Another evolutionary mystery – the Stegosaur’s plates

April 10, 2009 • 4:12 am

by Matthew Cobb

Stegosaurs were large herbivorous dinosaurs that flourished in the Jurassic (roughly 170-145 MY ago). Only one taxon – Wuerhosaurus – made it through the end-Jurassic mass extinction. These were the dinos with the great big plates on their back, and vicious spikes at the end of their tail. And no, they never fought with T. rex, which was around much later.

stegos

Common myths are that they had “two brains” (one in the tail) and were intensely dim (see below). They only had one brain, which was about the size of a cat’s, although differently organized (they were reptiles, not mammals). And they weighed anywhere up to 2 tons, which is pretty hefty for any animal.

larson1

The tail spikes – now officially called thagomizers (honest! Paleontologists do have a sense of humour!) – were presumably used in defence, and some species from China, like Kentrosaurus, had funky spikes on their shoulders (i.e. at the top of their front legs – see above). Of course, these shoulder spikes explain why Kentrosaurus and its fellows went extinct – they couldn’t get through the door of Noah’s Ark…

The most striking thing about Stegosaurs, however, were the plates, or osteoderms.

What exactly were they for?

Maybe they weren’t ‘for’ any one thing – different species had different-shaped plates, as can seen above. But in the classic Stegosaurus, the plates are large and flat.

The most obvious suggestion is defense, although if the plates were upright, as on most reconstructions (see above), they would not protect the legs or the head, which is where any self-respecting predator would attack first. Even if the plates were flat against the body, they would not cover any of  the key parts of the animal’s anatomy.

In the 1970s, it was suggested that the plates may have had a thermoregulatory function – if stegosaurs were “cold blooded” like their reptile relatives, they would have to use the sun to warm up and get moving in the morning. By having lots of blood vessels running through the plates, they could be used as a kind of solar panel to gather heat.

This hypothesis conjours up the image of herds of stegosaurs stumbling bleary-eyed into a red Jurassic dawn, climbing slowly to the top of the nearest hill and standing in a north-south direction, their plates exposed to the rising sun. Alternatively, the animals might have had to dissipate heat generated by fermentation in their vast stomachs, and therefore used the plates to radiate excess heat, creating columns of rising warm air that would attract dancing midges.

However, in 2005 a group of researchers looked carefully at the anatomy of the plates and found that the internal canals were not connected with those to be found on the outside, suggesting that there as no way for blood to circulate around them, thereby arguing against the thermoregulatory hypothesis. Instead, they suggested, the plates were used in species or herd recognition, although there was no clear evidence presented to support this idea.

Often when evolutionary biologists encounter an inexplicably bizarre structure, they lay the blame at the door of sexual selection, normally by females. For sexual selection to explain the stegosaur plates, however, there would need to be some sexual dimorphism present. Sexing Stego skeletons is a tricky business, but there is no suggestion that the sexes had different-shaped or -sized plates. Maybe they were different colors?

Last month, doctoral student Shoji Hayashi of Hokkaido University kindly sent me a reprint of an article he has just published in the Journal of Vertebrate Paleontology, which looks at the relative growth rates of the plates and the main skeleton in a number of Stegosaur skeletons of different ages. He concludes that the plates carried on growing faster, later, than the rest of the skeleton, as shown in this figure:

f06_123

Shoji and his colleagues conclude: “It is probable that the plates and spikes have an intraspecific display function and reflect social status and/or sexual attraction. Individuals having the largest osteoderms would have appeared much larger to rivals and/or potential mates. Judging from the morphological variations of plates within Stegosauria, it is reasonable to suppose that plates have a species recognition function as previously suggested by Carpenter (1998) and Main et al. (2005). Stegosaurus plates may also have a function as thermoregulation by virtue of their arrangement and the flat and tall morphology, although this function is doubtful in other stegosaurian dinosaurs possessing only small plates and spikes.”

