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Hot Wild Dragons Set Sex Through Temperature Not Genes

At room temperature, a bearded dragon’s sex depends on two chromosomes. If they have two Z chromosomes, these lizards develop as males. Those with a Z and a W become females. But raise the thermostat up a few notches, and something different happens. If a clutch of dragon eggs are incubated at 34 degrees Celsius, their bodies ignore the usual instructions from their sex chromosomes. Even if half of them are genetically male (ZZ), all of them will hatch as females.

Clare Holleley from the University of Canberra found some of these “sex-reversed” ZZ females in the wild, and bred them with the usual ZZ males. All the offspring from these crosses should have two Z chromosomes, so you might guess that all of them would turn out male.

You’d be wrong. In fact, their chromosomes didn’t matter at all. Instead, their sex depended entirely on the temperature at which they are incubated. Warm clutches produced females; cooler ones produced males. In a single generation, these lizards had evolved a radically different way of determining their sex—one in which their genes completely cede control to the heat of the world.

No one knew that these flips could happen so quickly, but they must surely happen. After all, animals are incredibly varied in their ways of determining sex. In humans, other mammals, and some insects, females usually have two of the same chromosomes (XX), while males tend to have different ones (XY). In birds and some reptiles, including the bearded dragons, the female is the one with different chromosomes (ZW), while the male has identical ones (ZZ). Other reptiles ignore chromosomes altogether and rely on temperature. Turtle eggs are more likely to hatch as males at cooler temperatures, and as females in warmer ones. In crocodiles, this pattern is reversed.

Scientists used to think that these two strategies—genetic sex determination (GSD) or temperature sex determination (TSD)—were mutually exclusive. Animals could use one mode or the other, but never both. That idea was put to rest in 2007, when a team led by Jennifer Marshall Graves experimentally switched bearded dragons from GSD to TSD by raising them at high temperatures. “But we didn’t know if this was something that happens naturally or if the sex-reversed females are fertile,” says Holleley.

Her team solved both mysteries by capturing 131 wild bearded dragons from eastern Australia, and identifying 11 ZZ females among them. They do exist in the wild, they can mate with males, and they can themselves be mothers of dragons. When Holleley incubated their eggs at 30C or under, they all hatched as males. At 36C, they were all female. At intermediate temperatures, she got a mix. In just one lab-bred generation, the W chromosome had completely disappeared, and the dragons had switched completely to TSD.

“It is often thought that once a species veers down the path of chromosomal sex determination, there’s no going back,” explains Melissa Wilson-Sayres from Arizona State University. That’s because the two chromosomes develop sex-specific versions of important genes, and one of them—such as the Y in humans—loses so many genes that it becomes small and stunted. This supposedly creates an inescapable rut. “But this paper suggests that not only is it possible for a population jump out of the chromosomal sex determination rut, but that it actually occurs in the wild,” says Wilson-Sayres. “It’s  fantastic because it shows how much variation can exist right below our noses.”

“This makes me think of the statements I’ve seen about trans individuals not being “truly male” or “truly female”, because of their (presumed) set of sex chromosomes,” she adds. “This research tells us that even with chromosomal sex determination, exceptions occur all the time. In the bearded dragon, the exception may even be a benefit, as ZZ females lay more eggs that ZW females. This tells us that we’re thinking much too simply if we say with confidence that only XX is female and XY is male.”

Holleley doesn’t understand exactly how the lizards flip from GSD to TSD because the genetics of sex determination are still a mystery in reptiles. In mammals, the Y chromosome has a gene called SRY that acts as a master sex-determining gene—if individuals have it, they’re usually male, and if not, they’re usually female. In birds, the equivalent gene is called DMRT1. But the reptilian counterpart is still a mystery. “We’ll have to figure out what the gene is, and how it’s regulated by temperature,” says Holleley.

The consequences of her discovery are also unclear. In the wild, the switch from GSD to TSD would be more gradual than what Holleley saw in the lab. Still, “there are many reasons why we think that if you get an extreme climatic event, and sex reversal starts happening, it’ll snowball,” she says.

First, as Wilson-Sayres noted, the sex-reversed ZZ females produce twice as many eggs as the usual ZW ones. Second, the offspring of the ZZ females are more likely to reverse sexes themselves—that is, males will hatch as females at lower temperatures than their mothers did. The third reason is more complicated. In a warmer world, ZW individuals all still become females but so do some ZZ individuals. This means that males become rarer. It also means that mothers become more successful if they have more sons, since those sons face less competition and can find mates more easily. So evolution should push mothers to have more ZZ male offspring.

