Why seven and not 24?

2 08 2007

PZ of Pharyngula has a new column in SEED about why humans (and by implication, many mammals) have seven neck vertebrae and not 6. The article is a quick read, giving a fair overview of why people who have six neck vertebrae (the seventh becoming the 1st vertebrae to bear ribs and therefore be “thoracic”) aren’t as numerous as those with the “normal” number, but the opposite side of the question isn’t answered. Why don’t we have nine like some sloths? How did that sloth even get nine? In the case of ancient reptiles like icthyosaurs that also exhibit hyperphlangy and increase in the number of digits, how do you increase the number of vertebrae in the thorax & tail and is that related to the genes that would have regulated the fins? How did plesiosaurs end up with so many neck vertebrae? Obviously fossils are much harder to study, and while I can’t say I knew about the detrimental effects of having only 6 neck vertebrae, I do wonder about what the mammalian constraint is on seven (and if anyone has been ever found with more than seven and what advantages or disadvantages would come with that). Indeed, even though some mammals have long necks (giraffes and camels come most readily to mind), why did they experience an elongation of the existing vertebrae instead of addition? Yet another question I have to look into…


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11 responses

2 08 2007
Zach Miller

Yes, this is a question I’ve pondered before. Mammals seem awfully conservative in their skeletal structure, so vertrebrae shifts really fascinate me. Why DO sloths have nine cervicals? Why not just eight? Why not TEN? I suppose there comes a point at which excessive cervicals simply don’t do anything useful, but are they adding new vertebrae or just “shifting” upwards, so there’s one less caudal?

2 08 2007
Zach Miller

Also (sorry for the double-post), I respectfully submit my most recent post about the feeding habits of theropods on my blog to the Boneyard. Thought I’d get it in before Friday.

2 08 2007
laelaps

Thanks again for the insightful comments Zach! I’ve got a lot of reading to do before I can catch up to you in experience!

Indeed, I’m especially interested in addition of cervicals in very large animals like some of the Asian sauropods. Why did they keep adding on the neck vertebrae (and what did this mean in the rest of skeletal structure that would have to balance this out?). To me, the question of “Why not 10? or 11? or 12? etc.” is more fascinating, but like has already been discussed, mammals seem to have a definite constraint.

2 08 2007
Julia

Want to know something really weird? I had this very conversation with my mother at the weekend. My mother is a 50-something retired radiographer, and she finds dinosaur osteology fascinating (even if she does like to “correct” my nomenclature so it’s the human anatomy format). And she decided to ask me why giraffes only had seven vertebrae when sauropods had so many more. I told her that different groups of organisms had different ways of achieving the same aim. So a long neck confers an advantage? A longer neck could arise from a mutation causing elongation of the vertebrae, or a mutation on the Hox gene (as far as I’m aware), affecting how many “segments” a vertebrate has. But as I think I’ve told you, I failed Vertebrate Structure…

2 08 2007
laelaps

Being that I’m fairly certain that Hox genes lay out the organization of many animals from head to tail, I think you’re right Julia. I’m just curious as to why dinosaurs and archosaurs in general seemed to be more prone to adding vertebrae because of Hox genes in development than mammals, and if there was any disadvantage to doing this. I also wonder, like I noted above, if characteristics like hyperphlangy are related to this, being that icthyosaurs varied in their number of vertebrae and number/length of their digits. Still, even if just one individual creature added vertebrae then it would be easy for those mutation to get lost (or at least it’d take a lot to bring it out in a population), so I wonder if environment/nutrition/other epigenic factors have some role to play here. Unfortunately for us, the fossils seem mum on the subject.

3 08 2007
LH

Hi Laelaps,

A curious question indeed. The first thing I would do is to compare the Hox gene expression pattern in mouse (representative mammal) and in chick (nearly a dinosaur).

And guess what – someone has already done this, two years back:

http://ravensara.blogspot.com/2005/06/modeling-correspondences-among-hox.html

The second thing I would do is to check the fossil record to see if neck vertebrae number is more “evolvable” in dinosaurs compared to mammals.

Maybe employing the same sort of approach in this study:

http://www-news.uchicago.edu/releases/07/070726.webster.shtml

3 08 2007
laelaps

Thanks for the links, LH! I’m definitely not as up-to-date on my genetics and evo-devo as I probably should be, so I appreciate the help. It does seem that archosaurs have historically been more malleable in terms of adding vertebrae/digits depending on adaptation, although we don’t have anything like them still around (birds are good, but not quite sauropods).

Still, I think attempting to answer this question could make for good interdisciplinary study, and hopefully someone will take up the question (as there’s little I can do other than just read papers as a student). Thanks again for the resources, and for all the links you’ve put up on your blog, too!

