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Tetrapod Zoology

Tetrapod Zoology

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Thor Hanson s Feathers: The Evolution of a Natural Miracle

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ResearchBlogging.org

The complex structure, development and growth of feathers can, to paraphrase one expert on the subject, be seriously damaging to your mental health. Feathers are just crazy, almost certainly the most complex structures to ever grow out of any animal’s external surface.

Yet for all their marvellous complexity, for all the interest that people have displayed in their evolutionary origins and diversity, for all their role in bird behaviour and ecology, and for all their economic and cultural significance to humans, it doesn’t seem that any one book has ever been devoted to feathers and feathers alone. Thor Hanson’s 2011 Feathers is thus a rather significant book, and very nice it is too.

Hanson, a Washington State-based conservation biologist, previously wrote The Impenetrable Forest: My Gorilla Years in Uganda. He’s published technical research on such topics as the ecology of tropical trees, forest fragmentation and its impact on bird nest predation, the impact that warfare can have on biodiversity hotspots, and the behaviour of Neotropical monkeys and birds.

Hanson might not be a feather specialist, or even a dedicated ornithologist, but his many encounters with feathers, and with their structure, role, significance and uniqueness, obviously created the urge that culminated in this book. And Feathers is not the provincial view of someone only interested in ecology or conservation biology; on the contrary, this is a remarkably well-rounded review of the subject. Chapters look at the evolution and origin of feathers, their use in flight, thermoregulation and display, and their importance to humans.

Feathers from the Mesozoic

♥ Alan Feduccia.

A discussion of feathered non-avialan theropod dinosaurs, and a review of feather origin theories make up early sections in the book. Hanson is very much up to date (as of 2011!), providing appropriate discussion of Richard Prum’s feather origins hypothesis (Prum & Brush 2002) and of Xu Xing and his numerous exciting feathered theropod discoveries. As per usual, we get the theropod origins model pitted against Alan Feduccia’s idea that birds just cannot be theropods, nor dinosaurs at all, but an independent derivation from non-dinosaurian diapsid reptiles of some sort. Feduccia is quoted as saying in the book that “If it has feathers, it’s a bird” (Hanson 2011, p. 57), which of course means that feathered oviraptorosaurs, deinonychosaurs and so on are now birds according to Feduccia. As Hanson notes, and as Prum and others have said before (Prum 2003), this means that Feduccia is now contradicting decades of bold assertion in which he has insisted that deinonychosaurs and other Mesozoic theropods are nothing whatsoever to do with birds. It must be understood that Feduccia's opinion is not a valuable, informed alternative or anything like that; rather, it relies on deliberate obfuscation and misinformation and ignorance with respect to what we actually know. I cannot see that he and his colleagues have done anything but add confusion, contradiction and erroneous interpretations to our understanding of bird origins and early evolution. Hanson was able to see through this, as have previous authors who approached the topic of bird origins as apparent outsiders (e.g., Shipman 1998).

It’s often difficult to entangle the difference between the origin of birds and the origin of flight, but recently discovered small non-avialan maniraptorans and the distribution of vaned feathers in maniraptoran phylogeny may well show that flight of a sort was present in whatever taxa were ancestral to both the bird and deinonychosaur lineages. While the majority of non-avialan theropods were non-climbers, could climbing or branch-leaping, or some other way of acquiring height, help explain the earliest steps in the evolution of flight?

One of Ken Dial's WAIR diagrams. This figure shows that wing stroke is nearly invariant to gravity; the red lines represent the wing-tip trace in WAIR while the blue lines represent the wing-tip trace in level flight.

Hanson provides nice coverage of this topic, and is especially keen on Ken Dial’s Wing Assisted Incline Running (WAIR) hypothesis (Dial, Bundle & Dial 2003, Dial et al. 2006, 2008). As Hanson notes, WAIR has been embraced rather enthusiastically by palaeontologists “as the best flight evolution story to date” (p. 129). It has appeal in explaining how small, incremental steps might explain how terrestrial theropods with an early form of feathering could have evolved into scansorial animals with larger, increasingly complex feathery surfaces on the limbs. I think that WAIR is a pretty neat hypothesis. However, I note that some who work on flight origins are highly sceptical of it, stating in particular that non-ornithurine birds lacked the bony architecture required to permit the vigorous, high-amplitude flapping needed for WAIR to function.