These three effects all spell trouble for the W chromosome and—provided the climate stays very hot—should eventually eradicate it. In the end, all the dragons should be ZZ and all of them should rely on TSD.

What happens next? It all depends on the threshold temperature at which all-male broods give way to all-female ones. If the threshold is a sensible one, and keeps in step with environmental changes, then the dragons will be fine. They’ll just stick with TSD as many other reptiles do. But if the threshold is too low, and the world keeps getting hotter, then trouble looms. “They could potentially get more and more female-biased, and if you end up with all females, you’ll go extinct,” says Holleley.

Reptiles have obviously lived through many extreme fluctuations in climate, and they’re still around. Indeed, Holleley’s discovery does suggest that “reptiles may have greater capacity to cope and compensate for climate change than previously appreciated.” Then again, the current rate of warming is far steeper than what they would have encountered in the past. It’s a brave new world; how reptiles will fare in it is anyone’s guess.

Reference: Holleley, O’Meally, Sarre, Graves, Ezaz, Matsubara, Azad, Zhang & Georges. 2015. Sex reversal triggers the rapid transition from genetic to temperature-dependent sex. Nature http://dx.doi.org:10.1038/nature14574

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Water Automatically Leaps Off Lizard’s Self-Cleaning Skin

It started when Jolanta Watson put a frozen box-patterned gecko on a glass slide. The lizard’s skin is adorned with beautiful auburn and tan blotches, and Watson wanted to study it under a microscope. But as she reached for a scalpel, she noticed that tiny water droplets had formed on the slide. The longer she looked, the more droplets there were. Where were they coming from?

The microscope revealed the answer. Through its lens, Watson saw that droplets would condense on the gecko’s skin, roll into each other, and jump off under their own power. That’s why the slide was wet. The box-patterned gecko’s skin can actively repel water even if it’s dead and immobile. And when it’s alive, it can use this phenomenon, which Watson calls “geckovescence”, to clean itself with no effort.

High-speed footage slowed down 13x shows dewdrops being propelled off a gecko’s skin, a phenomenon that may help keep bacteria and fungi at bay. Credit: Dr. Gregory Watson

There are some 1,500 species of geckos, which are best known for their sticky feet. Their toes are covered in thousands of microscopic hairs that allow them to cling to seemingly flat surfaces—including the walls of Watson’s Australian home. As she and her husband Gregory watched these lizards, they realised that scientists had largely ignored the rest of the gecko’s body. Their toes were cool, but what about the rest of their skin? In particular, how does it deal with water?

The box-patterned gecko lives in the Australian desert, where rainfall is rare and water is scarce. Still, chilly nights and humid mornings can produce a lot of dew, some of which condenses on the gecko’s skin. That’s a problem: water-logged skin is a breeding ground for microbes and fungi, which could potentially cause diseases.

Fortunately, as the Watsons found, the gecko can automatically dry itself. When they looked at the lizard’s skin under the microscope, they saw that its scales are like rounded domes. Each of these is covered in miniscule hairs, just a few millionths of a metre long, about the size of a small bacterium. They’re densely packed too: thousands of them would fit in the cross-section of a single human hair.

Close-ups of the gecko's skin, showing the scales (left) and hairs (right). Credit: Watson et al, 2015. Interface.
Close-ups of the gecko’s skin, showing the scales (left) and hairs (right). Credit: Watson et al, 2015. Interface.

Many natural structures, including springtails, leafhoppers, lotus leaves, and guillemot eggs, use similar microscopic textures to waterproof themselves. The principles are always the same: there are raised sections, like the gecko’s hairs, that trap pockets of air and stop water from seeping into the spaces between them. When droplets form, they sit on top of the raised bits as nigh-perfect spheres, rather than flattening out as they would do on a tabletop or on your skin.

The Watsons saw exactly this when they cooled gecko skin to the point when dew started to condense. Spherical droplets appeared, and grew. When they touched each other, they merged. And when they merged, they would occasionally fly off. Why? Because when two droplets unite, their volume stays the same but their combined surface area—and thus, their surface energy—goes down. They convert some of that surface energy into kinetic energy, and if the trade-off is substantial enough, they can launch themselves into the air.

