Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
The Naturmuseum Senckenberg in Frankfurt am Main, one of Europe’s great natural history museums, has announced the discovery in Laos of one of the world’s largest known daddy longlegs by Senckenberg researcher Peter Jager. The apparently new species is now being studied by Jager and his Senckenberg colleague, Ana Lucia Tourinho. Daddy longlegs are also called harvestmen, although I grew up with lots of daddy longlegs, and never knew them to be called harvestmen except in books.
Many people mistake daddy longlegs for spiders. While both spiders and daddy longlegs are eight-legged arachnids, daddy longlegs have a more compact body with the abdomen and cephalothorax not separated by a constriction as in spiders, and their legs are invariably thread-like. The new giant form from Laos, which has not yet been formally described as a new species, has a leg span of over 33 cm. The record is 34 cm for a South American species.
Although science fiction films abound in giant arthropods or arthropod-like aliens (my favorite: Attack of the Crab Monsters), it is in fact hard for an
Attack of the Crab Monsters (1957), original poster from Wikipedia.
arthropod to get very large. The size limitation of arthropods is due to constraints on their chitinous exoskeletons and their ability to breathe. The University of California, Berkeley, website Understanding Evolution has great animations explaining these size constraints. Both kinds of constraints arise from the disproportion of size and shape as animals get bigger. Basically, many of the functions of animals (including their weight bearing skeleton and their respiratory surfaces) increase as the square of their increase in size, whereas their need for the functions increases as the cube of their size. So, if you double the size of an animal, without changing its shape, you will quadruple its surface area, but its volume will increase by a factor of eight. Thus if you have a physiologically important surface area (say the lining of your respiratory system), you are going to be lacking by a factor of two, as your oxygen needs are set by your volume. The “square/cube problem” in biology has long been known, and J.B.S. Haldane, one of the founders of modern evolutionary theory, wrote an influential popular article on the subject, “On being the right size“, in 1926. (Haldane, by the way, a geneticist-physiologist-soldier-pacifist-communist-Hindu-atheist-patriot-expatriate, was the original most interesting man in the world, as we’ve noted before here at WEIT.)
The largest insects today are not very big. They grew larger in the distant past, and this is thought to be related to a higher concentration of oxygen in the atmosphere at that time (35% during the Carboniferous period, compared to 21 % today). The largest known arthropod ever, also an arachnid, but an aquatic one, a sea scorpion or eurypterid from the Devonian, was discovered by Simon Braddy and colleagues a few years ago (pdf).
Giant arthropods from the fossil record compared with the average height of a (British) human male; (a) the eurypterid Jaekelopterus rhenaniae, Early Devonian, Germany; (b) the trilobite Isotelus rex, Late Ordovician, Manitoba, Canada; (c) the dragonfly Meganeura monyi, Late Carboniferous, France; (d) the millipede Arthropleura armata, Late Carboniferous, Europe. Scale bar (a–d), 50 cm. (e) Chelicera of the giant eurypterid J. rhenaniae from the Early Devonian of Willwerath, Germany, PWL 2007/1-LS. Photograph, the disarticulated fixed (above) and (rotated) free ramus (below). Scale bar, 10 cm.
They were 2.5 m long, and longer if you stretched out their claws (and imagine the size of the turds produced by d!). This is, as shown in the figure above somewhat larger that the typical British male (who, I must say, appears admirably buff and well-muscled in this outline drawing; perhaps it’s all that wrist-wrestling, or is it elbow-bending, down at the pub).
I don’t know what this insect is, but I’m sure one of my readers does. But first you have to see it! It took me a while to spot it, but of course that’s why it has evolved.
I hate to give attention to my Chicago colleague James Shapiro’s bizarre ideas about evolution, which he publishes weekly on HuffPo rather than in peer-reviewed journals. His Big Idea is that natural selection has not only been overemphasized in evolution, but appears to play very little role at all. Even though he’s spreading nonsense in a widely-read place, I don’t go after him very often, for he just uses my criticisms as the basis of yet another abstruse and incoherent post. Like the creationists whose ideas he appropriates, he resembles those toy rubber clowns that are impossible to knock down. But once again, and for the last time, I wade into the fray. . .
