way in with yours and it is proving to be readily digestible. No, scratch that, I’m being too dry – it’s awesome!

    However, despite your claims to the contrary, it has become clear to me that you must have knocked this up in the last day or so after reading all the comments on the previous post. I think you’ll need to add some more names to the list of authors!

  3. Mike Taylor Says:

    Taylor Reints wrote: “If they were not used for sexual selection why was their such a degree in variance among sauropod neck length?”

    For the obvious reason: variance in feeding strategies. This is an idea that goes back a long way: most recently Whitlock (2011) proposed different feeding regimes for different diplodocids based on tooth wear and muzzle shape, but earlier forms of this idea include Fiorillo’s (1998) paper on resource partitioning between Morrison sauropods on the basis of tooth-wear.

    One of my recurring bugbears is that we really need to get away from saying “sauropods did this” or “pterosaurs did that” — as in “sauropods were terrestrial” or “pterosaurs” were dynamic soarers. As we pointed out in the paper, we’re talking here about huge, diverse, disaparate, geographically widespread, long-lived clades — behaviour will have varied enormously. Really, “sauropods were terrestrial” makes about as much sense as “mammals are terrestrial”. Sure, most are; but there are exceptions, and we need to be on the lookout for them in the fossil record. (It’s not hard to imagine some sauropods with a hippo-like lifestyle, for example.)

    References

    Fiorillo, Anthony. R. 1998. Dental microwear patterns of the sauropod dinosaurs Camarasaurus and Diplodocus: evidence for resource partitioning in the Late Jurassic of North America: Historical Biology 13:1-16.

    Whitlock, John A. 2011. Inferences of diplodocoid (Sauropoda: Dinosauria) feeding behavior from snout shape and microwear analyses. PLoS ONE 6(4):e18304. doi:10.1371/journal.pone.0018304

  4. David Hone Says:

    Agreed Mike (and I point I make in my own post). However, I would say that in order to deal with the exceptions, you have to know what the norm is so you can try to work out not just how it differs but why. For things like sauropods and pterosaurs, getting that much agreed upon is still happening (like this paper!) so yeah, now we can go looking for exceptions, but that’s all but meaningless unless you know what it is an exception to.

    As for neck length variabilty, look at extant taxa, as we say in the paper, you find very different neck and browsing heights in Africa – dik-diks, giraffe, kudu, gerenuk, zebra, eland, lechwe etc. are all doing slightly different things in slightly different ways and the morphology reflects that.

  5. DDeden Says:

    I guess sauropods got longer necks because their food got higher, with those species that retained short necks becoming more isolated in forest openings which opened due to storms etc. similar to relatively short forest elephants and okapis. Giraffes have galloping capability, so their legs have lengthened proportionately, sauropods didn’t so didn’t.

  6. Matt Wedel Says:

    If they were not used for sexual selection why was their such a degree in variance among sauropod neck length?

    Mike already answered this, I just wanted to point out that the assumption underlying this argument is that any character that varies across a clade must be under sexual selection. The criteria for sexual selection are a bit more stringent than that–as we discuss in the paper.

  7. Jura Says:

    Wow, the tortoise example from the previous post turned out to be a bit more prophetic than I intended. Good paper guys.

  8. Matt Wedel Says:

    Wow, the tortoise example from the previous post turned out to be a bit more prophetic than I intended. Good paper guys.

    Thanks! I admit that I had a low-level freakout when I saw your comment about the tortoises, but in rereading the paper I see that we (fortuitously) said that they used their necks in dominance displays, without specifying whether those contests were over mates, food, etc. So I think we’re still on the right side of the law.

  9. Brad McFeeters Says:

    Good paper, but how did you go about establishing that something like the number of cervical vertebrae in diplodocines cannot possibly ever be sexually dimorphic?

  10. Mike Taylor Says:

    Brad, in the paper we acknowledge that sexual dimorphism simply can’t be assessed in sauropods because we don’t have enough specimens, of good enough quality, of any single species. (Or at least, not that have been published. There might be Camarasaurus or Shunosaurus graveyards out there with enough good specimens to establish bimodality of some feature or other, but if they’re there, no-one’s done that work.)

    So there’s a paucity of evidence — Senter made the same point back in his 2006 paper, which is why it’s one of the two lines of evidence that the considered uninformative. We didn’t attempt to show that they “the number of cervical vertebrae in diplodocines cannot possibly ever be sexually dimorphic”; we only showed that there is no evidence that they are and that anecdotal evidence doesn’t support the idea.

  11. Matt Wedel Says:

    Does anyone know of any examples of tetrapods in which the precaudal vertebral count is sexually dimorphic? I say precaudal because tail length is probably more labile than neck or trunk length. I know there is often variation in vertebral counts within a species, but I’ve never heard of variation between the sexes. If you have, please let us know!

  12. Darren Naish Says:

    There are some plethodontid salamanders (Batrachoseps) where females have longer bodies and a higher number of presacral vertebrae than males. See…

    Jockusch, E. L. 1997. Geographic variation and phenotypic plasticity of number of trunk vertebrae in slender salamanders, Batrachoseps (Caudata: Plethodontidae). Evolution 51, 1966-1982.


