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Re: Speed Potential in Tyrannosaurs (long) [resent plain text]



[message got truncated before, I hear, so here's plain text version, which hopefully will make it thru]:

Hi folks,
I'll provide a reply to GS Paul's comments here, but don't intend to get drawn into another longwinded debate. As I've said before, the details as usual will be worked out over the next few years/decades in peer-reviewed journals. But it's always important to talk informally to non-specialists and specialists alike about scientific debates. To me, the paper is ancient history already, and I have long since moved on to other projects, including more realistic 3D models of dinosaur locomotion, and the usual experimental + modeling studies on living animals too. But I understand if people want to have their say about this controversy, and I welcome disagreement or agreement. [Sure, I like the latter more than the former; I'm only human!]
First off, I appreciate Greg's point of view. Again, I welcome debate as long as it is about the science itself and is presented in a collegial fashion. No one in their right mind has fun when scientific arguments turn nastily sarcastic or personal. I like Greg's hypothesis that tyrannosaurs could run fast; it's provocative and creative, and has some data consistent with it, and Greg has done a good job of marshaling evidence and presenting logical arguments. As I've said often, I'll be ready to admit I'm wrong if someone shows me data that prove me wrong. I haven't seen it yet, though, not in Greg's post or any others since our paper came out. Maybe Bakker's SVP talk will show some unambiguous fast-running tracks, whose speeds can be bounded with a confidence interval to lie at or above 11m/s for a large tyrannosaur. I won't be at SVP this year as I am attending biomechanics/biology conferences for once this year, but it sounds like a wild time there in OK.
OK, on to business. I still think the fast-running tyrannosaur hypothesis is incorrect, and new independent studies I've done continue to show support for what Mariano and I found in the Nature paper. I agree with a lot more of Greg's comments in his post than he probably thinks, but we do disagree on the fundamentals, it seems. The new paper, which should be out sometime next year, is longer and explains things a lot more clearly, with more data and taxa too. It should help people understand what we did; most of the arguments I've seen against our paper stem from a lack of understanding and/or an a priori dismissal of our methods. And yes, my conclusions do not change significantly, but I also have some new conclusions that are broader in scope.
Specific replies to Greg's post:


1. Small tyrannosaur: I think Greg is overemphasizing this "error." As we noted in the paper, and have since substantiated with new approaches, the rather high T value for the small tyrannosaur was for a crouched pose; T dropped to 5% for a more straight-legged pose, and I am now sure it could be even lower than that. The Coelophysis model could drop to less than 1% via the same strategy, but we showed how Tyrannosaurus couldn't get nearly that low. This, and the models of a human, chicken, and gator already fulfilled (to our satisfaction, and the reviewers') Greg's demand for "noncontroversial examples" and that "the method should have been adjusted until reasonable results were obtained." I've done the ostrich model now and, surprise, it's a runner! Glad I did it, though, now we have a 3D ostrich model based on a real animal, and many more to come. Greg comments that "the belief that increased leg flexion results in increased leg muscle mass has yet to be demonstrated by measurements, especially of animals of similar size and locomotary potential." I'm not sure what he means here; surely if animals bend their legs their muscle mass does not increase, violating the laws of energy/mass conservation. What happens when animals bend their legs, as Andrew Biewener and others have shown, is that the ratio of the GRF moment arm (R) to the muscle moment arm (r) increases, decreasing effective mechanical advantage and increasing the effort required from the muscles. This is extremely well substantiated in the literature; I know of no contradictory evidence and the method is soundly based on first-principles. Now, increased muscle effort (force) could require more muscle mass if the muscles were exerting themselves maximally before the limbs were bent, but increased muscle mass can only be attained through exercise or evolution. Humans "Groucho running" with bent legs do require ~50% more energy precisely because the ratio of R/r is higher; look into it and see; it follows directly from the biomechanics. Thus it's not "meaningless", even though it is not "optimal".

2. Energetic arguments: Our paper focused on the mechanics, not energetics, of running tyrannosaurs, although as a side comment we did note that a fast-walking tyrannosaur would have needed a surprisingly high % of active muscle volume. Regardless, the paper stands or falls based on the mechanical arguments: in order to run fast, an animal's muscles must be able to exert enough force to prevent limb collapse, and that force is dependent on muscle cross-sectional area, and hence mass. I don't think we know enough to say that "If there is one thing we can be certain of, it is that adult Tyrannosaurus moved with the same, low level of mass specific power output observed in elephants." Read the energetics scaling literature (which has a better size range for walking than for running, unfortunately) and notice the strong secondary signal for many animals. They don't all fall on the line. Therefore we should not expect scaling "laws" to dictate exactly what the metabolic cost of locomotion was in a particular animal, although with an energetics-based model Greg could estimate it and do sensitivity analysis to see how sure he is.