In his post of August 12, “Does natural selection really explain what makes evolution succeed?” (his answer, of course, is “no”), Shapiro simply recycles some discredited arguments used by creationists against evolution. The upshot, which we’ve heard for decades, is the discredited idea that natural selection is not a creative process. I quote:
“Darwin modeled natural selection on artificial selection by humans. He ignored the inconvenient fact that human selection for altered traits has never generated a truly new organismal feature (e.g., a limb or an organ) or formed a new species. Selection only modifies existing characters. When humans wish to create new species, they use other means.”
This is the old canard that artificial selection doesn’t create “new features.” His definition of a “new organismal feature” is, of course, one that hasn’t been generated by artificial selection, so it’s all tautological. Of course we haven’t seen whole new organs or limbs arise in the short term, for people have been doing serious selection for only a few thousand years, and have not even tried to create new organs or limbs. But we can create a strain of flies with four wings, breeds of dogs that would be regarded as new genera if they were found in the fossil record, and whole new biochemical systems in bacteria. Both Barry Hall and Rich Lenski, for example, have demonstrated the evolution of brand new biochemical pathways that have evolved to deal with new metabolic challenges. Now that is a “new organismal feature”!
Often new species are created by hybridization, but Shapiro forgets that that hybridization is often followed by either natural or artificial selection for increased interfertility of the new hybrid form, so it truly becomes an interbreeding population that characterizes a species. And that, of course, gives a crucial role to selection, as it did in the experiments of Loren Rieseberg and his colleagues on hybrid sunflowers.
Finally, we have selected for increased reproductive isolation in the laboratory, showing that full speciation is possible via artificial selection. My own student Daniel did this, as did Bill Rice and William Salt in lab experiments on Drosophila, which in effect created—by artificial selection—new species from a single original species.
What Shapiro fails to offer is an alternative mechanism for the origin of those features of organism that appear “designed”? Was it God? What turned an artiodactyl like Indohyus into a whale—a transition that is fully documented in the fossil record? Was it simply the “self-organization of the genome” that somehow fortuitously moved the nostrils atop the head, turned the front limbs into flippers, got rid of the hair and external ears, and wrought many other morphological and internal changes? How exactly did this happen, Dr. Shapiro? Might natural selection have played a role? Or was it “spontaneous genome organization,” whatever that means?
“Unlike most followers, Darwin acknowledged later that significant, sudden changes could occur in a fundamentally different way. He wrote about ‘… variations which seem to us in our ignorance to arise spontaneously. It appears that I formerly underrated the frequency and value of these latter forms of variation, as leading to permanent modifications of structure independently of natural selection’ (Origin of Species, 6th edition, Chapter XV, p. 395, emphasis added). So a way to rephrase my question is to ask: Have we learned since 1859 about processes that can lead to organism change “independently of natural selection?” The answer is overwhelmingly positive.Two fields principally illuminated the basic mechanisms of heredity and variation:
cytogenetics (the study of chromosome behavior in heredity using both genetic and microscopic methods) and
molecular genetics (using DNA analysis to identify the nature of genome change).”
Neither of these_though they can lead to organism change (i.e. “mutation”)—can also produce adaptation. As always, Shapiro is ducking the whole question of how organisms acquire those features that make them thrive in their environments.
If he’s going to respond to this post at HuffPo, which he will, here’s a challenge for him: what role, precisely, do you think natural selection plays in evolution—especially the kind of evolution that produces the “adaptive” features that so excite our wonder? How on earth do cytogenetics or molecular genetics alone explain the transformation of fish into tetrapods, deerlike animals into whales, or account for cryptic coloration, mimicry, and adaptive behaviors? They can’t, for there has to be some process that winnows out the variation that arises. That process is natural selection. How did the ancestral marsupial produce descendants like marsupial “flying squirrels” and “moles” in Australia that look very much like placental mammals? Did that have anything to do with natural selection? If not, explain how you think it happened.
Shapiro’s latest post, “Cell mergers and the evolution of new life forms: symbiogenesis rather than selection,” is just about as bad. Here he merely reprises Lynn Margulis’s argument that symbiosis was important in evolution: both mitochondria and chloroplasts (which respectively produce energy and photosynthesis), were the result of ancestral cells taking up and using symbiotic bacteria. And yes, that’s correct, and was a huge contribution of Margulis.