  13. [...] our recent paper on how the long necks of sauropods did not evolve primarily due to sexual selection (Taylor et al. 2011), one of the ideas we discussed is that sexual dimorphism between the necks of [...]


  14. [...] on from my post on sauropod neck lengths (and indeed that of the SV-POW! boys and Tet Zoo too), at the end I made the inevitable comment that necks (and indeed other structures) [...]

  15. ech Says:

    Total layman here; it was nice to be able to read a paper and be able to pretty much understand everything. Great job!

  16. Mike Taylor Says:

    ech, I can’t tell you how delighted I am to read that! Many thanks. (You might find that the same is true of our 2009 paper on habitual neck posture, BTW.)

  17. John Scanlon, FCD Says:

    Does anyone know of any examples of tetrapods in which the precaudal vertebral count is sexually dimorphic?

    This occurs in elongate squamates. E.g. Greer (1989: 171) has a table of pre- and postsacral counts for some long-bodied skinks, where males and females usually have overlapping but distinctly shifted ranges (females tending to be longer-bodied, presumably advantageous for holding more/bigger eggs). He also tests for sexual dimorphism in some other groups (e.g. varanids) and finds no evidence for it. I can’t think of a similar tabulation for vertebral counts in snakes, but the number of ventral scales is thought to be a good proxy for somite number and is very often dimorphic, females mostly longer-bodied (and shorter-tailed, as males need more space to keep their hemipenes) and often bigger, except in species with male-male combat (e.g Shine, various papers) where males have high ventral counts as well as body size. There are also plenty of species with no obvious dimorphism in ventral count or body size.

    Personally, I wouldn’t be surprised if any or all of these patterns occurred in one or another group of sauropods. Unfortunately that means I’m too open-minded to come up with a testable hypothesis.

    Greer, A.E. 1989. The Biology and Evolution of Australian Lizards. Surrey Beatty & Sons, Sydney.

  18. Matt Wedel Says:

    Many thanks for all the info, John.

    Given the occasional sexual dimorphism in precaudal vertebral counts in salamanders and squamates, the difference in cervical count between Diplodocus and Barosaurus is probably not the strongest evidence we could have cited for their generic separation. But it’s not useless, either. Even among salamanders and squamates, sexual dimorphism in precaudal vertebral counts is the exception rather than the norm, so the null hypothesis would still be no dimorphism in vertebral count.

    To overturn the null, one would have to look at other lines of evidence. And there is other evidence that does not support the hypothesis that Barosaurus and Diplodocus are sexual dimorphs. The two taxa differ in anatomical details that have nothing to do with neck elongation (see McIntosh 2005 in The Thunder-Lizards). The comparative rarity and restricted geographic range of Barosaurus compared to the more common and cosmopolitan Diplodocus is also problematic for the dimorph hypothesis.

    The biggest problem with all of this is that the sexual selection hypothesis was advanced by Senter (2006) to explain the evolution of long necks in all sauropods. Even if we’re wrong and Barosaurus and Diplodocus are sexual dimorphs (which still seems unlikely for the reasons discussed above), that doesn’t explain the lack of evidence for sexual dimorphism in neck length in other sauropod taxa. If sexual dimorphism was rampant, most sauropod taxa should come in pairs that overlap geographically and stratigraphically and differ mainly in neck length–but they don’t.

    Darn it, I wish we’d gotten all of this into the paper!

  19. Tony Says:

    Is it possible that the long necks had something to do with energy conservation? seems like it would be a lot easier to swing just a neck around than to have to move an entire body when feeding. perhaps that kind of adaptation is only benefitial once they got to be so massive?

    I’ve been doing a little writing about evolution & sexual selection lately too. I’m by no means an expert, but any feedback any of you would care to give would be appreciated:
    http://earthasweknewit.org/pages/sexual-selection/

  20. Mike Taylor Says:

    Is it possible that the long necks had something to do with energy conservation? seems like it would be a lot easier to swing just a neck around than to have to move an entire body when feeding. perhaps that kind of adaptation is only benefitial once they got to be so massive?

    Quite possible. This was first proposed by John Martin in his 1987 Mesozoic Terrestrial Ecosystems paper, and is now widely accepted, as for example in the recent sauropod palaeobiology review of Sander et al. (2010).

    Myself, I am still waiting to be persuaded. It’s not obvious to me that the energy you save by walking less is more than the energy you spend on growing, maintaining, posing and breathing through a giant neck. I have yet to see even an attempt at a qualitative analysis of this.


  21. [...] For more on sauropods and the “necks for sex” debate, see these posts by Darren Naish, Dave Hone and Mike Taylor. [...]


  22. [...] is a very useful thing.  In our recentish paper on how sauropod necks were not sexually selected (Taylor et al. 2011), we wanted to mention in passing (as part of a much more involved argument [...]


  23. [...] Wedel and Naish 2009 on habitual neck posture; Taylor, Wedel and Cifelli 2011 on Brontomerus; and Taylor, Hone, Wedel and Naish 2011 of sexual selection) but of all the many Mike-’n'-Matt projects we’ve started, this is the first to make it [...]


  24. […] thought you guys said that sauropod necks weren’t sexually selected.” Actually we made slightly different point: that the available evidence does not suggest that sexual selection was the primary driver of […]


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