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Sexual selection in the fossil record

The views expressed are those of the author and are not necessarily those of Scientific American.


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Sexual selection in action. Tyrannosaurids engage in a courtship ritual (an azhdarchoid and neornithine make guest appearances). It's not a great picture, but I've seen worse (it's by Darren Naish).

Sexual selection – the phenomenon in which organisms compete over and choose mates on the basis of desirable traits – is one of the fundamental driving processes of evolution. It’s all around us, seemingly explains an enormous amount of the morphological and behavioural variation observed in the natural world, and has been shown to be tightly linked with speciation, adaptation, and even the susceptibility of a species to extinction. Accordingly, it is inherently logical to assume that sexual selection shaped the evolution of extinct animals as well as modern ones.

The latest contribution on this subject – that is, on the possible presence of sexually selected traits in fossil animals – has just been published by Robert Knell, myself, Joseph Tomkins and Dave Hone in Trends in Ecology and Evolution (or TREE). The article is only out as a digital preprint at the moment (Knell et al. 2012). Given that this article is about fossil animals in general (and not just about tetrapods) it includes some discussion (and illustration) of arthropods, fishy things and other animals (Knell et al. 2012). It’s a general review of this large and complex subject, pointing to the areas we might consider when thinking about sexual selection in fossil animals.

Select fossil vertebrates with 'exaggerated morphologies', from Knell et al. (2012). a, Achelousaurus. b, Olorotitan. c, Protoceras. d, Akmonistion. e, Diceratosaurus. f, Diplocaulus. Images by Darren Naish (redrawn from various sources. Not to scale).

Those pesky archosaurs

The antlers of fossil deer, the horns of fossil bovids and rhinos, and the eyestalks of fossil stalk-eyed flies can – I think we can say with some confidence – be reliably interpreted as sociosexual organs just like those of living deer, bovid, rhino and stalk-eyed fly species. It’s those pesky archosaurs that are such a problem.

I’m of the opinion that many (or even most or even all) of the exaggerated structures present in dinosaurs and pterosaurs evolved under sexual selection pressure. I think this because I find the structures concerned (we’re talking cranial crests in hadrosaurs and pterosaurs, horns and frills in horned dinosaurs, and so on) analogous to sexually selected structures present in living animals. But the world is complicated, organs rarely have a single function, and other (sometimes competing) functions are not always absolutely ruled out.

Similicaudipteryx adult and juvenile, showing difference in integument between adult and juvenile. Image by Xing Lida and Song Qijin.

On that note… feathers. The presence of elaborate plumes, tail-fans, streamers and racquet-like structures in some Cretaceous birds and bird-like maniraptoran theropods suggest that feathers were widely used in sexual display among these animals, as they are in birds today. As we note (Knell et al. 2012), data on the growth changes seen in oviraptorosaur forelimb feathers (Xu et al. 2010), and on the presence of feather iridescence in the deinonychosaur Microraptor (Li et al. 2012) add further support to the idea that these dinosaurs were using feathers in display. This doesn’t mean that everything about the early evolution of feathers was linked to display; nevertheless, the possible role of sexual selection in the early evolution of feathers is intriguing. It has been brought to my attention that we should have cited Richard Cowen and Jere Lipps’s book The History of Life in the context of this discussion, since they devoted a whole section of their book to the hypothesis that feathers may have originated as display structures and should be credited for explicitly proposing this hypothesis.

Incidentally, the article includes a reconstructed skull of the Asian lambeosaurine hadrosaur Olorotitan, redrawn from the original description. With classic timing, the monograph on Olorotitan has just appeared (Godefroit et al. 2012), and it makes the skull look somewhat different – it has a slender, more gracile snout and a deeper cranial crest. But that’s ok, the crest still has the elaborate form I was aiming to portray.

Our new article is one of several sexual selection projects I’ve been involved in. There’s the whole ‘the long necks of sauropods did not evolve primarily under sexual selection’ project (Taylor et al. 2011), and the mutual sexual selection one (Hone et al. 2012). Ontogenetic changes seen in the exaggerated structures of pterosaurs and other animals, coupled with the sheer extravagance and ‘non-functional’ nature of certain crests and other structures have also led me to argue that sociosexual display was the primary force directing the evolution of such structures (Naish & Martill 2003, Martill & Naish 2006).