The problem for Shapiro, as it was for Margulis, is that they went on to suggest think that symbiosis is a replacement for natural selection. It isn’t. In fact, symbiosis occurs hand-in-hand with natural selection, because following the origin of an organism like a lichen or a chloroplast-containing cell via symbiosis, one finds natural selection acting on the “combination” organism, modifying both components. In fact, neither chloroplasts nor mitochondria can survive on their own outside of cells: both have been modified by natural selection to become part of an integrated and adapted cell. It is the whole vehicle—the symbiotic combination organism—that undergoes selection, with the best combinations leaving more offspring. In fact, Shapiro unwittingly alludes to this when he says this:
In all these cases, there is active DNA transfer between genome compartments. Typically, DNA sequences travel from the organelle genomes to the nuclear genome. Thus, the nucleus actually encodes most of the proteins in each of its organelles, even though they have their own genomes and protein synthesis machinery.
Restructuring of both nuclear and organelle genomes is an important aspect of evolution. Some groups of organisms are actually identified by the organization of their mitochondrial DNA.
Yes, and how does that “active DNA transfer” happen? It’s because those cells that best reapportion the genomes between “host” and “symbiont” DNA leave more offspring. And that’s natural selection.
I wouldn’t go after Shapiro except that he spews this anti-evolutionary nonsense at HuffPo, and naive readers might get the impression that biologists are beginning to doubt that natural selection is important. Well, as far as evolutionary biologists regard adaptations, it is: natural selection is the only game in town.
Yes, we now know of a whole host of new mechanisms to generate genetic variation, including symbiosis and the ingestion of DNA from distantly related species. But to produce adaptation, something has to winnow out the wheat from the chaff: those variants that reduce reproduction from those that enhance it. And that’s natural selection. There is no alternative, and Shapiro, despite his endless series of “blogs,” has never suggested one. His never-ending attacks on natural selection and neo-Darwinian evolution should be an embarrassment to HuffPo, which will apparently publish anything since they don’t have to pay for it; but they’re also an embarrassment to me, for Shapiro works at my university and, in my view, his writings impugn our reputation for excellence in evolutionary biology.
So again, I tender my challenge: tell us, Dr. Shapiro: you’re always banging on about new sources of genetic variation, but you never seem quite able to tell us how that variation is translated into adaptive evolution. If it’s not natural selection, what is it?
From the Flicker page of Nicky Bay, a photographer from Singapore, we have this beautiful example of mimicry: a spider (not an insect) mimicking a ladybug (“ladybird beetle” to Brits, which is actually more accurate since these insects are members of the order Coleoptera—beetles—rather than that of the “true bugs”, Hemiptera).
Photo by Nicky Bay (used with permission)
Ladybugs are brightly colored with what we biologists call aposematic (“warning”) coloration: a warning to predators to avoid them because they’re bad tasting (ladybugs contain toxic and foul-tasting alkaloids). Such coloration is common: other examples include black-and-orange striped bees and wasps, the orange-and-black monarch butterfly, and the striking pattern of the noxious striped skunk. (I once had a pet striped skunk for several years—descented, of course. It was a lovely animal, bred in captivity, tame, loving and litter-box trained, but I still feel a bit bad about having a pet whose genome was adapted to living in the wild.)
Once an aposematic model species is in place, there is an advantage to tasty and nontoxic species to evolve the patterns and colors of the model, for by so doing they avoid predation. This form of imitation is called Batesian mimicry after the British naturalist H.W. Bates. Here’s one example of a model (this one American) which the spider is likely imitating:
There’s no problem in explaining the evolution of Batesian mimicry, but how aposematic coloration evolved in the models has always been an evolutionary puzzle.
The system works now because predators learn to recognize and avoid the bright coloration after a bad experience tasting the prey (predators may occasionally evolve an innate, genetically-based aversion). But imagine the first mutant ladybug that is somewhat brightly colored. It won’t be avoided by bird predators because they’ve had no experience with the coloration, and will stick out because its color is conspicuous. This mutant individual may attract the attention of predators, and thus be more likely to be eaten than less-colorful individuals in the same species. In other words, natural selection would seem to work against the initial evolution of such colors, even though individuals benefit once the pattern becomes common. But how does it get to be common?