In other words, we still don’t know! And to be honest, it is hard to see how we will find decisive evidence one way or another. That is partly the joy of paleontology: trying to reconstruct something that can never be fully known.

Citations:

Carpenter, K. 1998. Armor of Stegosaurus stenops, and the taphonomic history of a new specimen from Garden Park, Colorado.Modern Geology 23:127–144.

Farlow, J. O., C. V. Thompson, and D. E. Rosner. 1976. Plates of Stegosaurus: Forced convection heat loss fins? Science 192:1123–1125.

Main, R. P., A. de Ricqlès, Horner, J. R., and K. Padian. 2005. The evolution and function of thyreophoran dinosaur scutes: implications for plate function in stegosaurs. Paleobiology 31:291–314.

Hayashi, S., Kenneth Carpenter, K, and Suzuki, D. 2009. Different growth patterns between the skeleton and osteoderms of Stegosaurus (Ornithischia: Thyreophora). Journal of Vertebrate Paleontology 29:123-131.

WEIT in TREE

April 9, 2009 • 11:29 am

by Matthew Cobb

This review by Douglas J Futuyma is about to appear in the academic journal Trends in Ecology and Evolution (aka TREE).

What everyone needs to know about evolution

Douglas J. Futuyma, Department of Ecology and Evolution, Stony Brook University, Stony Brook, NY 11794, USA

In 1980, when I set out to write a defense of evolution against creationism [1], I discovered that there existed no book for general readers that described the evidence for, and nature of, evolution. The closest approximation was George Gaylord Simpson’s The Meaning of Evolution [2], which was really, as its subtitle stated, ‘a study of the history of life and its significance for man.’ In any case, it was then 31 years old and substantially out of date. Since then, the creationist movement, including its recent manifestation as so-called intelligent design (ID), has come to be recognized as a serious threat not only to evolution education, but also to science in general [3]. Yet although it sometimes seems that almost every evolutionary biologist is writing for a wide, nonprofessional audience on themes ranging from the fossil record to practical applications of evolutionary biology and the evolutionary dimensions of psychology, culture and the religious impulse, there still has not been a single book devoted to the simple task of showing readers the evidence for evolution and how it happens. We are fortunate that Jerry Coyne has exactly satisfied this need, with one of the very best and most important books on evolution for broad audiences in at least 50 years.

Although most of the themes of Coyne’s book will be familiar to evolutionary biologists, most instructors will learn some new examples or facets of familiar ones, or new ways of addressing questions. Moreover, Coyne carefully structures the evidence and theory, makes all the crucial points and presents the subject clearly, with allusions to human experience, sympathy for the uncertain or hesitant, and a scholarly yet almost conversational style that should make it easy for most people to read. The 254 pages of actual text are small in format, so the entirety can be read quickly. They are followed by notes, a glossary, an outstanding and valuable list of book and web resources, and a formal bibliography arranged by chapter. I might also note that Coyne, Drosophila geneticist although he be, delights in marshalling evidence from classical anatomy, embryology, systematics and the biogeography of diverse organisms, as well as contemporary molecular and developmental studies.

Coyne begins by describing the contemporary problem of opposition to evolution despite its importance and its status as ‘a theory that is also a fact’, and makes the usual and indispensable points about the nature of science, the testability of scientific hypotheses, and how hypotheses gain support from the correspondence between observation and predictions from a hypothesis. This is an important theme that he emphasizes throughout the book. The next three chapters, on the fossil record, ‘remnants’ (vestiges, embryos and bad design) and biogeography, present massive, well-chosen evidence for common descent and modification. Coyne emphasizes that fossils tell us of gradual change and of forms such as Tiktaalikthat demonstrate intermediacy, expected time of occurrence and evolution of new characters from ancestral ones. Vestiges, embryos and bad design include the multitude of morphological and molecular features that are inconsistent with any concept of ID but fully explicable from, and predicted by, evolution. And ‘the biogeographical evidence for evolution is now so powerful that I have never seen a creationist book, article, or lecture that has tried to refute it. Creationists simply pretend that the evidence doesn’t exist’ (p. 95). There follow three process-oriented chapters, on natural selection, sexual selection and speciation (Coyne’s special area of expertise), which all provide clear expositions of theory and evidence, and devastating points against ID. (For example, blood clotting, far from being ‘irreducibly complex,’ is based partly on the evolution of fibrinogen from a protein with a different function that was predicted, and then found, in sea cucumbers.) Chapter 8, ‘What about us?’, treats paleontological and genetic evidence on human evolution and briefly but clearly touches on patterns of genetic variation within and among human populations, and on gene–culture coevolution.

