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Surprising History of Glowing Fish

Black Dragonfish (Idiacanthus).
Black Dragonfish (Idiacanthus).
Photograph by Matt Davis

During the Cretaceous period, while flowers and tyrant dinosaurs were spreading over the land, and pterosaurs and birds were taking over the skies, in the oceans, fish were starting to glow.

Today, some 1,500 fish species are bioluminescent—able to make their own light. They have luminous fishing lures coming out of their heads, glowing stripes on their flanks, bright goatees dangling from their chins, flashing headlamps beneath their eyes, or radiant bellies that cancel out their silhouettes to predators watching from below.

They evolved their glow in a variety of ways. Some came to generate it on their own, through chemical reactions within their own cells. Others formed partnerships with luminous bacteria, developing organs for housing these microscopic beacons.

Despite the obvious diversity of bioluminescent fish, no one knew exactly how often these animals evolved their self-made light. “We thought it might be a dozen or so just by eyeballing a list,” says Matthew Davis from St. Cloud State University, “but the actual number was considerably higher.”

Together with John Sparks and Leo Smith, Davis built a family tree of ray-finned fish—the group that includes some 99 percent of fish species. By marking out the bioluminescent lineages, they report Wednesday in the journal PLOS ONE that these animals independently evolved their own light at least 27 times.

Illuminated Netdevil (Linophryne arborifera). Credit: Leo Smith
Illuminated Netdevil (Linophryne arborifera). Credit: Leo Smith

Of those 27 origins, 17 involve partnerships with glowing bacteria, which the fish took up from the surrounding water. Deep-sea anglerfishes housed the microbes in their back fins, which they transformed into complex lures. Ponyfishes kept the microbes in their throats, and controlled the light they produced by evolving muscular shutters and translucent windows.

But to Steven Haddock from the Monterery Bay Aquarium Research Institute, these partnerships are fundamentally different from cases where fish evolved their own intrinsic light. “If one species of bacteria evolves the ability to glow, then is eaten and proliferates in the guts of four different fishes, you could argue that bioluminescence evolved once in the bacterium,” he says. “To me, this is much less interesting than fishes that have their own chemical and genetic machinery.”

Those intrinsic light-producers have come to dominate the open oceans. There are around 420 species of dragonfishes, most of which have long bodies and nightmarish faces armed with sharp teeth. They include the bristlemouth, the most common back-boned animal on the planet; hundred of trillions of them lurk in the deep ocean. The lanternfishes are similarly prolific; the 250 or so species account for around 65 percent of the fish in the deep sea by weight. “They’re among the most abundant vertebrates on the planet in terms of mass, but the average person doesn’t know anything about them,” says Davis.

Silver Hatchetfish (Argyropelecus). Credit: Leo Smith
Silver Hatchetfish (Argyropelecus). Credit: Leo Smith

These groups aren’t just diverse, but unexpectedly so. In the relatively short time they’ve been around, they’ve accumulated far more species than is normal, and far more so than lineages that got together with glowing bacteria. Why?

Davis thinks it’s because they can exert greater control over their light. While ponyfishes have to rely on body parts that obscure the continuous glow of their microbes, lanternfishes and dragonfishes can turn their glows on or off, using nerves that feed into their light organs. That means they can flash and pulse. They can use their light not just to lure prey or hide from predators, but to communicate with each other.

Many scientists think that deep-sea fish could use bioluminescence as badges of identity, allowing them to recognises others of their own kind and to mate with partners of the right species. This might also explain why these fish became so extraordinarily diverse, in an open world with no obvious features like mountains or rivers to separate them.

“Biodiversity in the deep sea used to be viewed as somewhat of a paradox given the apparent lack of genetic barriers,” says Edie Widder from the Ocean Research and Conservation Association. Davis’s study hints at an answer. After evolving their own light, some fish may have effectively built luminous towers of Babel—different flashing dialects that split single communities into many factions. (Something similar may have happened among electric fish in the rivers of Africa.)

The same story applies to sharks. They’ve evolved bioluminescent at least twice, and these luminous species account for 12 percent of the 550 or so species of shark. And the groups whose light organs allow them to communicate with each other seem to be exceptionally diverse. As Julien Claes from the Catholic University of Louvain told me last year, “They’re some of the most successful groups of sharks. We discover new ones every couple of years.”

So forget great whites and makos, salmon and tuna, clownfish and angelfish. The most common and diverse fish in the world are the obscure ones that you probably haven’t heard about, swimming somewhere in the open ocean, basking in their own glow.


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This Squid Has Glowing Eyeshadow That Acts Like An Invisibility Cloak

Galiteuthis. Credit: MBARI

The oceans of the world are home to animals that render themselves invisible with glowing eyeshadow.

They’re called glass squid and, as their name suggests, they are largely transparent. They’d be impossible to see in the darkness of the open ocean were it not for their eyes—the only obviously opaque parts of their bodies.

These animals live between 200 and 1000 metres below the ocean surface, where water is mostly dark. Still, some sunlight penetrates to these depths, and this light is hundreds of times brighter than anything reflected horizontally or upwards. As such, any predator looking upwards at a glass squid would see the squid’s eyes in dark silhouette against a relatively light background.

To hide itself, a glass squid uses a trick that’s common among many oceanic animals: counter-illumination. Two organs under its eyes, known as photophores, give off a dim light, which perfectly matches the weak light coming from the surface. Their glow cancels out the squid’s silhouette so that, from below, instead of just being mostly invisible, it is completely invisible.

It helps that the glass squid’s eyes stick out from the side of its head, and are controlled by powerful muscles. No matter where the squid’s body is pointing, its gyroscopic eyes always stay in the same position, with the light-producing photophore beneath them.

But that still leaves a significant problem. Without any guidance, light would leave the photophore in every direction, making the squid hard to see from directly below, but very conspicuous from other angles. Its glowing invisibility cloak would also be a beacon, were it not for yet another cunning anatomical feature.

Amanda Holt and Alison Sweeney from the University of Pennsylvania have now reported in the Journal of the Royal Society Interface that a glass squid’s photophore consists of long, skinny cells that are shaped like hockey sticks—they run parallel to the eye, and then take a sharp downward turn. The walls of these cells are lined with reflective proteins that turn them into living optic fibres. They channel the photophore’s light along their length and then downwards, into the ocean’s depths.

“They’re a way of building a literal pipe for light,” says Holt.

But wait, there’s more!

When the duo first saw the fibres, they “thought it was going to be straightforward and boring,” says Sweeney. “Oh, there are little fibres. That’s cute. We’ll describe how they work and move on.” But when they looked more closely, they noticed that the fibres are really leaky. That is, they’re not perfectly reflective. A little light always pours out along their length.

They don’t have to be like that. A few easy structural changes would turn them into perfect light guides. Instead, “they’re really inefficient,” says Sweeney. “We struggled with that for a while, before realising: Oh, that’s part of the point.”

In the deep ocean, most light comes from directly above, but a small fraction still travels at oblique angles. So the squid’s counter-illuminating light also needs to work in many directions. That’s why the photophore fibres are leaky. They’re like the diffusers that you can stick on a camera to spread the light from a flash over a large area.

Holt confirmed this by creating simulating of the fibres and calculating how much light they send sideways and downwards. She also calculated the light levels in the squid’s mid-water habitat and, again, calculated the amount of light travelling sideways and downwards. The two ratios matched.

“I remember sitting in my office comparing the two, and my jaw dropped,” says Sweeney. “I thought there must have been a mistake and we couldn’t possibly have been that lucky the first time round. But we were.”

So the glass squid’s photophores are omnidirectional invisibility cloaks. They obscure the animal’s eyes by perfectly matching the light coming in from every direction (at least, in the lower half).

The squid shows how imperfections can actually be a good thing—a lesson that, according to Sweeney, engineers should pay attention to. “Over and over in biology, we see that evolution harnesses disorder in very clever ways to make better devices than what you’d get with highly ordered structures,” she says. “Thinking [about this] will help engineers to leverage the disorder in their systems rather than trying to get rid of it.”

