Mammals have short necks because of local maxima
September 30, 2012
Having taken time to discuss at length why we posted our neck-anatomy paper on arXiv, let’s now return to the actual content of the paper. You may remember from the initial post, or indeed from the paper itself, that Table 3 of the paper summarises its conclusions:
Table 3. Neck-elongation features by taxon.
Needless to say, we puny humans lack all seven of the features that were discussed as contributing to long necks, while sauropods have them all. But it’s interesting to look at the giraffe and Paraceratherium, the two longest-necked mammals, and see what they have in common. They share quadrupedal stance; the giraffe has elongated cervical vertebrae; and Paraceratherium has absolutely large body size. But they both lack all four of the other features:
- Small, light head
- Numerous cervical vertebrae
- Air-sac system
- Vertebral pneumaticity
And they lack them for the same reason: because they are mammals. The same is true of all mammals, and the individual reasons for those four missing long-neck features are all the same: because mammals have hit local maxima, and can’t evolve away from them.
Mammals’ heads, for example, are all set up for extensive oral processing of food — certainly among large herbivores. (I think pretty much all the toothless mammals are insectivores.) They’ve got very good at it, and there’s no evolutionary pathway that can take a giraffe from its current lifestyle to a sauropod-like crop-and-swallow strategy without passing through an adaptive valley on the way. That means they are stuck with big, solid teeth and heavily engineered jaws, which means they can’t have light heads.
In the same way, mammals have much more efficient lungs than those of their reptile-like forebears, the common ancestors that they share with birds. They have evolved to a point where their lungs are too complex and effective to easily evolve into a different shape — yet by doing so, they have cut themselves off from the yet more efficient avian lung (shared by sauropods) that is capable of extracting twice as much oxygen as our lungs.
And of course in the absence of an avian-style lung, there can be no soft-tissue diverticula or air-sacs, and so no pneumatic invasion of the vertebrae.
A final nail in the coffin of mammal neck length is that we seem to be strongly wired to have exactly seven cervical vertebrae — no more, no less. The exceptions are very few and far between: sloths and sirenians, and even then they don’t vary from the seven-cervical pattern by more than one or two vertebrae.
Skull and cervical skeleton of the three-toed sloth, Bradypus tridactylus, taken at the University Museum of Zoology, Cambridge (UK). Note the nine cervical vertebrae — the most of any mammal.
As for why we can’t get past seven, or at most nine, cervicals — that’s harder to answer. There’s no reason why seven should be an adaptive maximum, so it seems that the reason is genetic: the instructions to produce seven cervicals are part of the same gene complex that gives us an advantage in some other way. I have vague memories of an excellent talk at the Bristol SVP suggesting that cervical-count is linked to cancer resistance, but I can’t remember any of the details.
Anyone able to elaborate?
Anyway: this is how evolution works, and why it doesn’t make organisms (including us) as perfect as we might wish. It has no goal in mind — such as a long neck — and blindly follows the path that at that moment gives the organism the best chance of reproducing successfully. That means an animal like a giraffe, even though it is clearly selecting for neck length, is trapped on an adaptive hill and can’t get down across the valley to a higher peak.
Sauropod Vertebra Picture of the Week 
October 1, 2012 at 3:28 pm
While the article is certainly an interesting thought piece I can’t help but feel that every time you ask why X doesn’t have a longer neck, you guys are asking a misleading question. After all why should, say, a giraffe evolve an even longer neck? Is it really at the limits of mammalian “engineering” or is it where it is because it doesn’t need to be any longer? I mean, what’s it competing with besides other giraffes? Even the acacia trees that it eats seem fine with using thorns an ants to keep giraffes away rather than growing out of their reach.
As for the seven cervical vertebrae limit, I wonder if it has something to do with losing the cervical ribs. Sauropods seem to have acquired a fair amount of their cervical verts by “stealing” dorsal vertebrae, which come equipped with ribs. Mammals may not cervicalize as easily because of all the issues associated with the acquired ribs (namely, impinging on vertebral arteries) whereas reptiles have been dealing with neck ribs since their beginning.
Of course this doesn’t stop them from doubling actual cervicals. Maybe leaving those Hox genes on results in a cascade that is ultimately maladaptive. I wonder if there has been any Evo-Devo on this?
October 1, 2012 at 4:15 pm
Thanks, Jura, lots of interesting stuff there.
Are you seriously suggesting that giraffes are not selecting for neck length? If not, then how do you explain extant giraffes having longer necks than any of their ancestors?
Ah, this is funny. The very first substantive point I ever made in any palaeobiological discussion was just this, back in July 2001. I wrote to Matt (who at that point I’d never met), saying:
And Matt’s response:
Remember, animals don’t know what species they are a part of! They are competing with other individuals, of the same and different species. And some forms of competition (e.g. sexual display) pertain only within a species.
So the absence of other long-necked animals sympatric with giraffes is certainly no reason to expect the giraffes to stop evolving longer necks.
That’s an interesting thought. I wonder. My monitor lizard skeleton has no cervical ribs, but then most monitors don’t have very long necks (although see this sand goanna, Varanus gouldi). Birds of course have abject cervical ribs, but they do have the unossified equivalent in long hypaxial tendons, so perhaps their ability to multiply cervicals is indeed related to this.