The cranial crest of Pteranodon exhibits allometry well in excess of what is predicted for thermoregulation or most other 'functional' requirements. From Tomkins et al. (2010).

Across all these pieces of research, there remains a fundamental and nagging question: how do we test for the presence of sexual selection in fossil animals? As we discuss (Knell et al. 2012), this is one of the main problems surrounding this issue. As indicated by my comment above, palaeontologists have mostly relied on analogy – that is, proposing a sexual selection function for a structure based on its general appearance and lack of ‘mechanical function’.

Information from ontogeny, data from the allometric changes that structures underwent during growth (Tomkins et al. 2010), and the inferred costliness of some of the relevant morphological traits seen in fossil animals all, we argue, appear more consistent with sexual selection than other explanations (Knell et al. 2012). But multifunctionality is never excluded. If the insane boomerang-shaped head of Diplocaulus was used in sexual display (as suggested by its growth trajectory: Rinehart & Lucas 2001), that doesn’t mean that it couldn’t also function in improving swimming performance (Cruickshank & Skews 1980).

The problem(s) with sexual dimorphism

Sexual dimorphism is an important indicator of sexual selection, but – as Dave, Innes Cuthill and I showed in our work on mutual sexual selection (Hone et al. 2012) – it’s not required, and it’s absent in some living animals that use exaggerated structures in sexual selection. Furthermore, there are some organisms (like spiders and echiuran worms) where pronounced sexual dimorphism is not clearly linked to sexual selection.

Sexual dimorphism in the monofenestratan pterosaur Darwinopterus, as portrayed by Mark Witton.

Palaeontologists always have sexual dimorphism in mind when looking at specimens thought to belong to the same taxon. Occasionally, we get really, really lucky and discover ridiculous things like numerous specimens of a species where some individuals have crests, some don’t, and where at least one crestless specimen is preserved with an egg right next to its pelvic girdle (Lü et al. 2011)…. more normally, however, such evidence is lacking and sample size is a massive problem. We know from living animals that individuals of both sexes overlap in measurements and proportions, and can only be reliably distinguished in robust statistical terms when large sample sizes are available. Oh, and the specimens don’t just have to be numerous, but also nicely preserved, and often three-dimensional as well. Good luck, then.

Mesozoic dinosaurs may well have been ridiculous, just like living animals. Sauroposeidon, reconstructed with elaborate display organs, by Brian Engh of dontmesswithdinosaurs.com.

At the background of this research is the claim from some palaeontologists that the elaborate ornaments of dinosaurs and pterosaurs (and perhaps other fossil animals as well) functioned as ‘species identification’ badges, and that sexual selection fails as an explanation. Padian & Horner (2011a) argued that sexual selection couldn’t be at play in Mesozoic dinosaur lineages because sexual dimorphism appears to be absent: needless to say, this line of argumentation is dependent on the rejection of the existence of mutual sexual selection. Padian and Horner’s rejection of mutual sexual selection is based on the fact that Darwin regarded sexual dimorphism as a necessary prerequisite of sexual selection (Padian & Horner 2011a, b). I’m not alone in finding this an unsatisfactory position (Knell & Sampson 2011, Mendelson & Shaw 2012).

Furthermore, the ‘species identification’ hypothesis fails to credit the fact that ‘species recognition’ has yet to be supported as the key mechanism explaining trait evolution in modern animals that possess elaborate structures – instead, the structures concerned mostly have roles in sexual selection.

Excelsior!

Anyway, hopefully, Knell et al. (2012) will be seen as a useful review of sexual selection as it applies to palaeontology, and let’s hope that it inspires future research and the exploration of various of the problem areas we point to. There’s more to come.

Knell et al. (2012) has been discussed elsewhere in the blogosphere already. Dave Hone covered it here at his Archosaur Musings and Brian Switek just wrote about it here at Dinosaur Tracking. To see the original, uncropped version of the ridiculous brachiosaur reconstruction shown above, go here. For previous articles on some of the issues mentioned here, see…

Refs – -

Cruickshank, A. R. I. and Skews, B. W. 1980. The functional significance of nectridean tabular horns (Amphibia: Lepospondyli). Proceedings of the Royal Society of London, series B: Biological Sciences 209, 513-537.