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How to Survive a Fast, Venomous, Flesh-Destroying Snake

The boy was thirteen years old when, while hunting for bush rats, he stuck his hand down the wrong hole. He was bitten by a saw-scaled viper. The boy’s hand swelled up and his skin turned white. He started bleeding from huge open gashes in his knuckles and arms. Worse still, the flesh in his hand started rotting.

The saw-scaled viper or carpet viper, Echis carinatus, Photograph by imageBROKER, Alamy
The saw-scaled viper or carpet viper, Echis carinatus, Photograph by imageBROKER, Alamy

Kempaiah Kemparaju from the University of Mysore in India shows me photos of the venom’s handiwork, and they’re hard to stomach. By the final image, the boy’s hand is a red, pulpy mess, and two fingers are missing. It looks like he reached into some kind of industrial machine.

The graphic images are, sadly, commonplace. Most snakes are harmless to humans, and even dangerously venomous ones are unlikely to bite us or to inject much venom. But the saw-scaled viper is a rare exception. It’s aggressive and hard to spot. It’s common to parts of the world that are densely populated by humans. And it has a potent venom. Toxins in the venom can break down the membranes that line our blood vessels, and max out our ability to clot, leading to catastrophic bleeding.

But the venom doesn’t just kill; it destroys.

It devastates the tissues around the site of the bite, so that even if people survive, they can still lose fingers, toes, or entire limbs. It’s estimated that around 125,000 people die from snakebites every year, but around 400,000 more face amputations. Antivenoms don’t help. They consist of large antibodies that are too big to effectively move from the blood into tissues that are being attacked. They save lives, but not limbs.

But we’re a little closer to a solution because Kemparaju and his colleagues, Gajanan Katkar and Kesturu Girish, have finally discovered how the viper’s venom wreaks so much havoc.

The team knew that the immune system reacts to viper venom by deploying white blood cells to the site of a bite. They suspected that some of these cells—the macrophages—might be inadvertently damage tissue, so they started isolating them. In the process, they snagged another kind of white blood cell, too—the neutrophils. What the hell, they thought. Might as well study the neutrophils too.

Good thing they did.

Neutrophils can sacrifice themselves to kill microbes by bursting open and releasing a tangled mesh of their own DNA. These webs, which are loaded with antimicrobial molecules, immobilise and kill invading cells. Rather aptly, they’re called neutrophil extracellular traps, or NETs.

When Kemparaju’s team saw the DNA threads under a microscope, they realised that neutrophils were also releasing NETs in the presence of viper toxins. But there, they do harm. The mesh blocks blood vessels and trap venom toxins at the site of the bite, where they attack local tissues. Those tissues also starve of oxygen, quickening their demise. Indeed, when the team injected viper venom into mice with low levels of neutrophils, the rodents succumbed to the venom but didn’t show any signs of tissue damage.

This leaves an unenviable choice. The actions of the neutrophils destroy tissue. But without them, the toxins circulate all over the body, damaging more organs and potentially killing the victim outright. The latter, incidentally, is what cobra venom does. It contains an enzyme called DNase that slices through the NETs and releases the trapped toxins.

Saw-scaled vipers lack DNase, which is probably a good thing on balance. “If the venom did have DNase activity, the systemic toxins along with the tissue-degrading enzymes would damage vital organs in no time, and a victim’s chances of survival would have been feeble,” says Kemparaju. It’s like this, he says: “Instead of life, you give your limb.”

But there might be a way to save both life and limb.

When the team injected mice with venom and DNase at the same time, the rodents died more quickly than they did with venom alone. But if the team waited for an hour or two before injecting the DNase, they prevented tissue damage without reducing the rodent’s odds of survival. “With our mice, we have achieved 100 percent success,” says Kemparaju. “Even if you administer the DNase three hours after the venom, you can prevent the loss of limb.”

“The results are very exciting, as they open up a potential new therapy for treating the debilitating, horrific, and destructive effects of certain snake venoms,” says Nicholas Casewell at the Liverpool School of Tropical Medicine. Still, the team must run clinical trials to ensure that DNAse treatments are safe and effective in people. Timing is everything, and the enzymes can do more harm than good if given at the wrong moment. And people would still need antivenoms to deal with the toxins already circulating in their blood.

“It will also be very interesting to see whether DNases will also reduce the local tissue effects caused by other snakes, such as puff adders and spitting cobras, thereby providing a generic treatment,” adds Casewell.

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This Frog Uses Its Spiky Face to Deliver a Venomous Headbutt

When Carlos Jared was first ‘stung’ by the venomous face of the Greening’s frog, he didn’t realise what had happened. He had picked up one of the small creatures, and it started thrashing about as if trying to headbutt his hand. At first, it felt like being abraded by rough sandpaper. Then, Jared quickly developed an intense pain, which radiated up his arm and lasted for five hours. And since he was four hours away from any major city, he just had to grin and bear it.

Jared didn’t initially connect the pain to the little frog he had picked up. After all, there was no obvious injury. Only later did he figure out what had happened: The frog had released toxins from its skin, and used the small spines that line its skull to drive those poisons into his hand. He had received a venomous headbutt.

Greening’s frog, also known as the casque-headed tree frog, was first described in 1896. It’s a palm-sized animal with mossy green skin and a distinctive flattened head. The many bones of the top of its skull have fused together, and merged with the lower layers of skin to create a single, sturdy plate.

Partly, this is an adaptation against dehydration. Greening’s frogs (like all of them) depend on water, but they also live in the extremely dry Caatinga forests of Brazil. Their solution is to shuffle backwards into holes and then plug the entrance with their bony helmets. That seals their bodies in humid spaces and stops them from losing too much water.

Since the skull is both flat and well camouflaged, it’s very hard for predators to spot the frogs, let alone pull them out of their hollows. And if any predators are tempted to try, the lip region of the skull is covered in wicked-looking spines. With all the flesh removed, the skull looks a bit like a medieval mace.

The skull of Greening's frog. Credit: Carlos Jared/Butantan Institute
The skull of Greening’s frog. Credit: Carlos Jared/Butantan Institute

Jared experienced firsthand what these spines can do when he started collecting the frogs in Caatinga. He was lucky to only get hours of pain. He soon discovered that the frogs can release a white, toxic mucus from glands in their skin, which can be lethal when swallowed. These glands are especially large in the frog’s face, where they sit over the skull spines. So, when threatened, the frogs can slather their own face with toxic mucus, and then drive it into an antagonist’s flesh using their own skull—a tactic that’s aided by their unusually flexible necks.

So are these frogs poisonous or venomous? Poisonous animals have no way of injecting their toxins, although they might be able to secrete them. That’s the case for most toxic amphibians. “They have a passive defense, only offering their own bodies (or parts of them) to be bitten by the predator or aggressor,” says Jared. “The predator is responsible  for its own poisoning.” By contrast, venomous animals can actively deliver their toxins into an enemy or victim using stings, spurs, or fangs. That’s the case for spiders, snakes, and scorpions.

The casque-headed frogs clearly fit in both categories. They can release toxins through their skin—a passive defence. They can also introduce those toxins directly into wounds using their spines—an active defence. Regardless of semantics, what matters is that the defence works.

When tested on mice, the toxins from Greening’s frog proved to be twice as lethal as those of the notorious fer-de-lance snake, and other pit vipers that share the same forests. A second closely related frog—Bruno’s casque-headed frog—is even more dangerous. Its poison glands are smaller and its skull spines are shorter, but its toxins are 25 times more lethal than pit viper venom.

Bruno's casque-headed frog Credit: Carlos Jared/Butantan Institute
Bruno’s casque-headed frog. Credit: Carlos Jared/Butantan Institute

“During all these years that I’ve lived with these animals in their environment, I’ve never seen any sign of predation, or aggression by predators,” says Jared. He imagines that for a snake, swallowing these frogs would be like trying to wolf down a poisoned cactus.