In the final chapter, Coyne acknowledges that evidence for evolution often cannot prevail against ‘the emotional consequences of facing [the] fact’ that we evolved from apes. He makes a good case that we need not fear ‘the beast within,’ the oft-imagined genetically determined selfishness and immorality that are thought to be inherent in ‘Darwinism,’ for the empirical evidence shows that we have immense capacities, unmatched by any other species, for empathy, kindness, and self-sacrifice. As Coyne notes, human sacrifice has disappeared, and ‘[i]n Roman times, some of the most sophisticated minds that ever existed found it an excellent afternoon’s entertainment to sit down and watch humans literally fighting for their lives against each other, or against wild animals. There is now no culture on the planet that would not think this barbaric’ (p. 251). Coyne admits that he cannot replace the comfort that so many find in conventional religion, but that he ‘can at least try to dispel the misconceptions that frighten people away from evolution and from the amazing derivation of life’s staggering diversity from a single naked replicating molecule’ (p. 253).

I can think of few changes that I would make beyond correcting a few proofreading lapses (Linnaeus’s great work was exactly a century after the date given) and minor errors. (Male stag beetles have sexually selected mandibles, not horns, and when will we ever stop hearing about the ‘peacock’s tail’? The historical contingency of evolution is exemplified by the variety of elongated display feathers among birds, which do include tail feathers in many other species, but also the flank feathers ofParadisaea birds of paradise, the secondary wing feathers of the great argus pheasant, head feathers in sage grouse and herons – and the back feathers, i.e. the train, of peacocks.) Coyne is clearly skeptical of most of evolutionary psychology while granting that some human universal characters might well be ancestral evolved traits, but I think one might make more allowance for the possible validity of hypotheses in this field (some of which do seem to make testable, and sometimes supported, predictions). Reassurances that evolutionary interpretations of human behavior are dubious will not allay fears of ‘the beast within’ if these interpretations prove to be well supported – and the empirical evidence Coyne presents, that we are not condemned by our genes to be unethical or immoral, should make such cautions unnecessary.

Why Evolution is True succeeds in being fully accessible to any reader who has even a vague idea of what DNA is. The publisher should issue an inexpensive paperback edition as soon as possible that should be stocked in every bookstore, sent to friends and relatives, and assigned as supplementary reading in introductory biology courses at both the high-school and university levels. It should also be widely translated. It is a book that needed to be written and needs to be read.

References

1 D.J. Futuyma, Science on Trial: The Case for Evolution, Pantheon (1982).

2 G.G. Simpson, The Meaning of Evolution, Yale University Press (1949).

3 National Academy of Sciences, Science, Evolution, and Creationism, The National Academies Press (2008).
Original version here (needs subscription).

Happy Birthday, Lungless Frog!

April 9, 2009 • 2:47 am

by Matthew Cobb

A year ago, a brief paper appeared in Current Biology, written by researchers from the National University of Singapore (NUS) and from Indonesia, describing the Bornean flat-headed frog, Barbourula kalimantanensis. The frog was first described in 1978, and on the International Union for the Conservation of Nature (IUCN) Red List it is described as “endangered”.