All of this happens without help from any external forces, but external forces can help. In fog, water droplets in the air collide with those on the gecko’s skin, increasing the odds that they will jump off. Here’s a series of images showing one such jump. Wind helps too; it blows droplets into each other, and carries the airborne drops away from the lizard.

All of this makes for effortless auto-cleaning skin. As the droplets form and merge, they carry dirt, spores, and other foreign material with them. When they leap away, they remove those contaminants from the gecko. Other animals probably use a similar trick, including a type of cicada that the Watsons studied a few years ago.

“There are a number of potential practical applications,” says Watson. “Keeping surfaces clean and free from dew or other small droplets may reduce the growth of bacteria and fungi. We are currently investigating a number of properties on replicated gecko skin architecture.”

Reference: Watson, Schwarzkopf, Cribb, Myhra, Gellender & Watson. 2015. Removal mechanisms of dew via

self-propulsion off the gecko skin. Interface http://dx.doi.org/10.1098/rsif.2014.1396

More on water-repellent nature:

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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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What Disco Fog Taught Us About Iguana Lungs

Froggys Fog Swamp Juice is billed as “the world’s greatest fog”. According to the manufacturers, it produces a long-lasting artificial mist that has been used in haunted houses, nightclubs, skating rinks, theme parks, and even police and fire departments.

Colleen Farmer used it to study how an iguana breathes.

She threaded an endoscope—a tube with a light and a camera at the end—into the lizard’s nose, while allowing it to inhale the artificial smoke from a fog machine. The smoke, though harmless, contained small particles, and the camera could detect these they worked their way into the iguana’s lungs.

And to Farmer’s surprise, no matter whether the lizard breathed in or out, the smoke particles only moved in one direction.

To understand why that’s weird, consider how your own lungs work. When you inhale, you suck in fresh air so that oxygen can pass into your blood. When you exhale, you expel the stale air back in the same direction. Air moves through your lungs like the tides: in and out, in and out.

A bird’s lungs work very differently, and this image explains it well. Think of them as a radiator with air sacs at either end. As the bird inhales, it draws air into the rear air sacs. It exhales, and this air is forced through the radiator-like lungs, where oxygen passes into the blood. The next inhalation drives the stale air into the front air sacs, and the next exhalation drives it out of the bird. It takes two breaths for any batch of air to circulate through the bird’s body. More importantly, whether the bird is inhaling or exhaling, air only ever flows one way through its lungs,.

All of this was discovered in the 1970s. During the following decades, biologists thought that this set-up was unique to birds. It presumably helped them to extract as much oxygen as possible from their breaths, to fuel their high-octane, warm-blooded, fast-flying lifestyle.

“Then in 2010,” as Matt Wedel wrote on his blog, “Colleen Farmer and Kent Sanders of the University of Utah blew our collective minds by demonstrating that alligators have unidirectional flow-through lungs, too.” Alligators aren’t warm-blooded and, last I checked, they can’t fly. That was a strong hint that one-way lungs evolved for different reasons than the ones people had assumed. The discovery also meant that these lungs probably evolved in the common ancestor of the archosaurs—the group of reptiles that includes birds, crocodiles, all the extinct dinosaurs, and pterosaurs.

But Farmer wasn’t finished. Last year, her team showed that a lizard—the savannah monitoralso has one-way lungs. Again, everything changed. Monitors are fast hunters that can run down mammalian prey; perhaps that’s because of their lungs. They aren’t archosaurs either. They belong to a different group of reptiles called the lepidosaurs, which includes all snakes and lizards. So, either the archosaurs and lepidosaurs evolved one-way lungs independently, or the set-up actually existed in their common ancestor and is much older than anyone thought.

That brings us back to the iguana and the Froggys Fog Swamp Juice. After the monitor lizard discovery, Farmer wanted to study another lepidosaur, and one that doesn’t have the same energetic lifestyle. The common iguanas were perfect. They can actually sprint faster than monitors, but a weird anatomical quirk cuts the blood flow to their legs when they try. They soon have to stop because they build up too much lactic acid. They have no stamina.

Their lungs are also incredibly simple. There are no complicated sacs or radiators like in birds, alligators or monitors. Each lung consists of just two chambers. Air enters and leaves each of these through a single hole. Aside from the holes, the chambers are completely airtight, with no bridges between that. Farmer’s team, including Robert Cieri and Emma Schachner, proved this by pumping air into the front chamber of a surgically removed lung and sealing the hole with latex. They then put the lung underwater and tried to squeeze the air into the back chamber. No dice.