Reference: Jared, Mailho-Fontana, Antoniazzi, Mendes, Barbaro, Rodrigues & Brodie. 2015. Venomous Frogs Use Heads as Weapons. Current Biology http://dx.doi.org/10.1016/j.cub.2015.06.061

See also: ‘Wolverine’ frogs pop retractable claws from their toes


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You’ll Find the Biggest Male Appendage in the World—at the Beach

So tell me: Of all the males on our planet—and I’m talking all, scale be damned, from the littlest insects to the biggest of whales—who’s got the most impressive appendage? Who (you should pardon the expression) is our Biggest Daddy?

And I don’t mean just sex organs, impressive as they sometimes are. I’m talking about any outstanding male appendage—whatever guys have that thrills the ladies, the obvious non-penile example being the peacock tail, which as we all know when fully displayed makes peahens dream of George Clooney.

Picture of a peacock with its feathers fanned out
Photograph by Richard I’Anson, Getty
Indian Peafowl, or Peacock, displaying tail feathers.. Photograph by Richard I'Anson

Impressive? Yes. The problem being that between trysts, a superheavy tail must be a drag to lug around. It costs energy to maintain and more energy to get up and unfurled. But the drive to reproduce is a powerful thing, and sexual selection, as Darwin taught us, just keeps pushing the limits of bigness.

Obviously, there is such a thing as too big. I imagine it gets awkward to be a walrus with tusks curling dangerously close to your chest.

Picture of a pacitic walrus with very long tusks lying on rocks
Photograph by Yva Momatiuk & John Eastcott, Minden Pictures / National Geographic
Pacific Walrus (Odobenus rosmarus) bull portrait on coastal rocks in haul-out cove, summer, Round Island, Bering Sea, Bristol Bay, Alaska. Photograph by Yva Momatiuk & John Eastcott / Minden Pictures / National Geographic

And the weight of these things? Not the absolute weight, but the proportional weight—that’s another limitation. How much can a guy carry? According to Douglas Emlen in his wonderful book Animal Weapons: The Evolution of Battle, while the rack of bone that sits atop a male caribou is hugely impressive (those antlers can weigh 20 pounds and stretch five feet across) …

Picture of a caribou with large antlers amid a lush green landscape on a misty day next to an equal sign that says ''8%''
Photograph by Bob Smith, National Geographic Creative
Close up portrait of a male caribou, Rangifer tarandus. Photograph by Bob Smith, National Geographic Creative

… as big as they are, they account for only 8 percent of the male’s total body weight. Moose and elk have even larger antlers, but even the biggest elk antlers equal only 12 percent of its body weight. So that’s doable. Once upon a time, 11,000 years ago, there was a deer (called Megaloceros giganteus, or the Irish elk) that wandered Europe and Asia, and those guys had boney tops so insanely branched, so crazily big, that our prehistoric humans painted them worshipfully onto the cave walls at Lascaux.

Picture of an Irish elk painted onto the wall at Lascaux cave next to a ''less than'' sign that says 20% next to it
Photograph by Sisse Brimberg, National Geographic Creative
Prehistoric artists painted a red deer on a cave wall. Photograph by Sisse Brimberg, National Geographic Creative

But even these antlers, 12 feet across and wildly branched, weighed less than 20 percent of the total animal.

Go Small to Get Big

You have to drop down to the insect family, to a group of horned beetles, to find a big appendage that approaches a third of the animal’s body weight. There are several beetles that have fighting, clamping horns almost the size of their bodies, and a thing like that growing out of your skull, writes Emlen, “is a little like having your leg sticking up out of your forehead.” You feel it.

Picture of a rhinocerous beetle on wood
Photograph by altrendo nature, Getty
Rhinoceros Beetle, male, Columbia, South America, Photograph by altrendo nature

But if you’re looking for the male who wears the crown, whose appendage is so big, so startling, so colorful, so attractive, so monstrous, and therefore unequaled in the animal kingdom—if you’re looking for the champ? Well …

He’s not in the African savannah. He doesn’t have a tusk. He’s not especially large. You have to look down to see him, down near your feet when you’re at the beach. This is him:

Picture of a fiddler-type crab standing on sand and waving a claw in the air
Photograph by Michael Nichols, National Geographic Creative
Loango National Park, Gabon. A fiddler-type crab with claw raised standing outside its burrow, Photograph by Michael Nichols, Getty

He’s a fiddler crab. And that appendage is his claw. And while sizes vary from crab to crab, the biggest fiddler crab claws weigh roughly half the body weight of the animal. Half! That’s nature’s biggest appendage, says Emlen. And what is it for? Not for feeding. The claw is useless at mealtime. Males eat with their other, smaller claw only. But, says Cornell biologist John Christy, the claw’s bright colors definitely attract female attention. It can also snap down and inflict real harm, so they’re potential weapons. But mostly, he discovered, males use them—I kid you not—to wave.

“Up and down, up and down, again and again,” writes Emlen, “they raise their claws high and drop them. Dozens of times each minute, thousands of times per hour, hour after hour … ” They look a little silly doing this, like a lonely fan trying to start a stadium wave. Check out this fellow:

Why are they waving? It’s a warning. “Look what I’ve got!” the male is saying to any male who would trespass into his burrow. “This thing is going to pound you if you come near, so stay away!” In effect, Emlen writes, “fiddlers are employing their claws as warnings rather than instruments of battle.” And it works. “An overwhelming majority of contests end before they ever begin, without anything even resembling a fight. A mere glance at a big claw is sufficient to deter smaller males.”

The male has built a tunnel, which leads to a nesting burrow. His claw has attracted a lady, and she’s down below raising his family. His job is to stay on top, waving till she’s done or he drops. It’s a tough life, lifting that gigantic appendage over and over, using up energy, constantly getting bothered by would-be challengers. The male can’t eat. Not while he’s guarding. His food is at the water’s edge, which is down lower on the beach. So he stands there, day after day, getting hungrier, until eventually, Emlen says, “Even the best males run out of steam and are forced to abandon their burrows to go feed and refuel. The instant they leave, others will claim their burrows.” And then they become challengers and have to start all over again.

So while it may be glamorous to top the list of Biggest Appendage Ever, what with the lifting, the waving, the straining, the not eating, the worrying about how long you’ll last, it might be better to have a medium-size claw and not have to be always worrying about the biggest bullies at the beach. Yes, the big claw does dramatically increase your chances of producing babies, which, as Darwin will tell you, is the whole point. But if I were a fiddler crab lucky enough to have the biggest appendage in the world, I think I’d get myself a nail file (claw file?), erase my genetic advantage, and spend lazy afternoons sipping pond scum by the ocean’s edge. I like a gentler life.

Douglas J. Emlen’s book, Animal Weapons: The Evolution of Battle, is a fascinating account of how animal weaponry, both offensive (claws, horns, teeth) and defensive (armor, shelter, thorns, claws again) parallel human weaponry, both offensive (arrows, lances, swords, missiles, A-bombs) and defensive (armor, castles, spying). It’s a compelling, fun, often scary analysis. And David Tuss’ drawings, especially his animals, made me jealous.

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Why Do Luna Moths Have Such Absurdly Long Tails?

You don’t need a field guide to recognise a luna moth. This large insect, found throughout the eastern half of North America, is unmistakeable. It has a fuzzy white body, red legs, feathery yellow antennae, and huge lime-green wings that can stretch up to 4.5 inches across. And at the end of its hindwings are a pair of long, streaming tails that can double the moth’s length.

In 1903, an entomologist named Archibald Weeks suggested that the tails direct predators away from the moth’s body. “Again and again may predator bat or bird, in an effort to capture a moth or butterfly, successively tear away sections of the tails, of which a sacrifice can be readily afforded, without disabling it or retarding its flight,” he wrote.