The 1911 Trans-Saharan Ostrich Expedition, Vegas showgirls, and other feather-based tales from the human world

Frontispiece to Richard Brookes's 1790 volume The Art of Angling.

While all of these topics might be quite familiar if you’re well-read on bird and feather origins, Hanson’s other chapters cover feather growth and moulting, avian thermoregulation, the down industry, the aerodynamic and water-repellent qualities of feathers, and the use of feathers in display (both among birds and among humans), fly-fishing and writing.

The anthropological sections were the most novel. We’re all familiar with the use (or former use) of feathers in hats, dusters, cloaks and so on, but overall this subject certainly isn’t something I’ve ever had the chance to read much about. Most people interested in birds know that the feather trade was so substantial during the early decades of the 20th century that egrets and other species were being slaughtered in their millions expressly for the purpose, and indeed that conservation bodies, laws and reserves were created in direct response to the scale and nature of this trade. Feathers were so valuable at this time that the more than 40 cases of plumes lost on the Titanic would have an insurance value of more than $2.3 million in today’s currency (Hanson 2011, p. 176).

Ostriches plucked of ALL their feathers. This image shows ostriches that belonged to Shua people in Nigeria; it's from Hugo Bernatzik's 1931 book The Dark Continent; Africa, the Landscape and the People (digitized by the New York Public Library).

One of my favourite sections in the book is that on the Trans-Saharan Ostrich Expedition, whereby Russel William Thornton led a 1911 expedition from South Africa to Nigeria in order to track down, capture and exploit the fabled Barbary ostrich. The plumes of this ostrich are (or were) ‘double-flossed’ and more luxuriant than those of other ostriches. I’m somewhat uncertain of the taxonomic status of this form but, then, I think that everyone might be, since the population concerned is apparently now extinct. It has at least been suggested that ‘Barbary ostriches’ were a local form of the North African, Sudan or Red-necked ostrich Struthio camelus camelus (Freitag & Robinson 1993), but I don’t know if this has ever been demonstrated. Anyway, you might be as surprised as I was to learn that captured ostriches were plucked of all their feathers.

Stuff I saw in Las Vegas. You should be able to see pink plumes on the photo of the showgirl at top right.

Hanson’s section on the role of feathers in display talks about birds-of-paradise, sexual display, the ‘sexy son’ hypothesis, Wallace and Darwin. But he goes on to discuss Las Vegas showgirls and the tradition of using showy, enormous feather plumes in dancing costumes. Some of the largest feathery pieces used in showgirl costumes might cost “tens of thousands of dollars” should they need to be replaced (Hanson 2011, p. 172). I was actually reading the book while staying in Las Vegas, and got to walk past the famous pink-plumed dancing girls of Bally’s Hotel and Casino on a twice-daily basis. It was weird… like everything else in Las Vegas.

Hanson also covers the massive use of birds by the Aztecs. The aviaries at Tenochtitlán were described by Hernán Cortés as being enormous, housing every kind of bird and requiring the services of 300 human attendants. They were completely burnt down during the Spanish siege of Tenochtitlán in 1521.

On vultures

Vultures play an important role in the book since Hanson says in the preface that they’re what got him interested in feathers in the first place. He relates a delightful tale from field research where he had to handle a rotting zebra caecum, destined for use at a vulture feeding station. It exploded, “blowing my hair back and coating me with a spray of old blood, strands of ropy goo, and flecks of half-digested bush grass. The smell was indescribable.” (Hanson 2011, p. 250).

Thermographic image of a vulture (a cathartid, probably Cathartes aura). Image by Arno/Coen, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

Hanson mostly includes vultures in the book because he argues that the reduced or absent feathering on their heads is an adaptation for carrion-feeding. This might be partly true, but in recent years it’s been questioned: some birds that regularly poke their heads into carcasses (giant petrels) get by fine with normal feathering, most vultures are not really naked-headed anyway, and it might be that reduced feathering on the head and neck is as much (or more) to do with thermoregulation than carrion feeding (Ward et al. 2008). Hanson also commits an oft-made mistake in writing that New World vultures are especially closely related to storks. This view was mooted a few decades ago but hasn’t been supported by any of the more recent studies published on bird phylogeny.