Godefroit, P., Bolotsky, Y. L. & Bolotsky, I. Y. 2012. Osteology and relationships of Olorotitan arharensis, a hollow-crested hadrosaurid dinosaur from the latest Cretaceous of Far Eastern Russia. Acta Palaeontologica Polonica 57, 527-560.

Hone, D. W. E., Naish, D. & Cuthill, I. C. 2011. Does mutual sexual selection explain the evolution of head crests in pterosaurs and dinosaurs? Lethaia 45, 139-156.

Knell, R. J., Naish, D., Tomkins, J. L. & Hone, D. W. E. 2012. Sexual selection in prehistoric animals: detection and implications. Trends in Ecology and Evolution DOI: 10.1016/j.tree.2012.07.015.

- . & Sampson, S. 2011. Bizarre structures in dinosaurs: species recognition or sexual selection? A response to Padian and Horner. Journal of Zoology 283, 18-22.

Li, Q., Gao, K.-Q., Meng, Q., Clarke, J. A., Shawkey, M. D., D’Alba, L., Pei, R., Ellison, M., Norell, M. A. & Vinther, J. 2012. Reconstruction of Microraptor and the evolution of iridescent plumage. Science 335, 1215-1219.

Lü, J., Unwin, D. M., Deeming, D. C., Jin X, Liu, Y., & Ji, Q. 2011. An egg-adult association, gender, and reproduction in pterosaurs. Science 331, 321-324.

Martill, D. M. & Naish, D. 2006. Cranial crest development in the azhdarchoid pterosaur Tupuxuara, with a review of the genus and tapejarid monophyly. Palaeontology 49, 925-941.

Mendelson, T. C. Shaw, K. L. 2012. The (mis)concept of species recognition. Trends in Ecology and Evolution 27, 421-427.

Naish, D. & Martill, D. M. 2003. Pterosaurs – a successful invasion of prehistoric skies. Biologist 50 (5), 213-216.

Rinehart, L. F. & Lucas, S. G. 2001. A statistical analysis of a growth series of the Permian nectridean Diplocaulus magnicornis showing two-stage ontogeny. Journal of Vertebrate Paleontology 21, 803-806.

Padian, K. & Horner, J. R. 2011a. The evolution of ‘bizarre structures’ in dinosaurs: biomechanics, sexual selection, social selection or species recognition? Journal of Zoology 283, 3-17.

- . & Horner, J. R. 2011b. The definition of sexual selection and its implications for dinosaurian biology. Journal of Zoology 283, 23-27.

Taylor, M. P., Hone, D. W. E., Wedel, M. J. & Naish, D. 2011. The long necks of sauropods did not evolve primarily through sexual selection. Journal of Zoology 285, 150-161.

Tomkins, J., LeBas, N., Witton, M., Martill, D., & Humphries, S. 2010. Positive allometry and the prehistory of sexual selection. The American Naturalist 176, 141-148.

Xu, X., Zheng, Z. & You, H. 2010. Exceptional dinosaur fossils show ontogenetic development of early feathers. Nature 464, 1338-1341.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Dave@reptileevolution.com 8:44 pm 09/7/2012

    Darren,

    Good blog. No doubt sexual selection is definitely at play, probably in all species, more or less. Bennett considered those Pteranodon with a small crest female. With regard to Pteranodon, when do we know we’re dealing with gender differences and not just the evolution of a big crest from a small crest? To that point, in Tapejara and Tupuxuara we don’t find any small female-type crests. Does that mean no females? Rather we get juveniles with crests just as large relatively as those in adults. In Darwinopterus the true female with egg had distinct proportions from the crested specimens, down to the pedal phalangeal ratios. So, how do we know any crested Darwinopterus is male, and not just a similar species with a crest? I think we may have to wait for a nesting site to give the gender thing a real go.