“No one in their right mind would think that there’s unidirectional flow in these lungs,” says Farmer. “If you were an engineer and tried to come up with a system with one-way flow, this is not what you’d think of.” And yet, one-way flow is exactly what her team found.

They studied the iguanas using several techniques. They watched synthetic smoke travel around the lizards’ lungs as they breathed normally. They implanted airflow meters in the animals. They pumped water full of pollen grains through surgically removed lungs, and watched the flow of the particles. And their colleague Brent Craven created a computer model that simulated the flow of air through virtual iguana lungs.

All of these techniques gave the same results. Air enters each chamber at high speed and jets straight to the back. It then branches off to the side, hugging the chamber walls as it moves back to the front. Eventually, it leaves via the same hole it entered. This means that the central part of each chamber is tidal—air moves in and out as the animal breathes. But along the walls of the chamber, air only ever moves in a single back-to-front direction. And the walls are where the blood vessels are—they’re the places where oxygen moves from the lungs into the bloodstream.

So, even though the iguana’s lung is incredibly simple, it achieves the same results as a hawk, crocodile, or Komodo dragon. At every part of its breathing cycle, its blood receives a steady stream of fresh oxygen.

“Nobody’s going to argue that this enables the iguana to be an endurance athlete,” says Farmer. Instead, she speculates that one-way lungs evolved because they allowed their owners to hold their breaths for long periods of time. That ability would have been especially useful to animals that hide from predators by blending into the background. Any movement would give them away, and breathing creates movement. If the animal holds its breath, its beating heart can still push air through its one-way lungs, allowing it to extract as much oxygen as possible.

These early one-way lungs probably worked like those of the iguana. Their owners could then have expanded on this simple structure by adding dividing walls in the centre of the lung where air flows tidally. Now, instead of a simple chamber, you have a series of connected sacs, as in birds and alligators. And these more efficient lungs allowed some groups like the birds and monitors to explore a more active existence.

“We’d expect to see better-developed unidirectional flow in species that rely heavily on crypsis,” she says, “whereas animals that are poisonous wouldn’t care.” She plans on testing this idea by studying the lungs of more reptiles. Chameleons, for example, rely on camouflage and have huge lungs. Do these organs have one-way flow? Farmer wants to find out.

But first, she has her eye set on a different target—the tuatara. This New Zealand resident looks like a lizard, but isn’t. Instead, it’s the only survivor of a largely extinct group of lepidosaurs, one that’s separate from the snakes and lizards. If it also has a one-way lung, that would really strengthen the case that such a structure was present in the ancestor of all living reptiles.

Reference: Cieria, Craven, Schachner & Farmer. 2014. New insight into the evolution of the vertebrate respiratory system and the discovery of unidirectional airflow in iguana lungs. PNAS http://dx.doi.org/10.1073/pnas.1405088111

Australia’s Giant, Venomous Lizard Gets Downsized

From time to  time, I’ve been accused of being a fossil killjoy. I pulverize childhood dreams like Diatryma crushed seeds (and not little horses). I’m not sure how true that is. I’ve yet to quantify how much of my writing destroys dreams versus geeking out over new discoveries. But today I have to own up to being a downer. In case you hadn’t heard, Australia’s extinct, giant monitor lizard wasn’t as monstrous as traditionally thought.

In 1858, when paleontology was still a young science, the anatomist Richard Owen read a paper before London’s Royal Society on some astonishingly large lizard bones that had been collected from Ice Age deposits in Australia. Specifically, Owen described a trio of vertebrae that measured three inches long and two inches high. These were far bigger than any lizard then known – the Komodo dragon wouldn’t be recognized by science for another 54 years – and, through a bit of rough anatomical math, Owen expected that this huge “land lizard” would have reached 20 feet from snout to tail.

Owen dubbed this gargantuan lizard Megalania prisca, and he had a little fun imagining that such a lizard might still clamber through the bush. “Whether among the vast and unexplored wildernesses of the Australian continent any living representative of the more truly gigantic Megalania still lingers, may be a question worth the attention of travellers,” Owen told his audience, although the hard-nosed scientist did concede that the lizard was most likely extinct. Either way, Owen concluded, the huge saurian “must have been carnivorous, and, by its bulk and strength, very formidable.”