White-backed vulture (Gyps africanus) in flight. Image by Mark Rosen, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

I can’t leave this mention of vultures without referring to the continuing decline of these magnificent birds due to human greed, stupidity and laziness. The die-off of Indian vultures due to use of the veterinary drug diclofenac is well known: the decline has been so rapid that, in 2008, some experts were predicting the extinction of certain vulture species within a ten-year period. Despite being banned from sale in 2005, diclofenac is still being sold illegally and some reports say that it’s continuing to have a deleterious effect. Vulture ‘restaurants’ set up by the Indian government have not been adequately funded or maintained, and an article from February of this year said that these feeding stations haven’t benefited vulture conservation at all. A symposium on the continuing decline of Asian vultures was in fact held just a few days ago (early May 2012).

The problems in India (which mostly effect the Slender-billed vulture Gyps tenuirostris, Indian vulture G. indicus and White-rumped vulture G. bengalensis) have received a reasonable amount of press interest. Less well known is that massive declines seem to have occurred in Africa as well, with some data from Mali and Niger indicating that vulture species have either disappeared entirely, or have declined by something like 98%. Here, diclofenac is less of a problem. Habitat destruction and degradation, poisoning, and killing for use in traditional and quack medicine – known as muti – have all contributed to decline. Particularly infuriating is the belief that smoking vulture brains will give people supernatural powers when gambling (yes, I said smoking vulture brains will give people supernatural powers when gambling). The demand for vulture products used in muti has soared in step with the growth of national lotteries and with events such as the 2010 World Cup, hosted by South Africa (Langley 2011). Clearly, the health of vulture populations across many areas is now a major cause for concern and urgent measures are needed.

Anyway… I digress, back to the book review. Other highlights include the discussion of the peregrine feather dropped on the surface of the moon by Commander David R. Scott in 1971, and the section on snarge. Yes, I said snarge, and will leave it at that.

Feathers is a handy, compact volume, well illustrated throughout, and with a compelling, enjoyable prose. It's fully referenced, with footnotes (arranged at the back) providing details that would have derailed or slowed the main flow of the text. Appendices discuss and illustrate the different kinds of feathers. I appreciated and agreed with Hanson’s perspective on all the areas he discusses, I enjoyed and respected the enthusiasm and sense of wonder that he conveys when talking about the natural world, and I congratulate him on thorough research and on being up-to-date on such a fast-moving topic. In short, Feathers is required reading for anyone interested in bird biology or evolution, and I strongly recommend it.

Hanson, T. 2011. Feathers: The Evolution of a Natural Miracle. Basic Books, New York, pp. 336. ISBN 978-0-465-02013-3. Hardback, index, refs. Here on amazon. Here on amazon.co.uk.

For previous articles relevant to feathers, early bird evolution and other topics mentioned in this article, see...

Refs - -

Bundle MW, & Dial KP (2003). Mechanics of wing-assisted incline running (WAIR). The Journal of experimental biology, 206 (Pt 24), 4553-64 PMID: 14610039

Dial, K. P. 2003. Wing-assisted incline running and the evolution of flight. Science 299, 402-404.

- ., Jackson, B. E. & Segre, P. 2008. A fundamental avian wing-stroke provides a new perspective on the evolution of flight. Nature 451, 985-989.

- ., Randall, R. J. & Dial, T. R. 2006. What use is half a wing in the ecology and evolution of birds? BioScience 56, 437-455.

Freitag, S. & Robinson, T. J. 1993. Phylogeographic patterns in mitochondrial DNA of the Ostrich (Struthio camelus). The Auk 110, 614-622.

Hanson, T. 2011. Feathers: The Evolution of a Natural Miracle. Basic Books, New York.

Langley, N. 2011. Africa’s emptying skies. World Birdwatch 33 (2), 16-18.

Prum, R. O. 2003. Are current critiques of the theropod origin of birds science? Rebuttal to Feduccia (2002). The Auk 120, 550-561.

- . & Brush, A. H. 2002. The evolutionary origin and diversification of feathers. The Quarterly Review of Biology 77, 261-295.

Shipman, P. 1998. Taking Wing. Weidenfeld & Nicolson, London.

Ward, J., McCafferty, D. J., Houston, D. C. & Ruxton, G. D. 2008. Why do vultures have bald heads? The role of postural adjustment and bare skin areas in thermoregulation. Journal of Thermal Biology 33, 168-173.

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

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