2397503833_778e831ff3jpg

This frog, of which only a few specimens are known, is noteworthy because it has no lungs, but instead breathes through its skin. This is a trick ordinary frogs can do, too – in nearctic regions frogs may spend the winter hunkered down in the bottom of ponds, slowly respiring through their skin.

But losing your lungs is an amazingly rare event. Among all the thousands of tetrapod species – mammals, birds, reptiles, amphibians – lunglessness has evolved only in amphibians. Indeed, it has only been observed three times previously – in two families of salamander and a single species of caecilian. (Caecilians are legless amphibians that are not snakes; most of them live underground.)

Why has this species of frog lost its lungs? The authors of the paper – David Bickford, Djoko Iskandar (who first described the frog, but did not realize its lunglessness) and Anggraini Barlian – point to how the frog manages without lungs (it lives in oxygen-rich fast-flowing water, presumably has a low metabolic rate (the water is very cold), its flat shape increases the surface area for oxygen absorption), but beyond the only positive reason they can come up with is that being lungless would decrease the frog’s buoyancy . Furthermore, the closely related species Barbourula busuangensis has lungs, just like all other known frogs.

What progress has been made over the last year? Had to say – a quick scouring of the scientific literature brings up no more publications, and there’s no indication on David Bickford’s NUS website that there’s been any further discoveries. The evolution of lunglessness in this odd frog may end up in the list of adaptations we don’t fully understand, such as the zebra’s stripes (discussed here) or the stegosaur’s plates (we’ll deal with that one tomorrow, so don’t chip in until you’ve read the blog!).

In a blog discussion last year over at nusbiodiversity, one of the readers made an astute comment. “JD” wrote “Why would a lungless frog have nostrils? Where do they lead to?” Presumably it retains the nostrils for its sense of smell – which will also work under the water. Ordinary frogs breathe by swallowing (they have no diaphragm), so my bet would be that this lungless variant still gulps to get air flowing over its olfactory receptors.

The photo is taken from the Current Biology article, which can be found here.

The billion trillion zillion trees of life

April 8, 2009 • 1:45 am

by Matthew Cobb

It’s a notorious fact that there are no pictures in Darwin’s On the Origin of Species. Or rather, there’s a single diagram, showing how Darwin saw the evolution of species , with whole families (‘branches’) going extinct at irregular intervals of time.:

darwins_tree_of_life_18591

This wasn’t Darwin’s first stab at the question, though – in his notebooks during his voyage on the Beagle he drew a coral-shaped diagram and wrote “I think” by the side:

darwins_first_tree_of_lifejpg3

Since this time, there have been thousands of evolutionary trees drawn up. Unlike previous trees of life they are not simply about classifying organisms, they are also hypotheses about how related organisms are to each other.

For a long time, phylogenetic trees were the restricted domain of bearded professors arguing about the significance of the porpoise’s left thumb, or whatever. With the advent of molecular data and the growth in computing power, scientists have become very interested in them once again.

Part of the reason for this is that there is plenty of room for argument – something scientists put up there with ‘discovery’ on the scale of what’s really important. The thing is, evolution took place in just one way. There was just one true sequence of how the species on our planet split, evolved and died out. And given the number of species that have lived on the Earth over the last 4 billion years, there are more ways of arranging those species in a tree than there are atoms in the Universe. Really.

For example, if you have just 10 taxa (a neutral word that could mean a species or any group), there are about 3,600,000 possible trees. Because of the way the mathematics works, if you double the number of taxa you far more than double the number of possible trees – it ends up at 2,000,000,000,000,000,000 trees. And how many species have been around over the last 4 billion years? We have no idea – anywhere from a dozens of millions upwards.

Within those zillions of possible trees, there is just one that is true. No one imagines we’ll ever find out which one it is (how could we know?). All we can hope is to try and get the Big Picture right (are humans more related to goats or dogs?), and to fiddle around with the fine detail at the ends of the various branches (the relations between various modern species of fruitfly are a particular favorite).