He was roughly right. More than a century on, Jesse Barber from Boise State University has shown that the luna moth’s tails are the equivalent of eyespots on fish and butterflies. These distinctive markings are typically found on dispensable body parts like tails and outer wings. They serve to draw a predator’s attention away from more vulnerable regions; better to lose a tail than a head.

Eyespots are visual defences, and bats—the main nemeses of moths—are not visual hunters. They find their prey with sonar—they make high-pitched squeaks and visualise the world using the rebounding echoes. To divert a bat, you need something that makes distracting echoes.

That, according to Barber, is what the luna moth’s tails do. They are “auditory deflectors”. Bat distractors.

Luna moth close-up. By Oliver Dodd. CC-BY-2.0
Luna moth close-up. By Oliver Dodd. CC-BY-2.0

Barber pitted luna moths against bats in a dark room, and filmed their encounters with infrared cameras. Under normal circumstances, the bats only managed to snag 35 percent of the moths. But if Barber cut off the insects’ tails beforehand, the bats caught 81 percent of them. That’s not because they become worse fliers—in fact, the tails don’t seem to affect their aerial abilities at all.

When bats aim their sonar at insects, they analyse the rebounding echoes for the distinctive signatures of beating wings. But the luna moths tails, which spin behind them as they fly, also produce echoes that resemble wingbeats. To the bat, they either sound like a very conspicuous part of their target, or like a different target entirely. As a result, they fumble their attacks.

When bats attack, they usually use their wings and tail to scoop an insect towards their faces, so they can deliver a killing bite to their victim’s body. But when bats attack luna moths, they aim about half their attacks at the tails. That’s a mistake—only 4 percent of those attacks succeed. Sometimes, the bat misses the moth entirely (see above). Other times, it bites off a tail while the moth escapes—down one inessential body part, and still alive (see below).

The tails also make the luna moths bigger, which might make them harder for the bats to handle and dispatch. But when Barber pitted bats against the polyphemus moth—an even bigger species that lacks tails—he saw that the predators killed 66 percent of their targets. The luna moths, despite being smaller, were harder to catch. “Clearly, tails provide an anti-bat advantage beyond increased size alone,” Barber wrote.

It’s possible that female moths also judge the health and quality of a male by looking at the size of his tails. But this doesn’t fit with the moths’ behaviour. Female moths spend most of their time hiding in protected nests and drawing males to them by releasing pheromones. They also mate with the first males they find, so there’s no evidence that they’re choosy—much less that they choose on the basis of tail length.

Luna moths belong to a group of large moths called the saturniids—a group that contains members like Copiopteryx and Eudaimonia, with even more extreme tails. By comparing the tail lengths of 113 saturniid species, Barber showed that these moths have evolved long tails on at least four separate occasions. He now wants to know if these other species are also good at foiling bats.

Reference: Barber, Leavell, Keener, Breinhoff, Chadwell, McClure, Hill & Kawahara. 2015. Moth tails divert bat attack: Evolution of acoustic deflection. PNAS http://dx.doi.org/10.1073/pnas.1421926112

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Falling Leaf, Flying Dragon

In the canopy of a Malaysian rainforest, a little lizard scuttles to the end of a branch and launches itself into the air. It doesn’t, however, fall to its doom. Instead, it extends two flaps of skin from its flanks, supported by unusually long ribs. The flaps look and work like wings, allowing this lizard—the aptly named flying dragon—to glide to safety. They are so adept in the air that they almost never come to the ground. Why bother, when they can travel for 20 to 30 metres between treetops, without losing much altitude?

There are 42 species of flying dragons, or Draco as they are formally known, and they all glide on extended flaps of skin or patagia. But Danielle Klomp from the University of New South Wales thinks that there’s more to the patagia than gliding. They are also beautifully coloured and Klomp has shown that, in at least one species, these hues match those of falling leaves from the local area. This, she says, is no coincidence. She thinks that the lizards have evolved to mimic falling leaves, to avoid the attention of birds.

“The locals we would chat to would often describe the lizards as looking like falling leaves,” she says. “We spent a lot of time walking around the forest trying to find them, and we often confused a gliding lizard for a falling leaf out of the corner of our eyes.” She would also find fallen leaves on the floors of many different rainforests that looked like the patagia of the dragons that lived in the area. This called for a more systematic study.

Klomp focused one species—Draco cornutus—which lives in Borneo and comes in at least two varieties. The individuals that dwell in coastal mangrove forests have rusty red patagia, and the dominant trees there jettison similarly coloured leaves. Elsewhere, in the lowland forests, the lizards’ patagia are a dark greenish-brown, and so are the falling leaves of the local trees.

Patagia of Draco cornutus from coastal mangrove forests (top) and lowland forests (below)
Patagia of Draco cornutus from coastal mangrove forests (top) and lowland forests (below)

The resemblance is striking to human eyes. To quantify it, Klomp collected both dragons and leaves from the two forests, and analysed the light reflecting from all of them. She showed that the contrast in colour was smallest when she paired the dragons with falling leaves from their own habitat, and higher when she compared them to standing leaves, or falling leaves from a different area.

Flying dragons glide around four times an hour and although they excel at it, they aren’t more manoeuvrable than birds. With plenty of hungry beaks around, it behoves them to have some way of avoiding attention. Mimicking falling leaves is one possible solution and not a far-fetched one, either. Some birds might do the same. The black fairy hummingbird, for example, does a weird gliding flight whenever it leaves its nest. It opens its wings and tail so that its body is horizontal to the ground, and it spins on its way down, recovering just a couple of metres before crashing. The movement looks a lot like a falling leaf.

Other scientists have suggested that the flying dragons use their brightly coloured patagia as billboards for signalling to mates. But Klomp’s team have filmed many of these lizards in the wild, and their 30 hours of footage rarely shows the animals using their wings in displays.

But absence of evidence isn’t evidence of absence, and Jim McGuire at the University of California in Berkeley, who has studied these lizards extensively, has often seen the males displaying with their patagia (here’s some video). They’ll sometimes open just the wing that’s closest to the female.

Other lines of evidence support the idea that the dragons communicate with their patagia. In most species, males have more vividly coloured wings than females, even though both sexes would presumably benefit from mimicking leaves. The colours are almost always species-specific too, and different species with distinct colours often live in the same area amid the same trees.

“There’s no doubt in my mind that patagial colours play an important role in species recognition,” says McGuire. “If it’s possible to evolve a colour pattern that would at once be conspicuous to [other Draco individuals] and simultaneously cryptic to predators, this would be a win-win. However, it’s also possible that Draco could be mimicking something other than leaves, like unpalatable [stick insects] or butterflies. And, of course, Draco may not be mimicking anything at all.”

To support her hypothesis, Klomp needs more data. So far, she has only compared wings and leaves in two populations of Draco from one species. Anecdotally, she has seen that several other species resemble like their local leaves but “this needs to be done properly,” she says. She also wants to test her prediction that species that live in more open habitats, or in places with a single dominant tree species, might benefit more from mimicking leaves.

Reference: Klomp, Stuart-Fox, Das & Ord. 2014. Marked colour divergence in the gliding membranes of a tropical lizard mirrors population differences in the colour of falling leaves. Biology Letters http://dx.doi.org/10.1098/rsbl.2014.0776


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Why Does This Weird Insect Flash Warnings After An Attack?

Kate Umbers was hiking through Australia’s Snowy Mountains in the autumn of 2008, when she saw her first mountain katydid—a thumb-sized insect with the colour and texture of a dead leaf. “I recognised it from the guide books and picked it up excitedly,” she says. “It immediately vomited and flashed its bright colours.”

Emphasis on bright. The insect’s dull brown wing casings flew apart to reveal vivid bands of red, black, and electric blue. The inconspicuous leaf suddenly transformed into a garish Christmas bauble.