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  2. 2. naishd 8:50 pm 09/7/2012

    Thanks, Dave. Darwinopterus is a complex case and not all the evidence is published yet (Dave Unwin is talking about it next week, I believe). In this case, I think there are too many coincidences to discount the hypothesis that those crested specimens are males (pelvis morphology, for one thing). And it will be interesting to see what bone histology reveals…

    Darren

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  3. 3. way2ec 12:29 am 09/8/2012

    Great article. I admit my eyes glaze over when the vocabulary gets “technical” but that reflects on my being out of my league, not on the writer nor the science. I like that at the same time we get to enjoy the “ridiculous” brachiosaur reconstruction, I was immediately back in my time machine reminding myself that these animals had millions of years to do their mating dances and why wouldn’t we find extreme examples just as we do today. And to think how much we are missing without soft tissue evidence. Given that sexual selection seems to explain the sometimes huge breasts and buttocks of the female of our species, and if mutual selection is at work, some of the males also seem to have more than is needed for reproductive success, yet we have nothing in the fossil record to “prove” or even track the development of these structures. Are we not “some living animals that use exaggerated structures in sexual selection?” I would guess that our pubic hair patterns, and I include beards and mustaches as “pubic hair” (as in puberty), might we expect to find many other “ridiculous” examples if we could but better travel back in time. Oh well, plenty of living examples around today.

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  4. 4. danarel 12:53 am 09/8/2012

    Awesome post, just think about how amazed Darwin would be at all of this. He was amazed by the peacock, now we are talking about dinosaurs!

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  5. 5. Heteromeles 1:58 am 09/8/2012

    Fun, and great to see something on this topic outside the paywall.

    I’m still chewing over how to disentangle sexual selection and functional adaptation. On the surface, you’d think it would be possible to disentangle which was which, wouldn’t you? Then I thought about Americans, car buying habits, and sexual selection vs. adaptive significance, and realized that yes indeed, for one of the best known examples of sex selection (men and their cars, although it’s really been men, women and their cars for about two decades now) is actually fraught with ambiguity and contingency. For example, some monster SUVs are bought to actually transport families of eight or nine routinely (rather than showing off one’s credentials as a massive consumer with right wing tendencies), while some very expensive sports cars (cf Top Gear) are so geekishly ugly and specialized that they don’t even serve as adequate status displays.

    Now I have a better appreciation of the complexity of fossil interpretation. Bravo for making it this far, Darren, and your coauthors too!

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  6. 6. Dartian 2:06 am 09/8/2012

    Darren:
    It has been brought to my attention that we should have cited Richard Cowen and Jere Lipps’s book The History of Life in the context of this discussion, since they devoted a whole section of their book to the hypothesis that feathers may have originated as display structures and should be credited for explicitly proposing this hypothesis.

    I’m not familiar with Cowen & Lipp’s book, but, just as a general comment: Anyone who proposes a novel hypothesis about some very specific subject should do it somewhere where the right people can reasonably be expected to see it. A book with a super-generic title such as The History of Life is not such a place.

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  7. 7. MA-writer 4:11 am 09/8/2012

    Feathers probably originated for practical physical reasons, such as insulation and, almost immediately, camouflage. Display structures and hues would be later elaborations.

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  8. 8. naishd 5:42 am 09/8/2012

    Thanks for these great comments.

    Dartian (comment 6): you’re right – there is a general feeling that it’s ‘wrong’ not to credit something when it’s the focus of a technical paper. In this specific case, I believe that the authors tried to get the idea out there in a technical paper but eventually gave up after several rejections.

    MA-writer (comment 7): you may be right, but the evolution of feathers involved numerous incremental steps and it isn’t clear that all of those stages were driven by selection for insulation and camouflage. The relevant stages as goes sexual selection involve the evolution of enlarged, vaned feathers on the limbs and tail – was that _really_ driven by selection for insulation or camouflage? A role in locomotion is of course plausible, but display may have been involved as well.

    Darren

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  9. 9. Heteromeles 11:42 am 09/8/2012

    Does sexual selection cover better incubation of eggs? That might be a use for vaned feathers on the body.

    Speaking of which, what are the intermediate steps between depositing eggs in a warm nest and incubating them against a body? Squatting guard over eggs due to high predation rates?

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  10. 10. dicklipke 2:17 pm 09/8/2012

    It took nature and the evolutionary road to discover the best and only way to keep these experimental wonders reproducing in an attempt to maintain their population.
    I bet the number of failures along the way would stagger our imagination.
    Wish I could have seen some of those wondrous and bizarre attempts that will remain unknown and hidden for ever.