One suggestion is kin selection: mutant individuals may occur in broods of relatives, so an individual “sacrifices” its life to perpetuate the colorful genes of its brothers and sisters, now presumably protected by a bird that’s learned its lesson. And indeed, some aposematic caterpillars tend to stay together as groups of relatives more often than individuals of related species that aren’t colorful. But other evolutionary models show that the bright color could result from individual selection, particularly if the colorful individual isn’t really killed, but only tasted and released. There are other, more complicated models as well. For the time being, the evolution of such coloration remains a bit of a mystery.
And here’s a video showing another “ladybug spider”; I’m not sure if it’s the same species.
Here, from the website “What’s that bug?” is another example of what is likely to be a Batesian mimetic spider.
I found it on Arkive as well; it appears to be a rare, sexually dimorphic species in which the males are Batesian mimics and the females, much larger, aren’t. Here are both sexes with an egg case.
The attractive ladybird spider (Eresus sandaliatus) is one of the rarest in the UK. The males have a bright orange or vermilion back with four large black spots and two smaller ones, and superficially resemble a ladybird. Females and juvenile males are black and velvety. Both sexes and immature individuals have obvious large bulbous heads. Photograph by David Fox.
New species of mammals aren’t found very often, but if one is, chances are it will be a bat. Bats are secretive, often nocturnal, and numerous. With more than 1200 species in the order Chiroptera, they represent 20% of all mammalian species (red in the pie chart below); the only bigger order is the rodents:
In 2005 a new species of bat was discovered in the cloud forests of Ecuador (perhaps reader Lou Jost has seen this one): Anoura fistulata, otherwise known as the tube-lipped nectar bat. (The short paper describing it, by Nathan Muchhala et al., is free for download here, and the reference is at the bottom of this post.) As National Geographic News reported last week, its unusual method of feeding—inserting an absurdly long tongue into flowers and lapping up the nectar—has just been filmed for the first time.
What is unusual about the bat is that its tongue is longer than its body!
The creature is only about two inches (five centimeters) long, but its tongue is nearly three and a half inches (nine centimeters) long—one and a half times longer than the bat’s body.
When not collecting nectar from the Centropogon nigricans flower, the bat’s tongue is retracted and stored in the animal’s rib cage.
That’s the longest tongue relative to its body size of any mammal, even longer than that of Gene Simmons! Just as a comparison (no griping, readers), if my own tongue were of similar relative size, it would be eight and a half feet long.
A. fistulata is distinguished from other species in the genus by having a lower lip that is elongated, extends further than the upper lip, and is rolled into a tube covered with papillae. And of course there’s that tongue. Both of these features are shown in the figure below, taken from the paper.
Fig. 2 from Muchhala et al. Lip morphology of Anoura fistulata. A) Lateral view of noseleaf, lip, and partially extended tongue with papillae; and B) dorsal view of noseleaf and lip.
But of course what we want to see is how they use that tongue. This was filmed just recently by a National Geographic crew. As their article reports:
In the new high-def video—which aired Sunday as part of the National Geographic Channel’s Untamed Americas documentary series—the bat is shown feeding on the wing. (The Channel and National Geographic News are affiliated within the National Geographic Society.)
“These bats can hover,” said biologist Nathan Muchhala, who helped discover the species in an Andean cloud forest. “They’re like hummingbirds in that sense.”
In a close-up, the animal’s tongue slithers, snakelike, down the flower’s long neck. When the tongue reaches the pool of sweet nectar at the bottom, the tip transforms, becoming suddenly prickly as hairlike structures called papillae extend outward.
“Just before the bat retracts the tongue, the [papillae] stick straight out sideways,” said Muchhala, of the University of Nebraska-Lincoln. “That maximizes the surface area, allowing it to act like a mop and sop up as much nectar as possible.”
Here’s the video, which shows the bat hovering like a hummingbird (this, by the way, is an example of “convergent evolution,” since these bats and hummingbirds independently evolved nectar feeding, long tongues, and hovering):
The narration is very good here: I love the line “necessity is the mother of evoluiton.”