Many animals do something similar. When a threat gets close, they flash bright colours, show off distracting eyespots, strike aggressive poses, and spray off-putting chemicals. They hiss, rattle, puff, and arch. These spectacles are called deimatic displays and they are supposedly meant to distract or intimate predators. Bright colours, in particular, are often messages that scream: “I AM TOXIC; DO NOT EAT ME.” For some animals, these claims are bluffs. For the mountain katydid, they are genuine warnings—this insect is full of foul-tasting chemicals.

But Umbers noticed something unusual about its displays: the katydid only flashed its colours after an attack.

“I was struck by how easy it was to catch them and how little resistance they put up,” she says. “They waited until they had been grabbed before revealing any defences.”

This isn't even my final form. (Credit: Kate Umbers)
This isn’t even my final form. (Credit: Kate Umbers)

Together, with Johanna Mappes, whose work I’ve written about before, Umbers carefully tested 40 captive katydids. She found that they almost never used their displays when she tapped a pen near their heads, blew gently on them, or waved a book overhead to simulate a passing bird. They only flashed upon contact, when she poked them or tried to pick them up.

That made no sense. Animals are meant to use deimatic displays to avoid attacks. It’s no use screaming, “I’m dangerous,” when beaks are already grabbing you or teeth are already sinking into your flanks. “Not only does this seem like a pretty bad strategy, it is counter to current thinking on how deimatic displays work,” says Umbers. “There is no theory to allow for such an adaptation, and yet there it is.”

“It’s a neat study, which suggests that we might have misread some kinds of animal signals, and misunderstood the different uses that startle defences can have,” says Mike Speed from the University of Liverpool.

Mountain katydid in the field. Credit: Kate Umbers.
Mountain katydid in the field. Credit: Kate Umbers.

Umbers suspects that the insect might prioritise stealth over shock. It blends into its surroundings, and if it revealed its colours, it would instantly break its own camouflage. That would be worthwhile if the colours worked as intended. But if predators don’t encounter these katydids very often, they might not know what the warnings mean and attack anyway. And while many animals use deimatic displays as distractions, to give themselves time to escape, the katydid is terrible at fleeing. It lacks the powerful jumping legs of its relatives, and the females can’t even fly. It does, however, have a very tough shell.

So, the katydid’s strategy seems to be: hide for as long as possible, rely on a tough shell to withstand a first strike; and hope that a mouthful of foul chemicals will deter a second one. “The species is combining the best of both worlds by walking softly and carrying a big stick,” says Tom Sherratt from Carleton University

But why the colours? Umbers suggests that they might reinforce the off-putting nature of the insect’s foul taste. Sherratt agrees, and notes that other animals couple chemicals and signals in this way. Some caterpillars, for example, vomit noxious substances and make clicking noises, when touched.

Umbers is now trying to work out what actually eats the katydids—the most likely candidates are ravens and magpies—and how they react to different aspects of the insects’ defence.

“Startle displays seem quite common in nature but are very much understudied,” says Martin Stevens from the University of Exeter. “What makes them effective—unexpectedness, novelty, anomaly, conspicuous colours, and so on—isn’t clear. The current study is therefore a nice start in understanding this.”

Reference: Umbers & Mappes. 2014. Postattack deimatic display in the mountain katydid, Acripeza reticulata. Animal Behaviour http://dx.doi.org/10.1016/j.anbehav.2014.11.009

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Spider Hides From Spider Among Spider-Eating Ants

Most spiders only eject silk from glands in their rear ends but Scytodes—the spitting spider—is an exception. It can also shoot silk from its mouthpart. It does so with great force, and it impregnates these strands with venom to create a sticky gum that both poisons and traps its victims. It’s the closest natural equivalent to Spider-Man’s web-shooters.

If the prospect of a spider with a long-range weapon freaks you out, you are not alone. Even other spiders are wary of Scytodes.

In the Philippines, the spitting spider will readily attack jumping spiders and its web is often littered with arachnid carcasses. Ximena Nelson and Robert Jackson from the University of Canterbury have shown that it often targets a black-and-lemon species called Phintella piatensis. Scytodes will build its nest directly over a Phintella nest and ensnare the jumping spider as it enters and leaves its home. Sometimes, it even taps on the nest with its legs, perhaps to check if anyone’s home.

But Phintella is not entirely defenceless. Nelson and Jackson also found that it protects itself by nesting in the company of the weaver ant Oecophylla smaragdina.

When the duo placed leaves with Phintella nests in a chamber, and wafted in the smell of weaver ants, they found that Scytodes avoided building its own web overhead. And Phintella, in turn, was more likely to build nests on leaves where the ants could be seen or smelled.

Weaver ant kills Phintella. Credit: Robert Jackson
Weaver ant kills Phintella. Credit: Robert Jackson

The reason is simple: the ants are voracious predators and spiders are on their menu. They’re so aggressive that farmers often deliberately use them to protect mango crops from pests. Even Scytodes’ trademark weapon is of little use: its spit will immobilise a couple of weaver ants, but it can’t pin down an entire group. When Nelson and Jackson housed a Scytodes with weaver ants, it was almost always killed.

Phintella, however, isn’t bothered by the weavers. It fashions a silken cocoon like most jumping spiders do, but it uses an especially tough and dense weave that the ants cannot tear open. It also builds doors! It has hinged silken flaps at either end of its nest which seal it away when it’s inside the nest, and which the ants rarely try to open. The ants do sometimes capture Phintella, as the image above shows, but this is relatively rare.

So, its ant-proof home allows Phintella to surreptitiously recruit the weavers as protectors, without succumbing to them itself. It’s like a person who seeks safety by walking into the most dangerous part of town in a Kevlar suit.

There are many cases where ants act as bodyguards for other species, including aphids, acacia trees, and some butterflies, in exchange for food and nutrients. But the relationship between Phintella and the weavers isn’t a mutualism, where both partners benefit. The ants don’t seem to get anything from Phintella’s presence. They don’t suffer either, so it’s not parasitism. Instead, Phintella is more of a commensal—a creature that benefits by living alongside another, without offering any advantages in return.

It’s also not the only jumping spider to have a connection with ants. Some eat them. Others mimic them to a spectacular degree. One mimics ants to avoid being eaten by spiders so that it itself can eat spiders. And Phintella, which neither looks like an ant nor eats them, lives alongside ants and avoids being eaten by them so it can also avoid being eaten by another type of spider, which ants can eat. Ain’t nature grand?

Reference: Nelson & Jackson. 2014. Timid spider uses odor and visual cues to actively select protected nesting sites near ants. Behav Ecol Sociobiol. http://dx.doi.org/10.1007/s00265-014-1690-2

More on Nelson and Jackson’s work:

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Toxic Halitosis Protects Tobacco-Eating Caterpillar

Way before people started inhaling nicotine in cigarette smoke, plants were using the chemical to defend themselves from animals.  Nicotine is a poison, and an exceptionally deadly one. It targets proteins that tell our muscles to fire when they receive signals from our nerves. At high enough doses of nicotine, these proteins force muscles to constantly contract, leading to paralysis and death. And since the same proteins are found in every animal with muscles, nicotine can kill cows and caterpillars alike.

The tobacco hornworm is an exception. As a caterpillar, this moth specialises in eating tobacco leaves, because it can cope with doses of nicotine that would kill other species. It gets rid of most of the poison in its waste but, adding insult to injury, it also co-opts a small fraction for its own protection.

Pavan Kumar and colleagues from the Max Planck Institute for Chemical Ecology in Germany have shown that it exhales the poison through pores in its skin, creating a toxic miasma that deters hungry spiders. They call it a “defensive halitosis”.

In 2010, Kumar’s  team, led by Ian Baldwin, raised tobacco hornworm caterpillars on genetically modified tobacco that doesn’t make much nicotine. They found that a gene called CYP6B46 was less active than usual in the guts of these insects, suggesting that it’s usually involved in resisting the effects of nicotine.

To test this idea, the team engineered tobacco plants that could deactivate the gene in any caterpillars that fed upon them, and planted them at a private ranch in Utah’s Great Basin Desert. They waited, and watched.