3 08 2007
daud.deden

Ichthy & Brontosaur types had long neck with many small vertebrae due to somewhat eel-like swimming, and hydrodynamic linearity-streamlining, very different origin than very terrestrial camelids and giraffes which got long necks due to selection for advantageous feeding combined with terrestrial quadrupedal locomotion (gravity more significant than streamlining). Bront. had long tail to counterbalance the long neck during terrestrial locomotion and feeding, Icthy. didn’t since it stayed in the water. AFAICT…

Interesting coincidences regarding human vertebrae/spinal/cranial nerves, harmonic division (equidistant tones, energy freq.) and sphere division (think ovum splitting geodesically), the numbers 12 & 31 seem to connect them, I wrote about it at my blog the-arc, (see the spine pic at bottom).

3 08 2007
laelaps

Thanks for the post daud. I know that there were different reasons for sauropods and icthyosaurs to develop long necks, but I guess the main question being driven at here is why does is seem like archosaurs and their descendants are more prone to adding on vertebrae (probably due to Hox activity) than mammals. The variation needs to exist for nature to work on in the first place, so while the pressures that created the long necks are important, I’m just wondering why mammals don’t seem to experience the same sort of additions that other groups of animals have shown in the past.

Indeed, you mentioned that icthyosaurs added vertebrae for eel-like swimming, which was more true when they were evolutionarily “young” as a group. As they became more advanced, they seemed to lose the eel-like swimming ability, having a body shape and tail shape closer to tuna and lamnid sharks than sharks like swim with an eel-like motion (i.e. catsharks). During this transition more vertebrae were added even though the mode of swimming became more “rigid,” moving away from eel-like swimming.

Likewise, sauropods got acheived longer necks and tails, so they would had to add on both ends in one way or another in order to stay balanced. What the genetic basis that allowed this to happen is, at least to me, the bigger question (and I still think sexual selection could have played a big role in the evolution of long necks, doubly enhanced by a wider range of feeding opportunity).

I’ll definitely check out your blog, too, and thanks for commenting!

3 08 2007
daud.deden

I might be quite mistaken, as I’ve never studied dinos specifically.
As you say, early ichth. were eel like, I think they eventually developed a head thrust & snatch method of predation, both from the surface (IF they had some form of controllable air sacs atypical of terrestrials) and while submerged, this would require More cerv. vertebrae due to the higher speed head thrust and greater manueverability. High speed head thrust might fit with sclerotic rings in the eyes (high pressure) and 2 sets of nostrils (pressure, or possibly equalization of internal skull cavities), and yet still result in a thunniform body shape with more vertebrae. None of these counteract each other, though no extant species has this combination.
Pachyostosis of the fins indicate slow diver (walrus, manatee), not a fast chaser (dolphin), the head-thrust set-up indicates that they were slower swimming than their prey, with less efficient metabolism than their prey.
Eels can make numerous sharp turns inside coral reefs, where thunniforms must retreat blindly. Not impossible that early icth. stuck their heads into coral reef caves? If so, they’d need their fins to pull them back quickly.

Sexual selection, as opposed to what? Asexual selection? All natural selection in sexually reproducing species is sexual selection, isn’t it?

4 08 2007
laelaps

Thanks for your thoughts on icthyosaurs and body shape daud. From what I can tell, there was a definitely change over from early eel-like forms to later tuna-like forms (regardless of whether they were fast or not). I don’t contest the addition of vertebrae early on, and I could definitely see some icthyosaurs inhabiting the niches of modern reef sharks (which do go after fish in crevices), but it seems that even as they became more derived, they still added some vertebrae, which is the bigger question I’ve got, I suppose. I’ll definitely have to look into it more.

And as far as my sexual selection comment, sexual selection in terms of how I’m using it are specific morphological features or behaviors that directly impact mate competition or the choice of the selective sex. For instance, not long ago I blogged about antlers in deer and horns in bovids, which (in males at least) are selected for size for use in sparring contests to get access to females. The other “textbook” example of female choice would be the tails of some tropical birds, where females certainly prefer longer tails over shorter ones. Who reproduces and who doesn’t has a lot to do with evolution, but when I say sexual selection I mean those features or behaviors that directly relate to mate competition or choice. In terms of giraffes, it’s been hypothesized that their long necks could have arisen out of the “necking” contests known among males today, although I think this hypothesis looks to much at current use and tries to make it work backwards. What I would like to find out, though, is if females prefer males with longer necks; in some animals, like swordtail fish, the size or length of a feature can definitely attract females. I’m doubtful as to how such an experiment could turn out, and Okapis would have to be tested along with giraffes, but I do wonder if there is any female preference despite the fact that the males compete for access to the females.

You do bring up a good point, though; “sexual selection” can be a somewhat ambiguous term, and any sexually producing species will have one method of selecting mates vs. another, the style in which this is done may allow for (or maybe even prevent in some cases) the selection of characters that leads to sexual dimorphism or even an overall change in the population (depending on how the selection is occurring and how the genes behind the features in question or inherited).

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