3. Scaling: Size factors into the scaling equation for the muscle mass to run fast, because if you collapse our equation 1, it reads simply as T ~ (L*R)/r. Therefore, T should scale as two linear dimensions divided by third linear dimension, or hence T ~ mbody^0.33. In a tricky way, then, T is not so much dependent on the exact body mass value that we input, but rather on the linear dimensions of the animal. So size matters, but body mass value does not matter in our equation, in answer to previous queries. Again, this all follows from simple mechanics and validation from decades of animal research. As we noted on our webpage (http://tam.cornell.edu/students/garcia/.trex_www/naturepaper.html) (caught it too late for fixing in the paper), the giant chicken scaling line was indeed off base a bit, because two joint angles in the giant chicken were accidentally put 5 degrees different from the small chicken (learned my lesson about having too many Excel files at once!). The scaling is relatively linear (not exactly, because of significant figure rounding) when the correction we noted is made. We've checked through all of the math in the paper now many times, and none of the corrections change our main conclusions. The scaling of extensor muscle fiber lengths (L) is an interesting question and we are looking at it; it's more complex than Greg portrays it.

4. Absolute speeds vs. relative speeds: Our paper was mainly about relative speeds (Froude number). While in some cases in mammals, maximum absolute speed does not change much with body mass, maximum relative speed clearly does decline. The interesting question to me is, when does max relative speed decline so much that a certain gait or absolute speed becomes unattainable due to biomechanical constraints? That's what our paper touched on, and new papers will continue to investigate. Greg says "Its up to those who think otherwise to produce data showing that relative leg muscle mass increases in animals of similar top speed as size increases. Good luck." When I hear statements like this, I can't help but think that people are too lazy or dogmatic to collect their own data and test their own assumptions and ideas. I think the burden is just as much on Greg et al. as it is on me to dissect real animals and show how leg muscle mass changes with speed/size. Looking at photos, running around the living room, or watching Hatari or PBS isn't a rigorous way to collect data or test hypotheses that involve the complex relationship between anatomy and dynamics. I agree, though, that ceratomorphs would be a wonderful group to look at in detail; it's been on my mind for years. Hopefully someday. As a biologist, I've always tried to base my work on a foundation of studies of living (or recently dead) animals; it's what most paleontologists do the worst job of. Hence I've worked hands-on in locomotor experiments with live birds, crocodiles, and elephants, and have dissected more tetrapods than I can shake a drumstick at. We'll see what can be done in the few decades that my career will span. [Correction to Greg's post: for short distances, human sprinters can reach 12m/s top speed, not 10, although the average speed over 100m is the very recent world record by Tim Montgomery, 9.78 m/s.]

5. Anatomy: "Not much faster than elephants" is the key phrase here. The question is, how important is anatomy, especially so-called "cursorial adaptations"? Does the anatomy of a tyrannosaur demand that it must have been able to run 10%, 50%, or 300% faster than an elephant of similar size? I am totally unconvinced by the anatomical arguments that put tyrannosaur speed at 11-20 m/s; they are based on a weak, correlation and analogy-based reasoning that has many untested assumptions about the relationship of anatomy and locomotor performance. The few biomechanical studies I've seen that put tyrannosaurs/big theropods at those speeds have worse flaws than our paper did; I've gone through the math and I understand where they went wrong. Our model included all of the tyrannosaur anatomy that was needed, and T.rex was still found wanting. Long legs, big muscles, big cnemial crests, big pelvis, strings and sealing wax and other fancy stuff... They did not change T enough to make tyrannosaurs into speed demons. My feeling is that people who have mainly studied anatomy have been seduced by tunnel vision into thinking that anatomy is a failsafe, or at least a very reliable, indicator of locomotor performance. Anatomy and performance are correlated, I am sure, but I see weak links in many parts of the correlation when I look at what quantitative effect the anatomy has. I think biomechanics is what is needed to test anatomical specializations and see how important they really are; the work has not been done to my satisfaction.

6. Wrapup: I hear criticism of computer models most frequently from people that do not understand them, and second most frequently from people that do not use them, or do not like quantification or math. Greg's post leads me to suspect that he does not understand how our model works. I doubt that he could accurately explain it, which makes me wonder how he can doubt its results despite the seemingly contradictory evidence he cites. Before I doubt a model, I go through it and see how it works out on paper (e.g., draw my Fig. 1 and make sure I can understand where the parameters fit in), and which parameters/assumptions are most influential (e.g., sensitivity analysis). I do not only use models; I use every line of evidence that I can find, including anatomy. I do not want to avoid or exclude any approach. But time is limited, and I take the approaches that I deem to be most relevant and efficient. I agree that we badly need more data from living animals, that's what my research program is all about. Computer models in isolation are not that great. But computer models will always be a useful supplement to any other line of evidence, particularly when it comes to extinct animals, because they allow you to test hypotheses indirectly without relying on time machines, analogy, or mere speculation and anecdotes. You just need to be cautious when using them. The fact that scientists have used the same methods we used in the Nature paper for decades of studying living animals gives me strong confidence in the applicability of these modeling methods to extinct animals, particularly when carefully approached with sensitivity analysis. Failsafe? No way! I'm not that deluded. I'm probably more anal-retentive about scrutinizing my models these days than I should be. No line of evidence is faultless, but modeling is not useless or untrustworthy either. Like any line of evidence, it's the Way that the evidence is collected, used, and evaluated that matters, and I'm glad that at least both Greg and I are committed to using science as our Way.

Have at it, folks.   :-)
John


=========================================== John R Hutchinson NSF Postdoctoral Research Fellow Biomechanical Engineering Division Stanford University Durand 209, BME Stanford, CA 94305-4038 (650) 736-0804 lab (415) 871-6437 cell (650) 725-1587 fax ===========================================