Soon, they noticed that hornworm caterpillars were more likely to die during the night if they ate the modified plants. A few nocturnal surveys revealed the cause of their deaths—wolf spiders. These powerful, fast-running hunters usually pose no threat to hornworms that eat nicotine-rich meals. However, they readily killed any caterpillars that ate the modified tobacco and had inactivated CYP6B46 genes. Why?

The answer seemed obvious at first. CYP6B46 is part of a large family of metabolic genes, which animals frequently use to detoxify the chemicals in the plants they eat. The team assumed that CYP6B46 was neutralising nicotine by breaking it down into safer substances. But, to their surprise, they couldn’t find any traces of these by-products in the caterpillars’ bodies or faeces.

Instead, they showed that CYP6B46 redirects a tiny amount of nicotine from the caterpillars’ guts to their haemolymph—the liquid that fills their bodies and acts as their bloodstream. From there, the caterpillars can vent the nicotine into the outside world by opening their spiracles—small breathing holes in their flanks, which allow air to enter and leave their bodies.

The caterpillars send just 0.65 percent of the nicotine they eat into their haemolymph. But even this tiny amount is enough to quadruple the concentration of nicotine in the air around them, creating an effective anti-spider spray.

When a wolf spider attacks, it first inspects its prey with chemically sensitive appendages. Here’s what happens when it approaches a caterpillar with a nicotine cloud.

And here’s what happens when it approaches a caterpillar with an inactivated CYP6B46 gene. The caterpillar can’t shunt nicotine from its gut to its haemolymph and can’t exhale the poison into the surrounding air. It pays the price for it.

The hornworm’s nicotine cloud probably works against other predators too. In earlier studies, when caterpillars are reared on tobacco, ants are less likely to attack them. Parasitic wasp larvae are also less likely to survive inside the caterpillars’ bodies, presumably because they are directly poisoned by the nicotine in their haemolymph. But the defence isn’t fool-proof. Kumar’s team showed that two predators—big-eyed bugs and antlions—will kill hornworms despite their halitosis. No one knows why.

Chemical theft is fairly common in the animal world, and many caterpillars store defensive poisons from the plants they eat. For example, the eastern tent caterpillar munches on plants that are loaded with hydrogen cyanide, which it then vomits onto marauding ants. But nicotine is too deadly to store. Instead, the tobacco hornworm has evolved a way of getting rid of it, which also doubles as a potent defence.

Reference: Kumar, Sagar, Pandit, Steppuhn & Baldwin. 2013. Natural history-driven, plant-mediated RNAi-based study reveals CYP6B46’s role in a nicotine-mediated antipredator herbivore defense. PNAS http://www.pnas.org/cgi/doi/10.1073/pnas.1314848111

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Termite Pest’s Faecal Fortress Is Part Of Its Immune System

Formosa is the Portuguese word for “beautiful”. It was the name given to Taiwan by sailors passing by in the 16th century. The name was changed in 1949, but it still lingers on in people’s minds, in the names of local businesses, and in one annoying global pest.

The Formosan subterranean termite (Coptotermes formosanus) was first described in Taiwan in the early 1900s and although it originated in China, the name stuck. From there, wandering humans took it all over the world. It’s what most people think of when they hear the word “termite”—a small, white insect that eats wood and sometimes, specifically, the wood in your house. Each individual is no more gluttonous than your average termite, but the million-strong colonies are so big that they inflict serious damage upon buildings.

There are around 3,000 species of termites, and while many build huge towers and castles, the Formosan termite’s kingdom is entirely underground. There’s a core nest and several satellite chambers, connected by long foraging galleries, which occasionally rise up to find houses, trees, and other wood sources.

Even as the Formosan termites are destroying your house, they are building their own. Their construction material is… well… your house, but chewed up, digested, and excreted out the other end. They use their faeces to coat the walls of their foraging galleries. And they pack it together with chewed wood and soil to create a spongy “carton material”, which fills the empty spaces of their nests and foraging sites. A termite is effectively a machine that converts a house or tree into an underground faecal fortress.

But the faeces are more than just a building material. They’re also part of the termite’s immune system.

A block of carton material, 6cm x 4cm. Credit: Thomas Chouvenc.
A block of carton material, 6cm x 4cm. Credit: Thomas Chouvenc.

The termites encounter many dangerous diseases in their underground incursions, including many insect-killing fungi.  Scientists have actually tried to use these fungi to kill termite colonies for 50 years, but none of the field trials succeeded. The insects are just too good at controlling the infections. They secrete antifungal chemicals, groom each other to remove spores, and bury dead infected nestmates.

Thomas Chouvenc from the University of Florida has been studying these strategies for many years. But recently, he started to realise that the nest itself might help to keep the insects healthy.

His team collected carton material from five Formosan termite nests and showed that they’re rich in Actinobacteria. These microbes are a regular part of insect healthcare, protecting them against fungal diseases. Indeed, Chouvenc showed that a single species of these bacteria—Streptomyces—slashed the growth of the deadly fungus Metarhizium anisopliaei by two-thirds.

The fungus grows inside the bodies of insects, consuming them from within before erupting out as a white mould. But Streptomyces secretes chemicals that stop the fungus from germinating or growing.

To see if this actually helps the termites, Chouvenc added 50 termites to small mini-nests, which he built using sterilised carton material. If he added M.anisopliae, around half the termites were dead within two months. If he added Streptomyces first, the survival rate shot up to 90 percent, rivalling that of colonies that never encountered the fungus at all.

The Formosan termite is an aggressive forager, always digging new tunnels to find food. Workers regularly come across fungal spores, which they can inadvertently bring back to the nest. But since the entire nest is laced with bacteria, and saturated by their antifungal chemicals, the spores rarely germinate.

And the nest itself is important. If Chouvenc used an artificial nest made of sand, the bacteria don’t protect the termites against the fungi. It seems that the bacteria need to feed upon the carton material in order to make their fungus-beating substances.

“Our new publication is a new nail in the coffin of biological control, as another layer of protection has now been identified,” say Chouvenc. Traps, baits, and pesticides can still work, but controlling termites with diseases looks like an unworkable option.

Leafcutter ants also use Actinobacteria  to protect themselves from fungi, but they house the microbes in special structures on their bodies. The termites lack such body parts; instead, they grow their beneficial bacteria in the walls of their nests. It’s the equivalent of spraying your entire house, and every bit of public transport you take, with living antibiotics.

Reference: Chouvenc, Efstathion, Elliott & Su. 2031. Extended disease resistance emerging from the faecal nest of a subterranean termite. http://dx.doi.org/10.1098/rspb.2013.1885

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Mercenary Ants Protect Farmers With Chemical Weapons

Humans have a long history of bolstering their armies by paying mercenaries to fight on their behalf. Now, Rachelle Adams from the University of Copenhagen has found that some ants do the same.

Some ants defend their colonies with special soldiers, which are bigger and wield more formidable weapons. But others lack a proper army. Sericomyrmex ants, for example, are fungus-farmers. They bring bits of vegetation back to their nests and use these to nourish a fungus, which they then eat. They are poorly defended, and a sitting target for raiders and pirates. That’s why they rely on other ants to fight for them.

Another ant called Megalomyrmex symmetochus forms its own colonies, complete with queen and workers, inside those of Sericomyrmex colonies. This guest is present in over 80 percent of the farmers’ nests and, at first glance, it looks like a parasite. It eats the fungus that the farmers so assiduously grow, without contributing any labour of its own. Worse still, it eats some of the farmers’ larvae and clips the wings of their young queens, so they contribute to the gardening rather than flying off to start their own colonies.

But M.symmetochus doesn’t just freeload off its hosts. In some cases, it can be their salvation.