As National Geographic reports, getting that video was no easy matter:
To get the super-tongue footage, National Geographic filmmakers flew to Ecuador, where Muchhala and his team were waiting with a bat they’d already netted. Filming took place in a special tent, in which the bat could freely fly and feed. To make the tongue visible to the camera, a small hole was cut at the base of the flower. “They put the camera behind the hole and got that amazing close-up shot,” Muchhala said. At first, the bats were bothered by the humans and the bright lights in the tent and would not approach the flower to feed, but they eventually adjusted.”They actually get so used to it that after a while,” Muchhala said, “you come into the tent and they come up to you and will land on your hand looking for nectar.”
The long tongue was first discovered before they knew anything about the bat’s ecology, but, like Darwin’s orchid and its pollinating moth, a long tongue in a nectar-feeding bat implies a flower with a long corolla tube. Muchhala published another paper in 2006 (reference below) verifying this and giving a diagram showing how the bat’s tongue is retracted into its abdomen with special muscles:
Dietary studies of Anoura in four reserves on the eastern and western slopes of the Andes confirm this prediction. During 129 nights of mist-netting in 2003–05, I captured 46 A. geoffroyi, 38 A. caudifer, and 21 A. fistulata, and identified the pollen on their fur and in their faeces. Pollen from Centropogon nigricans, which has corollas 8–9 cm long, was carried only by A. fistulata (Fig. 1d), as might be expected, given that other Anoura could not reach this flower’s nectar. During 55 hours of nocturnal and diurnal videotaping of 12 flowers of C. nigricans, ten bats were the only visitors. This observation, combined with the finding that A. fistulata was the only bat visitor, supports the conclusion that A. fistulata is the only pollinator of this plant.
Specialization on one species of pollinator is exceedingly rare in angiosperms, and C. nigricans is the only example known in flowers pollinated by bats. After the initial evolution of a glossal tube, the extreme tongue length of A. fistulata probably coevolved with long flowers such as those of C. nigricans. In an example of convergent evolution, pangolins (scaly anteaters) also have a glossal tube; despite their different diets, ant-eating and nectar-feeding animals face similar evolutionary pressures for highly protrusible tongues, and pangolins and A. fistulata have independently converged on a similar solution.
Part of rom the Muchhala paper in Nature: c, Ventral view of A. fistulata, showing tongue (pink), glossal tube and tongue retractor muscle (blue), and skeletal elements (white). d, Anoura fistulata pollinating the specialized flower of Centropogon nigricans; because of the long corolla, only A. fistulata can reach its nectar. (Fig. 1a, M. Cooper; Fig. 1d, N. M.)
LOL!
How did this evolve? Well, of course we’re not sure, but, as Muchhala (2006) suggests, it could be a form of “coevolution,” in which the bat evolved its long tongue in concert with the flower evolving its long corolla tube. A flower that is already being pollinated by bats (and whose nectar is being nommed) might accrue a reproductive advantage by making the nectar a little bit more inaccessible—a little farther away from the opening. That forces the bat to press its head up against the flower, ensuring a better contact between bat and flower (the pollen is carried between flowers on the bat’s head). And if the flower tube gets longer, that then gives an advantage to bats who have a longer tongue, since those individuals will be better at getting the nectar, and hence will be better fed and leave more offspring. And so tongue and flower, spurred on by each other’s evolution, mutually elongate.
True “coevolution” occurs when two species act as selective forces for each other, so that they evolve in concert. The word is often misused to imply one species adapting to another, as in certain forms of mimicry, but true coevolution involves reciprocal evolutionary adjustments between a pair of species. Although it’s probably common in nature, we don’t have a lot of good examples.
Will this system evolve until the flower gets three-foot nectar tubes and bats have three-foot tongues? Probably not. There’s a limit to how far this type of coevolution can proceed, presumably imposed by the evolutionary costs of making longer corolla tubes or producing longer tongues. For the same reason, the very long tails of male African widowbirds don’t become ten feet long, for although females prefer longer tails than males actually have, at some point the added sexual benefits of having a longer, more attractive tail are outweighed by the loss in fitness such a tail confers (it could, for example, seriously impede the male’s ability to fly).
There are no new evolutionary principles demonstrated by this example, but it’s nevertheless thrilling, for it shows us another unexpected way that natural selection has worked, producing adaptations that seem a priori inconceivable.