I was prompted to write this because Current Biology, a fortnightly research journal, is about to publish a new tree of ‘deep animal relationships’, which tries to see how bilaterally symmetrical animals (like us) are related to sponges, and jellyfish.

Based on the sequence of 128 genes, the new tree suggests that the four kinds of sponge are a single group (they are ‘monophyletic’) – suggesting that the earliest animals were not sponge-like – and that the comb jellies (‘ctenophores’) belong with the jellyfish (‘cndarians’). What’s fascinating is that this arrangement is precisely that taught many years ago on the basis of comparative anatomy, and challenged by previous molecular studies…

Is the tree right? I have no idea!

Citation:  Hervé Philippe et al. (2009) Phylogenomics Revives Traditional Views on Deep Animal Relationships. Current Biology – 02 April 2009

The article – which has not yet been published – can be found here, but you or your institution will need a subscription to read more than the abstract.


The Complete Work of Charles Darwin Online

April 7, 2009 • 8:40 pm

by Greg Mayer

I’m slipping in to make a quick plug here for one my favorite websites, The Complete Work of Charles Darwin Online, and its founder and director, John van Wyhe of Christ’s College, Cambridge. The website contains text and image copies of at least one edition of all Darwin’s works (and often of many other editions as well), an updated version of Freeman’s bibliographical handlist, and many, many other useful things; check it out yourself. It is undoubtedly the best scholarly website ever.  John is the rising star of the new generation of Darwin scholars.

Dr. John van Wyhe
Dr. John van Wyhe

For those of you in or near southeastern Wisconsin, he will be speaking on Wednesday, April 8 at 7 PM at the University of Wisconsin-Parkside, on Darwin: the True Story.  The talk is part of UWP’s Darwin 1809-1859-2009 commemoration of the  Darwin bi- and sesquicentennials, and is free and open to the public. Details here.

Of mosquitoes and the menopause

April 7, 2009 • 7:24 am

by Matthew Cobb

The biology of aging is one of the sexiest topics around. If we could work out how and why different animals have different life-spans, maybe we could work out how to extend human life-span. (Whether this is a Good Thing or not from the point of view of the individuals concerned, or the planet, is another issue.)

One of the interesting things about aging is that once you stop reproducing, your genes cannot be ‘seen’ by natural selection. Whatever choices you make – where to eat if you’re an animal, what to eat if you’re a human animal – it cannot have a direct effect on the survival of your genes in the next generation, because you’ve already passed them on.

In most organisms, this is generally irrelevant, as they carry on reproducing until they fall down dead or get eaten, or both. But in humans – or more specifically, in human females living since the rise of civilization – there is ample opportunity for living in a zone where natural selection cannot touch your genes. It’s called life after the menopause.

Although it seems unlikely that in prehistory very many women survived to enjoy this phase of their life (disease and hunger would probably have done for them earlier), finding a selective explanation of the menopause has intrigued evolutionary biologists down the ages.

One simple explanation is that although a woman’s direct fitness cannot be affected after she ceases to reproduce, her inclusive fitness can be altered. Her genes would also be present in her offspring or relatives, and they may well benefit from her being around, even though she is not able to reproduce.

The presence of old women (and men) may have been a great advantage to early human societies by enabling human groups to deal with rare challenges in the environment (what to do when drought comes, which rare mushrooms you can and can’t eat), because they remembered what happened years before.

Mosquitoes don’t live in societies, and don’t – as far as we are aware – have any kind of culture. They do reproduce like mad, though, and they are the subject of intense selection pressures as people try to kill them using pesticides, to prevent malaria transmission.