Megalomyrmex mercenary (top right) defends a fungus garden from a Gnamptogenys raider (bottom). Credit: Rachelle Adams
Megalomyrmex mercenary (top right) defends a fungus garden from a Gnamptogenys raider (bottom). Credit: Anders Illum

Sericomyrmex nests are often attacked by a third ant called Gnamptogenys hartmani—a sort of six-legged pirate. It raids the colonies of farming species, drives them out, usurps their nests and gardens, and eats any remaining larvae.

The farmers can do very little against these raiders, since they lack specialised soldiers and have mostly lost their stings. Their powerful jaws can deliver a strong bite, but at such close quarters, they risk getting stung and bitten themselves. When attacked, they’re much more likely to feign death or flee. That is, unless there’s a M.symmetochus colony living in their nests.

Back in 2011, Adams let some raiders loose upon a colony of Sericomyrmex farmers, which was already being parasitized by M.symmetochus. “To our surprise, the hosts hid and the parasites rose to the top of the garden to confront and kill the invaders,” she says.

Unlike their hosts, the parasitic ants are far from defenceless. They raise their stings and release a powerful venom directly into the air—an airborne chemical weapon that kills the raiders and befuddles any survivors. “Rather than uniting as an efficient infiltration squad they turn on each other and attack, sometimes killing their own kin,” says Adams.

M.symmetochus behaves like a colony of mercenaries. They can cause problems for the farmers during peace-time, but they provide an invaluable defence when an invading force arrives.

By pitting the three types of ants against each other, first in one-to-one battles and then in more realistic groups, Adams’ team showed that the mercenaries are much better at subduing the raiders than the farmers. It takes just two of them to overpower a single raider, while the same task requires at least eight farmers. On average, a pair of raiders can kill 70 percent of farmers in an unprotected nest, but just 10 percent of them if there are six mercenaries around.

And the raiders seem to know it. When the team gave them a choice between two nests, only one of which was defended by mercenaries, they were more likely to attack the unprotected nest. In the experiment, a wire mesh prevented the raiders from actually touching the colonies, so they were probably put off by the smell of the mercenaries’ chemical weapons. Just by living in the same colonies, the mercenaries protect the farmers by cloaking them in a defensive miasma.

The thing that clinches this incredible relationship is probably Megalomyrmex’s life cycle. They form a life-long bond with a single colony of farmers, and won’t leave to seek out a different host. They exploit their hosts but they don’t overexploit them, and if raiders attack, they mount a defence rather than abandoning the farmers to their fates.

Next, the team wants to study how this alliance varies over time and space. For example, it should be easier to find the mercenaries in the farmers’ nests in places where the raiders are more common. Similarly, a different farmer called Trachymyrmex zeteki should be relatively unbothered by raiders, since it also hosts a parasitic mercenary but in fewer than 6 percent of its nests.

For now, they very astutely compare the mercenary ants to a human disease called sickle cell anaemia. It’s an inherited genetic disease that warps the shape of red blood cells and causes a gradual building illness. The mutated gene behind the disease is common in Africa because two copies might cause sickle-cell anaemia, but one copy protects a person against malaria. Similarly, M.symmetochus is like a mild chronic disease that exerts a toll upon its host, while protecting it from an acute and more destructive infection—the Gnamptogenys raiders.

Reference: Adams, Liberti, Illum, Jones, Nasha & Boomsma. 2013. Chemically armed mercenary ants protect fungus-farming societies. PNAS http://dx.doi.org/10.1073/pnas.1311654110

Experimental manipulation of the tendon of the large jumping muscle of one hindleg

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These Microscopic Balls Protect Insects From Their Own Waste

The intricate soccer-ball structures in the image above are so tiny that you could pack few hundred of them into the width of a human hair. They’re probably the most beautiful non-stick coatings to have ever evolved.

These balls, known as brochosomes, were first discovered in the early 1950s. Scientists knew that they were found on the shells of leafhoppers—a group of colourful, sap-sucking insects that includes more than 20,000 species. They make the brochosomes within special glands in their guts, secrete them by the billions in a milky anal fluid, and spread them over their bodies using their legs. When the fluid dries, the brochosomes form a powdery coating, and the leafhoppers spread them even further using comb-like hairs on their legs.

Roman Rakitov from the Russian Academy of Sciences first saw the brochosomes when he put a leafhopper under a powerful microscope. His advisor (wrongly) told him: “Look, your leafhopper is coated with pollen!” “The published data on brochosomes were so scarce,” says Rakitov, “that pretty much everybody who later worked on them re-discovered much of the information about them independently.”

Eurymela distincta, Meehan Range, Tasmania, Australia. By JJ Harrison (jjharrison89@facebook.com)
Eurymela distincta, Meehan Range, Tasmania, Australia. By JJ Harrison (jjharrison89@facebook.com)

Rakitov became mesmerised by the brochosomes and wanted to work out what they were for—a mystery that had gone unsolved for almost 50 years. Since the 1970s, several scientists have noticed that the leafhoppers are great at repelling water, and suggested that the brochosomes might help. It was a nice idea, but Rakitov became the first to test it, with help from Stanislav Gorb from the University of Kiel.

If you put a drop of water on a table or plate, it will flatten out. The “contact angle” that it makes with the surface falls towards zero.  But a leafhopper’s wing is so good at repelling water that a drop will sit upon it as a nigh-perfect sphere. It forms a contact angle of around 170 degrees—one of the highest ever recorded for a natural surface.

The brochosomes are responsible. The droplets sit upon their tips, and are cushioned by the pockets of air between them. Without these spheres to add roughness and texture, the naked leafhopper shell has a contact angle of just 120 degrees.

Contact-angleOther natural water-repellent surfaces like lotus leaves or guillemot eggs work in the same way, but they are made that way. The leafhoppers, however, waterproof themselves by actively applying a rough surface to their shells. There are a few other similar examples: Christoph Neinhuis from Dresden University told me about some plants that coat themselves with spores for a similar reason.

But leafhoppers are land-living insects—why do they need such good waterproofing? The brochosomes might help to repel rain, or even spider silk, but the classic explanation is that they protect the leafhoppers from their own waste. After sucking the sap of trees, these insects excrete any extra sugar as a sticky liquid. This is a serious hazard. If the liquid contaminates their shells and dries, it could stick the insect to a leaf, or glue its body parts together. Some bugs deal with this problem by shooting the waste away from their bodies at high speed. Others coat their droplets with a waxy powder. Perhaps the brochosomes are the leafhoppers’ solution?

More brochosomes. Credit: Rakitov & Gorb, 2013.

Again, Rakitov and Gorb have tested this idea. They clipped the wings from some European leafhoppers (Alnetoidia alneti) and removed the brochosomes from half of them. They then placed the wings in a cage with 100 live leafhoppers, which rained a downpour of sticky waste upon them.

A week later, the duo found that 155 spots of dried waste had stuck to the wings, but only three had hardened on the ones with intact brochosomes. The naked wings had become dirtier; the brochosome-coated ones were almost totally clean. The duo had found strong evidence that the tiny spheres can indeed protect the leafhoppers from their own waste.

That’s one mystery down, but many more left to solve. How exactly do leafhoppers produce such intricate structures? Does their soccer-ball form make them more efficient at repelling water than other shapes? And how do they actually stick to the leafhoppers’ surfaces, and why do they often form long chains? They’re not glued, says Rakitov. “We don’t know but we plan to find out.”

Reference: Rakitov & Gorb. 2013. Brochosomal coats turn leafhopper (Insecta, Hemiptera, Cicadellidae) integument to superhydrophobic state. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2012.2391

Rakitov & Gorb. 2013. Brochosomes protect leafhoppers (Insecta, Hemiptera, Cicadellidae) from sticky exudates. Interface. http://dx.doi.org/10.1098/rsif.2013.0445

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Flies Use Alcohol to Protect Their Young From Body-Snatchers

If your babies are in danger of being usurped by grisly body-snatchers, there’s only one thing for it—ply them with booze as soon as possible.