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  11. 11. Andreas Johansson 4:54 pm 09/8/2012

    Padian & Horner (2011a) argued that sexual selection couldn’t be at play in Mesozoic dinosaur lineages because sexual dimorphism appears to be absent: needless to say, this line of argumentation is dependent on the rejection of the existence of mutual sexual selection.

    It seems to me there’s a more basic problem with the argument – how do we know sexual dimorphism was absent? For all we know, boy brachiosaurs were blue and girl ones were pink, right?

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  12. 12. Jerzy v. 3.0. 5:41 pm 09/8/2012

    Just to point, that there is a theory that dinosaurs and pterosaurs passed several different ecological niches during their growth. So theoretically excessive allometric growth of some structure may be related to change of ecology, not for sexual selection. This is what we don’t see in modern birds and mammals which are normally cared by parents until almost fully grown.

    Playing devil’s advocate – could growth of ceratopsian horns be strictly for defensive purposes, because juveniles could hide and adults had to defend themselves from theropods?

    BTW, this hypothesis is testable – check if crocodiles and komodo dragons have some structures with more than allometric growth. They are among few large animals which pass through several ecological niches during growth.

    BTW2 – what about extravagant structures which get resorbed during growth – like spikes of Dracorex – Stygimoloch?

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  13. 13. Heteromeles 11:28 pm 09/8/2012

    @Andreas: while I like the craziness of Brian Engh’s drawing, I’m not thrilled with either the colors or the inflatable display organs.

    Here’s the (potential) issue: why blue? Big ol’ bull male gets lost against the sky if his head’s blue.. Pink or red would have been much better, bright and high contrast against the vegetation or the sky.

    As for the inflatable pouches, I got the impression somewhere that there’s a lot of pressure involved in raising a sauropod head that high, and perhaps some of the air sacs in the neck acted much as the g-suits in jet fighter pilots did, using air pressure to regulate the blood pressure and keep the animal from blacking out. Whether this is true or not, I’m not sure whether having structures inflated by either air or blood would be the wisest thing, pressure wise. Either the display structures would inflate as taut as the rest of the neck, or the relatively fragile display tissues might explode under pressure.

    Personally, I suspect that if bull sauropods had colorful neck displays, they were probably more like oversize pink and red phalluses, rather than blue balloons.

    This is all speculation, of course.

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  14. 14. naishd 6:42 am 09/9/2012

    Jerzy (comment 12): yes, it seems that fossil archosaurs of at least some lineages did indeed act as several ‘eco-species’ during their lifetime. But I think you’re confusing things by implying that this is related to the strong positive allometry sometimes seen in extravagant structures linked with display, since the taxa exhibiting those positively allometric structures (e.g., Pteranodon head crest) are not the same ones with the ‘eco-partitioned’ lifestyles…

    Extant crocodiles and Komodo dragons: have checked.. they do not possess structures that exhibit the same sort of positive allometry as that seen in (e.g.) Pteranodon head crest. However, the snout of the paddlefish does, and it doesn’t seem to have a role in sexual selection (hmm, or does it?) – this probably shows that massive allometry during ontogeny is not always linked with a sociosexual role for a structure, but, then, nobody said that this was a ‘rule’ anyway. Indeed, as argued in the sauropod ‘necks not for sex’ project, sexually selected traits do not all exhibit positive allometry anyway, and some even exhibit negative allometry! For dinosaurs, this could mean that what we see in spiky pachycephalosaurs etc. is consistent with a role in sexual selection.

    Darren

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  15. 15. naishd 6:59 am 09/9/2012

    Heteromeles (comment 13): I think that, if we’re going to pretend that we can objectively evaluate the bright display colours speculatively illustrated for a sauropod, we might have to admit that there could be all kinds of factors you aren’t crediting. Maybe long-necked titanosauriforms want to be camouflaged against the sky when out in the open – maybe they only do their display when surrounded by foliage. Maybe the species (or population) concerned sticks to well-wooded habitats and never comes out into the open (not impossible in some parts of the Mesozoic world). Maybe it’s seasonally blue and another colour at other times of the year.