Stomatopods, also known as “mantis shrimp,” are an order of marine crustaceans. They’re a nasty piece of work; as Wikipedia notes:
Called “sea locusts” by ancient Assyrians, “prawn killers” in Australia and now sometimes referred to as “thumb splitters” – because of the animal’s ability to inflict painful gashes if handled incautiously[4 – mantis shrimp sport powerful claws that they use to attack and kill prey by spearing, stunning or dismemberment. Although it happens rarely, some larger species of mantis shrimp are capable of breaking through aquarium glass with a single strike from this weapon. . .
Around 400 species of mantis shrimp have currently been described worldwide; all living species are in the suborder Unipeltata. They are commonly separated into two distinct groups determined by the manner of claws they possess:
Spearers are armed with spiny appendages topped with barbed tips, used to stab and snag prey.
Smashers, on the other hand, possess a much more developed club and a more rudimentary spear (which is nevertheless quite sharp and still used in fights between their own kind); the club is used to bludgeon and smash their meals apart. The inner aspect of the dactyl (the terminal portion of the appendage) can also possess a sharp edge, with which the animal can cut prey while it swims.
Both types strike by rapidly unfolding and swinging their raptorial claws at the prey, and are capable of inflicting serious damage on victims significantly greater in size than themselves. In smashers, these two weapons are employed with blinding quickness, with an acceleration of 10,400 g (102,000 m/s2 or 335,000 ft/s2) and speeds of 23 m/s from a standing start, about the acceleration of a .22 calibre bullet. Because they strike so rapidly, they generate cavitation bubbles between the appendage and the striking surface. The collapse of these cavitation bubbles produces measurable forces on their prey in addition to the instantaneous forces of 1,500 newtons that are caused by the impact of the appendage against the striking surface, which means that the prey is hit twice by a single strike; first by the claw and then by the collapsing cavitation bubbles that immediately follow. Even if the initial strike misses the prey, the resulting shock wave can be enough to kill or stun the prey.
The snap can also produce sonoluminescence from the collapsing bubble. This will produce a very small amount of light and high temperatures in the range of several thousand kelvins within the collapsing bubble, although both the light and high temperatures are too weak and short-lived to be detected without advanced scientific equipment. The light emission and temperature increase probably have no biological significance but are rather side-effects of the rapid snapping motion. Pistol shrimp produce this effect in a very similar manner.
Smashers use this ability to attack snails, crabs, molluscs and rock oysters; their blunt clubs enabling them to crack the shells of their prey into pieces. Spearers, on the other hand, prefer the meat of softer animals, like fish, which their barbed claws can more easily slice and snag.
Here’s a spearer:
Squilla mantis
And here’s a smasher. Check out those second pair of appendages, known as “dactyl clubs”:
Odontodactylus scyllarus
Here’s a spearer in action:
I’m particularly interested in the smashers, since the way they get food is stunning. Here (via Faye Flam’s website, Planet of the Apes, which alerted me to this new research), is a video of a mantis shrimp busting open a clam. If you’re not amazed at how evolution could produce such a weapon, you are jaded!
How can they do this repeatedly without damaging their “clubs”? Granted, they grow new ones each year when they molt, but they have to do these strikes thousands of times per year. A new paper in Science by James Weaver et al. (reference below, see also the Science perspective on it by K. Elizabeth Tanner, “Small but extremely tough“) did microstructural analysis of the club and found that it has several unique features to protect it. The paper is extremely technical and difficult to read, so I’ll quickly summarize what they found. The club consists of three sections:
The striking surface is made of hydroxyapatite, an extremely tough mineral made of calcium and phosphorus. This is very rare in the exoskeletons of marine invertebrates, which are usually made of calcium carbonate. Hydroxyapatite is a component of teeth and bones in vertebrates.
Behind the striking surface are layers of “chitosan,” a polysaccharide (sugarlike molecule). This not only helps deflect some of the striking energy back to the surface, but also prevents the inevitable cracks from growing (as Tanner says, “any crack is forced to continually change direcction, retarding crack growth.
Finally, the second layer is wrapped on the outside by a layer of chitin, which keeps prevents the club from disintegrating during its strikes
Not much more need be said except to marvel again at what natural selection can produce. The force of the animal’s blow is more than 1000 times its own weight; that’s the equivalent of a boxer landing a 100-ton punch! Remember that all this evolved out of some simple, primal replicator through a blind and naturalistic process of gene sorting.