No idea. Anyone?
October 1, 2012 at 5:47 pm
Nice article! I remember reading an article by Solounias (1999) stating that the giraffe has 8 cervical vertebrae. The eigth vertebra has the same moprhology as the seventh thoraic vertebra in other mammals, and the 9th vertebra has the same moprhology as the first thoracic vertebra in other mammals.
Also there is one paper by Galis (1999) suggesting that developmental constraints in genes Hox control the number of cervical vertebrae of mammals. Mutations afecting the number of cervical vertebrae would have very negative effects such as an increased succeptibility to cancer.
Sources:
http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1999.tb00989.x/abstract
http://www.ncbi.nlm.nih.gov/pubmed/10327647
October 1, 2012 at 6:16 pm
Tony S. writes:
Yes, I’ve seen that article. But Badalangana et al. (2009:739) disagree:
For what it’s worth, giraffe axial columns don’t look dramatically different to those of other ungulates in my eyes, and there’s nothing about the 8th vertebra that screams “dorsal^H^H^H^H^H^Hthoracic!” to me. But I suppose the designation of vertebra 8 as T1 or C8 is rather arbitrary anyway. It’s a point of classification rather than of anatomy.
That sounds useful, I’ll look it up. Chimes well with my vague memories of the SVP talk. Thanks for the pointer.
Reference
Badlangana, N. Ludo, Justin W. Adams and Paul R. Manger. 2009. The giraffe (Giraffa camelopardalis) cervical vertebral column: a heuristic example in understanding evolutionary processes? Zoological Journal of the Linnean Society 155:736-757.
October 1, 2012 at 6:18 pm
‘I think pretty much all the toothless mammals are insectivores.’
Umm…<mysticete cetaceans anyone? They are macropredators, though I guess they do eat some arthropods, but marine crustaceans rather than terrestrial insects.
October 1, 2012 at 8:08 pm
Oh, sure, mysticetes. But they live in a different world. They are certainly not losing teeth as a result of selective pressure for a smaller, lighter head!
October 1, 2012 at 8:14 pm
A few points.
First, to everyone who is still upset that we addressed why sauropods have long necks: chill. We’re not saying that long necks are universally superior to short necks. Obviously sometimes there’s a selective advantage to having a shorter neck, even in sauropods (e.g., Brachytrachelopan, Isisaurus). But long necks are interesting to us. We could have looked at things from the other side and written a paper called, “Why sauropods have such small heads, and why mammals have such big heads,” and talked about oral processing and brain size and display structures (horns, antlers, tusks, etc, all of which sauropods lack), but that wasn’t the question we were interested in.
Second, yes, the reigning hypothesis for the almost invariant cervical count in mammals is that the developmental patterning genes governing cervical count have gotten pleiotropically bound up with the genes governing cell division. That would explain why variant cervical counts in humans and other mammals are so frequently associated with gnarly cancers. AFAIK so far the proposed mechanism has not been extensively tested. There’s a good plain-language summary and a couple of cited refs here.
I’m going to put my pedant hat on regarding giraffe neck length. I don’t think we can say that giraffes have been selected to have long necks, specifically. They’ve certainly been selected for height, but their necks are just barely long enough to reach the ground, so while we can imagine workable giraffes with longer necks, we couldn’t have giraffes with shorter necks, unless they had shorter legs as well. For all I know, maybe selection in the giraffe lineage was for longer legs, and the necks are just being carried along as part of that package. Extinct giraffids and giraffoids with shorter necks had shorter legs as well–as does the extant okapi. The same is not true in sauropods, where in almost all taxa the neck is MUCH longer than it needs to be to reach the ground.
This brings up an interesting question: how many long-necked animals actually have necks of the minimum length necessary to reach the ground, but no longer than that? Almost all of the runners-up we considered in the paper probably fall into that bin. Certainly things like ostriches, Gigantoraptor, and indricotheres. So maybe one thing we should consider is a fundamental division between those critters that could have shorter necks (sauropods, probably azhdarchids) and those that couldn’t (almost everything else); for the latter group, it may be loony to talk about selection for neck elongation at all, as opposed to leg length or sheer height or whatever.
Finally, although we mentioned a low and apparently fixed cervical count in mammals as a potential limit on evolving a long neck, I don’t think either of us puts much stock in it. For one thing, it’s possible to have a very long neck with a small number of highly elongate vertebrae. The longest azhdarchid verts are way, way, way longer proportionally than any known sauropod or giraffe vert (see the last image here), so we know that stupid-long vertebrae are not ruled out by any fundamental amniote developmental rules. And having a big heavy head full of teeth is probably much more of a constraint on neck length than is vertebral count.
October 1, 2012 at 8:21 pm
What?!
October 26, 2012 at 6:56 pm
For you guys if you missed this new research presented at this year’s SVP: http://vertpaleo.org/The-Society/SVP—Paleo-News/Society-News-and-Events/Extinct-Fossils-Reveal-their-Genetic-Pattern.aspx
October 26, 2012 at 6:59 pm
Thanks. Yep, I saw this, but I don’t have the genetics to really understand what it’s saying (and it also seems terribly low on details). I’ll look forward to seeing the paper when it comes out; or, hopefully, the pre-print before then!
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