    Inflatable sacs: the stuff that’s been published so far on sauropods and blood pressure is probably not accurate; unfortunately I can’t elaborate on this due to the contents of an in-progress manuscript. Given that the pneumatic system allows saurischian dinosaurs to push enormous quantities of air around the body (something sauropods had to do in order to deal with dead space in the trachea), I don’t automatically see a problem with the inflation of hypothetical display structures like those shown in Engh’s illustration. It’s admittedly extreme… but, then, sauropods were.

    Darren

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  16. 16. Tayo Bethel 9:35 am 09/9/2012

    Would it be possible to evaluate habitat preferences of extinct archosaurs based on fossils? If so then we could speculate that some species were sexually selected for vocal as well as or instead of bisual displays. No singing dinosaurs though LOL

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  17. 17. Tayo Bethel 9:36 am 09/9/2012

    sorry visual LOL

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  18. 18. Owl905 12:26 pm 09/9/2012

    The sacred cow of sexual selection as a key driver of evolution distorts many discussions. Sexual selection roots the stability of the species.

    Fill, Ill, Chill, and Kill, are the real pressure-points that drive the acceptance of unusual shapes (parents usually accept clones of themselves the best) and the consequential upheaval of evolution.

    Feathers are dried and splayed fish scales. Hairs are branch-less feathers. Efficiency of energy is a strong driver on that evolutionary path.

    Human bias about the dominance of sight masks the mesozoic foundations of sound, travel and camouflage.

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  19. 19. em_allways_right 1:01 pm 09/9/2012

    “modern animals that bear elaborate structures”

    bare – trans. verb; NOT bear -a large mammal.

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  20. 20. David Marjanović 6:15 pm 09/9/2012

    I see your Diplocaulus and raise you a Diploceraspis!!! It’s not quite a Texas Longhorn… but… close.

    I’m not familiar with Cowen & Lipp’s book, but, just as a general comment: Anyone who proposes a novel hypothesis about some very specific subject should do it somewhere where the right people can reasonably be expected to see it. A book with a super-generic title such as The History of Life is not such a place.

    Seconded.

    Feathers are dried and splayed fish scales.

    This is wrong several times over; I don’t even know where to begin. Learn some development genetics and some fine-scale anatomy.

    Hairs are branch-less feathers.

    Nope, hairs aren’t feathers at all.

    Human bias about the dominance of sight masks the mesozoic foundations of sound, travel and camouflage.

    Most birds are much more visual than most mammals. There’s clear evidence that mammals have deemphasized sight in the course of their evolution; for instance, all mammals have lost 1 or 2 (often 2, sometimes more) of the original 4 kinds of color receptor in the retina. Humans and other Old World monkeys have 3, but 2 of those have arisen fairly recently from a single one by gene duplication and a few mutations…

    bare – trans. verb; NOT bear -a large mammal.

    Oh no. English spelling is a much greater mess than you seem to believe. There are two verbs here, one spelled bare, the other spelled bear, both pronounced the same, and both transitive.

    Bare = to make bare; to bare one’s feet = to become barefoot. To remove covering (clothes in particular).
    Bear = carry, especially in abstract ways.

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  21. 21. naishd 7:00 pm 09/9/2012

    I opted to avoid the mess over the terms ‘bare’ and ‘bear’ by finding another term…

    Darren

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  22. 22. Owl905 8:46 pm 09/9/2012

    “This is wrong several times over; I don’t even know where to begin.”

    Agreed, you don’t know where to begin.

    “Learn some development genetics and some fine-scale anatomy.”

    “Most birds are much more visual than most mammals”

    You clearly just did not get it.

    And some evolutionary controversy about flight start with falling rather than jumping.

    “Hairs are branch-less feathers.

    Nope, hairs aren’t feathers at all.”

    That’s right. They aren’t feathers.

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  23. 23. BrianL 5:24 am 09/10/2012

    @Owl905
    It would be rather helpful if you elaborated some more on what you mean to say, instead of making bold, or might I say weird, statements matter-of-factly and then appearing mildly offended when someone points out that your statements are, to put it mildly, in need of being clarified.
    Oh and you don’t want to get into an argument like this with David Marjanovic. I suspect he might be Huxley, ‘Darwin’s bulldog’, reincarnated. The preciseness is strong in him. :)

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  24. 24. David Marjanović 10:13 am 09/10/2012

    You clearly just did not get it.