The result is inevitable – the population of mosquitoes evolves resistance. It only takes one fertilized female to show a slight resistance to a given insecticide for that character to sweep through the population, as shown in the figure below, which shows the rapid evolution of resistance in a population of mosquitoes in southern Mexico three years after the introduction of indoor spraying:

journal-pbio-1000058-g004

In an article that has just appeared in the Open Access journal PLoS Biology (‘Open Access’ means it is free to read for everyone), researchers from Penn State University and the Open University, UK, suggest that this evolution of insecticide resistance could be avoided if the mosquitoes were killed after they had reproduced. In other words, at a period of their life when natural selection couldn’t ‘see’ them.

This idea is not as far-fetched as it might seem – in general the mosquitoes that bite and transmit malaria are the older females, who have already laid their eggs. As the authors put it “Thus, in principle at least, public health advances can be achieved with minimal selection for resistance by an insecticide that kills after the majority of mosquito reproduction has occurred but before malaria parasites are infectious.”

So how do they propose to blindside the mosquitoes and get round natural selection? They put forward several ideas, from slowly-accumulating pesticides to fungal infections that would gradually kill the mosquitoes, hitting the most lethal phase just as they begin to bite.

This cunning plan combines evolutionary insights, a profound knowledge of mosquito/malaria biology and some great lateral thinking. If it can be realized, we will be able to treat mosquito populations with insecticides without natural selection getting in the way.

Citation (from whence the figure above is taken):

Read AF, Lynch PA, Thomas MB (2009) How to Make Evolution-Proof Insecticides for Malaria Control. PLoS Biol 7(4): e1000058

For the full article, go here: doi:10.1371/journal.pbio.1000058

Recent natural selection in human populations

April 6, 2009 • 10:18 am

A bunch of high-powered genomics people have just come out with a paper in Genome Research surveying the human genome for recent signs of natural selection (which you can assess by looking at the patterns of DNA variation around various genes).  You can get the original paper (reference and abstract below) here as a pdf file, or read a story about it in the April 2 New York Times.  The survey was based on DNA data from 938 people from 53 populations, making this the most extensive survey of selection in human groups to date.

I’m leaving shortly (see post below by Matthew Cobb, my replacement), but just a few conclusions from this survey:

1.  Geographically adjacent populations tend to have similar histories of selection: Europe, Central Asia, and the Middle East tend to share their genetic complements compared to populations elsewhere in the world.  This presumably reflects the history of migration and intermarriage between the former three areas.

2.  Genes that seem indubitably under selection include not only loci involved in pigmentation, but some loci of unknown function (e.g., the gene called C21orf34, which resides on our 21st chromosome).  As I note in WEIT, human skin pigmentation is one of the few “racial” traits whose difference between human populations is pretty clearly connected with natural selection.  We previously had a good “intuitive” explanation for this difference (in sunnier climes, darker pigmentation is selected to prevent melanomas, while in northern, less sunny areas, the skin becomes less pigmented to allow production of vitamin D), but now we have more direct evidence from looking at the genes that themselves produce pigment.  However, most of the differences in appearance between groups don’t have an evolutionary explanation so far: they may have changed via sexual selection. Note that in the chart below, the SLC genes are pigmentation alleles, with a pretty strong signal of selection in Europe, the Middle East, and Asia.

3.  A gene that is a risk factor for diabetes and celiac disase also shows signs of being positively selected in some populations.  Why?  Perhaps because, as the authors theorize, the “risk allele” is involved in some other, positively selected trait, or perhaps it is genetically linked to such traits by being nearby on the chromosome.

4.  As mentioned in an earlier post, a pygmy and nearby non-pygmy African population differ in their insulin growth factor genes, which may have been selected to have alleles for smaller height in pygmies. (See earlier post for possible reasons for this).  These data come from only two populations, however, and it would be interesting to see if pygmy populations in Southeast Asia and South America have similar forms of the IGF gene.

Note that much of this selection has to be fairly recent, since populations outside of Africa have differentiated only within the last 60,000-100,000 years.