Last year, Todd Schlenke from Emory University in Atlanta showed that the fruit fly Drosophila melanogaster uses alcohol as medicine. Like many insects, these flies are targeted by parasitic wasps that lay eggs in their bodies. To stop the newly hatched wasps from devouring them alive, the fly larvae consume alcohol at toxic levels. This kills many of the wasp grubs and causes crippling deformities in the survivors. But the flies, which live in a naturally boozy world fermenting fruit, have evolved to handle their drink. They suffer few side effects from their unusual medicine.

But the flies don’t just use alcohol as a treatment. Schlenke’s team has now shown that they can use it as a sort of vaccine. At the mere sight of a parasitic wasp, females will lay their eggs on alcohol-soaked food, giving the hatchlings an immediate source of anti-wasp protection.

Schlenke’s student Balint Kacsoh offered female flies two plates of food—one that was 6 percent alcohol, and another that was alcohol-free. The females chose the boozy plate 40 percent of the time if they were left alone, but 90 percent of the time if there were female Leptopilina heterotoma wasps in the cage too. And 6 percent alcohol? That’ll do, but the females opt for stronger brews if those are available. They’ll lay eggs on food that’s 12-15 percent—the highest concentrations found in nature.

The females’ choices saved the lives of their baby maggots. Normally, 90 percent of the larvae make it to adulthood. With body-snatching wasps around, that proportion falls to a measly 10 percent. But it rises back to 50 percent if the larvae were born on alcoholic food, probably because they’re better at curing their infections and less likely to become infected in the first place.

The flies do not head towards alcohol at the sight of just any wasp. They can tell the difference between females, which could impregnate their young, and males, which cannot. If they are housed with males, they don’t switch to alcoholic food. And their penchant for alcoholic nurseries only kicks in if they see wasps that infect fly larvae, and not those that infect pupae. After all, larvae will typically move away from the place where they hatch. By the time they start to pupate, their mother’s choice of hatching site will have little bearing on whether they fall prey to wasps.

That’s quite spectacular. The fly carries a mental image of a dangerous wasp that’s specific enough to tell females from males, and to ignore irrelevant species while reacting to diverse range of dangerous ones. This all depends on vision. The team found that blind mutant insects never made the switch to alcoholic sites, but mutants that lacked a normal sense of smell still did so.

Most people would stop here, but Schlenke’s team snowballed ahead and pieced together what happens in the brains of the females. These experiments showed that the insects’ attitudes to alcohol are intimately connected with their vision and memory, with the same molecules connecting these seemingly disparate mental traits.

For example, when flies see a dangerous wasp, they experience falling levels of a protein called NPF, in a part of their brains involved in recognising patterns. But a loss of NPF also boosts a fly’s tolerance and preference for alcohol. This molecule links the sight of a threat to a defensive behaviour. If the team artificially loaded the flies’ brains with more NPF, they never made the switch to alcoholic nurseries. If the team depleted NPF entirely, the flies preferred to lay on alcoholic patches even if there were no wasps around.

Another protein called Adf1, which is involved in long-term memory, controls the flies’ ability to remember their wasp foes days after they first see them. Without Adf1, the flies will still make the switch to alcohol when they see wasps—it’s an instinct they’re born with—but their memories fade over time. But Adf1 also controls the gene that’s responsible for breaking down alcohol. It allows the insect to both remember dangers that they can beat through booze, and to tolerate that booze in the first place.

The tolerance is important. High levels of alcohol would kill off a lot of developing insects, but D.melanogaster can resist its negative effects enough to reap its benefits. When Schlenke’s team tested six closely related species of Drosophila flies, they found that only three can tolerate high alcohol concentrations. And only this trio uses alcohol to vaccinate their young against wasps. Throughout the history of these insects, the ability to resist alcohol and to use it as medicine have evolved hand-in-hand many times over.

Reference: Kacsoh, Lynch, Mortimer & Schlenke. 2013. Fruit Flies Medicate Offspring After Seeing Parasites. Science http://dx.doi.org/10.1126/science.1229625

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Chinese Mantis Guts Its Toxic Caterpillar Prey

Whether we’re eating prawns or fish, chicken or sheep, we tend to remove the guts of animals before eating their meat. There’s another predator that shares our culinary practice: the Chinese mantis.

The mantis, a finger-sized animal found in the eastern US, is one of the few hunters that successfully eats the toxic caterpillars of the monarch butterfly. These larvae are poisonous enough to ward off ants and birds, but the mantis has a special trick for dealing with them—it guts them. It removes their intestines before eating the rest of their bodies in safety.

Monarch caterpillars take in toxic chemicals called cardenolides from the milkweed plants they eat. Rather than succumbing to these poisons, the caterpillars store and repurpose them for their own defence. And they advertise their chemical payload with warning colours—bright stripes of yellow, black and white, running down their flanks.

Monarch caterpillar feeds on swamp milkweed, by Derek Ramsey

Some predators can get around this. Birds like orioles and grosbeaks will sometimes eat the innards of the adult butterflies, avoiding the outer layers that are richest in toxins. Ants and ladybirds eat monarch eggs, or very young hatchlings that haven’t had a chance to build up their cardenolide stockpile. But the older caterpillars—the ones that have had a lifetime of storing cardenolides—are safer. Assassin bugs and hungry wasps will tackle them, but not much else.

That is, except the Chinese mantis (Tenodera sinensis). While doing unrelated experiments in a local field, Jamie Rafter from the University of Rhode Island noticed that mantises would gut monarch caterpillars before eating them. It’s not a delicate process. After grabbing a victim, the mantis starts nibbling at it, chews open a hole, and lets the guts fall away. Around 40 percent of the caterpillar goes to waste.

Chinese mantis guts a monarch caterpillar. Credit: Alex Allaux.

This isn’t typical behaviour for the mantis. Rafter showed that when they captured the non-toxic caterpillars of the greater wax moth or the European corn borer moth, they ate everything, guts and all. They only left 14 percent of these meals, and even then, only because bits of blood would messily dribble away when they ate.

The obvious explanation is that the mantis is trying to avoid the poisonous bits of the caterpillar’s body, but things aren’t that simple. Rafter found that the caterpillar’s guts have exactly the same level of cardenolides as the rest of its body, so the mantis isn’t actually avoiding the poisonous tissues. But then, why gut the monarchs, while eating the palatable moths whole? What’s going on?

Chinese mantis, by Luc Viatour / www.Lucnix.be

There are two possibilities. The mantis might only be vulnerable to some cardenolides and not others. The caterpillar’s body has around three times as many types of these chemicals as its guts, but at lower concentrations. This suggests it’s processing or breaking down the cardenolides that it gets from the milkweed before storing them in its other tissues. Maybe the mantis can tolerate these processed forms, but is trying to avoid the originals in the guts.

Alternatively, the mantis might just find the guts distasteful, or not worth the effort. These organs tend are usually filled with chewed-up plant matter and contain 58 percent less nitrogen than other tissues. So perhaps the mantis is just feasting on the richest tissues and discarding the nutrient-poor ones. And by happy coincidence, that reduces the total amount of poison that it consumes.

Rafter noticed one pattern that supports this second idea: she watched their mantises eating 21 caterpillars, and they only gutted 18 of them. The other three were all harbouring parasites! Two of them contained the larvae of a tachinid fly, which were slowly devouring them from the inside out. The third was heavily infested by a fungus. As a result of these body-snatchers, the caterpillars had barely eaten any milkweed. When the mantises broke into their bodies, they found no plant matter in their guts. So, they just ate the lot.

Either or both explanations might be correct. Regardless, Rafter’s discovery might explain why the Chinese mantis has done so well in the eastern US, since its introduction from China. Its culinary antics might have given it access to a source of food that other predators left alone.

Hat tip to Jeramia Ory for telling me about this paper

Reference: Rafter, Agrawal & Preisser. 2013. Chinese mantids gut toxic monarch caterpillars: avoidance of prey defence? Ecological Entomology http://dx.doi.org/10.1111/j.1365-2311.2012.01408.x