    Then explain it.

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  25. 25. BilBy 2:33 pm 09/10/2012

    @Owl905 “Fill, Ill, Chill, and Kill, are the real pressure-points that drive the acceptance of unusual shapes (parents usually accept clones of themselves the best) and the consequential upheaval of evolution.” Would you explain this please?

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  26. 26. BilBy 2:38 pm 09/10/2012

    @David Marjanovic – thanks for pointing out the bear/bare difference! My students often get this mixed up along with horde/hoard, pore/pour and for some completely unfathomable reason lose/loose.

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  27. 27. David Marjanović 11:46 am 09/11/2012

    for some completely unfathomable reason lose/loose

    Unfathomable? “Lose” is spelled in a, shall we say, unexpected way. It’s no wonder that people get confused when the spelling itself is confused. See also: read/read/read (present tense/past tense/past participle), lead/led/lead (present tense/past tense and past participle/metal).

    I’m with Opinion 16.

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  28. 28. BilBy 1:34 pm 09/11/2012

    It’s unfathomable because it is spelled differently, pronounced differently and means different things.

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  29. 29. Heteromeles 3:50 pm 09/11/2012

    I’m with Opinion 16.

    But David, how can you be for Opinion 16 if Opinion 20 is also true?

    I’m not going to argue that English is perfect, but if we’re willing to speak English and work on Windows (or use Latin root words for science), then complaining about how stupid English spelling is true but irrelevant.

    Personally, I like http://myjetpack.tumblr.com/post/29829452057?ref=nf

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  30. 30. David Marjanović 8:23 am 09/12/2012

    But David, how can you be for Opinion 16 if Opinion 20 is also true?

    o_O

    I don’t see how they’re related.

    complaining about how stupid English spelling is true but irrelevant

    It is, indeed, off topic. I replied to em_allways_right’s incorrection of bear. I don’t see what your point is.

    Personally, I like http://myjetpack.tumblr.com/post/29829452057?ref=nf

    That’s nice, but off even that topic.

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  31. 31. BuckSkinMan 4:12 am 09/13/2012

    Hmm, this looks like another degeneration of the Scientific Method into the “Ideological Method.” I mean of course that just because there are species (often widely separated taxonomically) that do not show evidence of spectacular dimorphism, it doesn’t mean that dimorphism isn’t used to give advantage to the “do’s or “don’ts” — If Evolutions says anything, it says: Whatever works!!

    Nothing is said in the article about this but I think we’ll find when there are many species within a genus, there will be more “visual difference” between the genders. Birds for example – look at the finches in Darwin’s examples. The main visual difference between genders is in color of plumage in addition to the shape of beaks.

    The less colorful females also have the better camouflage – so there are TWO things which give the species a better chance to reproduce and continue species survival. Bird species without male / female differences exist as well – and their survival is still based on some form of “recognition” between genders.

    But among people calling themselves “scientists” there’s just the difference of opinion. Making half-baked claims while criticizing the theories of others should be left to the politicians. All we know for sure about the many species on Earth is that they continue to exist and have only their bodies to use as tools of species survival.

    Link to this
  32. 32. David Marjanović 9:49 am 09/13/2012

    If Evolutions says anything, it says: Whatever works!!

    …Well, that’s it: in some environments, for some populations, sexual dimorphism works (sometimes spectacularly well), for others it doesn’t. Do you disagree?

    give the species a better chance to reproduce and continue species survival

    No, there is no such thing as species survival. Natural selection doesn’t recognize species. You mean “give the genes present in that species a better chance to reproduce themselves and continue their own survival”.

    Link to this
  33. 33. Heteromeles 6:16 pm 09/14/2012

    I think the counter example for species richness and gender dimorphism is probably the Tyrannidae. There are some flycatchers that are dimorphic, but there sure are a lot of Empidonax that are hard to tell apart.

    Certainly, once you get beyond birds, the whole idea falls apart. For example, think about all the reef fish that change sex as they age. That in itself should invalidate the idea.

    Link to this

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