02visuals_graph600-1

This chart shows the sites along the genome (listed at the left) at which natural selection has occurred in the genome of eight regional groups (shown at top). These are 1) Biaka pygmies, 2) Bantu-speaking Africans, 3) Western Europeans, 4) Middle Easterners, 5) South Asians (people of Pakistan and India),

6) East Asians, 7) Oceanians and 8. Native Americans. The colored bars show the degree of selection at each site, with yellow denoting a signal of clear but moderate statistical significance and red denoting high statistical significance. (Photo and caption courtesy of The New York Times)

Signals of recent positive selection in a worldwide sample of human populations

Abstract: Genome-wide scans for recent positive selection in humans have yielded insight into the mechanisms underlying the extensive phenotypic diversity in our species, but have focused on a limited number of populations. Here, we present an analysis of recent selection in a global sample of 53 populations, using genotype data from the Human Genome Diversity-CEPH Panel. We refine the geographic distributions of known selective sweeps, and find extensive overlap between these distributions for populations in the same continental region but limited overlap between populations outside these groupings. We present several examples of previously unrecognized candidate targets of selection, including signals at a number of genes in the NRG–ERBB4 developmental pathway in non-African populations. Analysis of recently identified genes involved in complex diseases suggests that there has been selection on loci involved in susceptibility to type II diabetes. Finally, we search for local adaptation between geographically close populations, and highlight several examples.

Joseph K. Pickrell et al., Genome Res. May 2009 ; published ahead of print March 23, 2009,

The wonders of camouflage

April 6, 2009 • 9:55 am

by Matthew Cobb

Hello everyone, Jerry has kindly (or foolishly) handed over the reins of the WEIT blog to me for the next 10 days or so. See if you can spot the difference!

Some of the most spectacular signs of evolutionary  adaptation are the many examples of camouflage shown by animals. Although many examples of camouflage are shown by prey animals seeking to avoid being eaten, predators also use camouflage to avoid detection. My good friend Professor Innes Cuthill of Bristol University, UK, studies animal camouflage, and has just posted this excellent audio slideshow on the BBC website.

Innes describes various cases of how camouflage works in different animal species, and there are some great pictures to go with it. Sometimes, changing color is not actually to do with camouflage – this is the case in chameleons, and  also in octopuses and squid, which can use rapidly changing patterns of skin color to communicate in ways we do not fully understand. And by ‘ways’ I mean both what they are communicating and how they change their color so quickly.

One of the most spectacular examples of cryptic camouflage can be seen in the octopus, in this video:

This is taken (without credit!) from a fantastic five-minute talk on underwater animals by David Gallo at Ted.com, which you can find here and which includes some great interactions between squid at around 2m 40s and some cuttlefish showing fantastic rapidly changing color patterns.

One example of camouflage given by Innes Cuthill is the zebra, which he suggests may have stripes because it disrupts their outline, making it more difficult for predators to decide where the zebra begins and ends. This may be true – but in reality we simply do not know what the adaptive advantage is. Indeed, it is possible that the stripes have nothing to do with what is really going on (they may be simply a side-effect of the true advantage), although that seems unlikely. A non-camouflage explanation is that zebra foals are born into a world of stripes, and that the stripes on their parents help to enable them to identify their fellow-zebras, and reinforce their herd identity.

The problem with all these explanations is that they are what the late Stephen Jay Gould called ‘Just So Stories’, after the children’s fables written by the British author Rudyard Kipling (‘How the elephant got his trunk’, and so on). They fit the facts, and they may be true, but they lack the decisive support that science alone can provide: experimental evidence.

In discussing this with Jerry last week, he pointed out that a simple test of the ‘disruption’ hypothesis to explain the zebra’s stripes would be to paint some all black or all white, and see what happens to the predation rate. I suspect that would not be possible, either ethically or practically, but some kind of experiments on zebras, lions, or both, will be required before we can be really sure why zebras have stripes. Post your explanations – and above all, think of a doable experiment that could test your hypothesis!