The research was partially funded by the U.S. Air Force, for it could have implications for designing not only aircraft frames but body armor for soldiers.
p.s. Be sure to check out Faye’s discussion of the physics of Ray Bradbury’s story “A sound of thunder.” You know the one: a hunter goes back to the past to kill a dinosaur that would already have been doomed, steps off the track, crushes a butterfly, and, in coming back to the present, finds that that one butterfly’s death dramatically changed the world. The discussion of why time has a direction is nice, but what particularly struck me was Bradbury’s convergent discovery of LOLspeak. When the hunter comes back to his present, he finds that one of the things that’s changed is language. The sign that was in English before he went back to the past now reads:
SEFARIS TU ANY YEER EN THE PAST. YU NAIM THE ANIMALL. WEE TAEK YU THAIR.YU SHOOT ITT.
Well, I can’t give a definitive answer to that question, but I can start by telling you that zebras are not white with black stripes, but black with white stripes. The ground color in embryos is black, and the white stripes appear later in development in areas where the deposition of melanin pigment is inhibited.
But of course you’re wondering why they have stripes at all. Various hypotheses have been suggested, the most famous being camouflage, confusion of predators, or even thermoregulation. None of these have been experimentally supported, for it’s hard to test them. (The camouflage story, though, doesn’t hold up since it depends on zebras hiding in tall grass, but they live on the savanna where tall grass is almost nonexistent.)
In short, experiments on Hungarian horse farms using model horses and other targets for flies suggest that the striped pattern reduces the kind of polarized light that attracts blood-sucking tabanid flies (“horseflies”). Those flies are attracted to horizontally polarized light, presumably because that’s a sign of water, and water is where female tabanids lay their eggs. Model horses and pans of fly-attracting oil painted with a dark color are very attractive to flies; white models less so. Surprisingly, striped models attracted the fewest flies, and the stripe width that was the most deterrent was that actually present on the heads and legs of all three existing species of zebras.
The authors theorize that the adaptive significance of the stripes is to deter tabanids in Africa, because fly bites can reduce grazing, and hence survival and fertility.
It’s a reasonable theory, but has a few problems. First, there’s some special pleading about the width of stripes: as I said, the most deterrent widths are those found on the zebra’s heads and legs, not on their backs, where the stripes are wider (see below). To explain this, the authors argue that the heads are important for survival because they contain vital sensory organs, and the legs are critical because they “are indispensable to escape from predators.” (The authors also claim the blood vessels are closest to the skin in these places.) But that doesn’t explain why the stripes aren’t the same fly-deterring width all over the body.
Here’s a Grant’s Zebra (Equus quagga boehmii, a subspecies of the Plains zebra). Note that the stripes are narrowest on the head and legs:
Matthew Cobb, quoted in the BBC piece, raises another problem:
Prof Matthew Cobb, an evolutionary biologist from the University of Manchester pointed out that the experiment was “rigorous and fascinating” but did not exclude the other hypotheses about the origin of zebras’ stripes.
“Above all, for this explanation to be true, the authors would have to show that tabanid fly bites are a major selection pressure on zebras, but not on horses and donkeys found elsewhere in the world… none of which are stripy,” he told BBC Nature.
Indeed! To add to that, there’s another fly that could be a serious pest on zebras in Africa: the tsetse fly, although the authors claim that tsetses don’t bite zebras as often as they do other mammals. But zebras are susceptible to the trypanosome parasite that causes human sleeping sickness (and makes horses chronically ill), and the authors didn’t test whether stripes deterred tsetse flies. (The experiments were, after all, done in Hungary where the tsetse doesn’t live.)
I find the authors’ hypothesis intriguing and the data somewhat convincing, but there could be other reasons for stripes that we just don’t understand. And, as Matthew noted above, we also need to explain why most species of equids lost their stripes, because it’s pretty clear that the ancestral equid was striped. (Darwin noted this in The Origin when he pointed to the existence of occasional stripes in domesticated horses as evidence for the occasional re-expression of an ancestral trait). Do unstriped equids not encounter horseflies? Or was a stripey pattern too conspicuous to predators?
And someone needs to investigate this problem in felids.