While the fossil trail took some confusing turns as Owen and succeeding generations of researchers puzzled over fragments of Australia’s prehistoric reptiles, by 1975 paleontologists had settled the image of Megalania as a truly gigantic monitor lizard that ripped into Volkswagon Bug-sized wombats between 4 million and 30,000 years ago. This was late enough that the first people to arrive on Australia may have encountered the lizard, and Megalania was definitely not a squamate to be trifled with. The skeletal reconstructions put up at the Museum of Victoria and other institutions looked like Komodo dragons pumped up to almost 20 feet long. Despite all that had changed since 1859, Owen was right about one salient point – Megalania was one formidable lizard.

A reconstruction of Varanus priscus at the Melbourne Museum. Photo by Cas Liber.
A reconstruction of Varanus priscus at the Melbourne Museum. Photo by Cas Liber.

But Megalania ain’t what it used to be. For one thing, the lizard’s bones are so similar to those of other monitor species – belonging to the genus Varanus – that paleontologists have taken to calling it Varanus priscus. And while it seems likely that the big lizard was venomous, recent size estimates have shrunk this “dragon in the dust.”

Let’s have a look at the traditional baseline first. In 2004, working with the relationship between vertebrae size and body length, paleontologist Ralph Molnar proposed that mature Varanus priscus could have been between 23 and 26 feet long, depending on the anatomy of the tail. But other researchers think such sizes are major overestimates. In a 2002 study that critiqued “the myth of reptilian domination” in prehistoric Australia, anatomist Stephen Wroe reanalyzed old body size data and calculated that the lizard probably averaged about 11 feet in total length and, citing earlier estimates from Molnar, wouldn’t have grown much longer than 15 feet.

Size estimates in a 2012 paper by paleontologist Jack Conrad and colleagues came out in between the extremes. While describing a new, large Varanus species that once lived in Greece, the researchers also took a look back at Australia’s ever-contentious lizard. Without the tail, the Varanus priscus specimen in their study had an estimated body length of almost seven feet, meaning that this individuals total length was almost certainly longer than the 11 foot average Wroe suggested. Especially large specimens, Conrad and coauthors noted, could have had bodies almost 10 feet long with the tails trailing behind, although these animals still would have been smaller than the monstrous lizards paleontologists used to reconstruct.

The entire back-and-forth over the lizard’s size is only a small part of the story, though. Since Owen’s days, paleontologists have viewed Australia as a place where the Mesozoic clung to life – a land full of marsupials where reptiles tenaciously clung to predatory dominance. That view is changing into a more complex Pleistocene vision, with the island continent’s huge monitor playing the role of ambush predator among a variety of carnivores – mammalian and reptilian – that stalked the Australian Ice Age. At 11 feet or 20, it’d be difficult not to be impressed by a lizard of such scale, but the rapacious role of the long-lost monitor has yet to be pulled from the scraps the reptile left behind.

Related Stories:

Of Dragons and Diminutive Elephants
The Demise of the Komodo Kings

[Top image by Peter Trusler, via Flickr.]


Conrad, J., Balcarcel, A., Mehling, C. 2012. Earliest example of a giant monitor lizard (Varanus, Varanidae, Squamata). PLoS ONE. 7, 8: e41767. doi:10.1371/journal.pone.0041767

Molnar, R. 2004. Dragons in the Dust. Bloomington: Indiana University Press.

Owen, R. 1859. Description of some remains of a gigantic land-lizard (Megalania prisca, Owen) from Australia. Philosophical Transactions of the Royal Society of London 149: 43–48.

Wroe, S. 2002. A review of terrestrial mammalian and reptilian carnivore ecology in Australian fossil faunas, and factors influencing their diversity: the myth of reptilian domination and its broader ramifications. Australian Journal of Zoology. 50: 1-24

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Chameleons Convey Different Info With Different Body Parts

If you describe someone as a chameleon, you probably mean that they’re great at blending in, at changing their behaviour to suit different social situations. You probably don’t mean that they make their heads really bright when they’re about to get in a fight. The latter, however, would be more fitting.

Chameleons are famed for their ability to change colour, and people usually assume that this helps them to camouflage themselves from predators or prey. But in 2008, Devi Stuart-Fox and Adnan Moussalli showed that chameleons probably evolved their dynamic palettes to be social rather than secretive, to stand out rather than blend in.

The duo studied 21 species and sub-species of South African dwarf chameleons and found that those that undergo the most dramatic colour changes show stronger contrasts between different body parts, and stand out more strongly against their normal environments. It was communication not disguise that drove their capacity for colour change.

But what are they communicating? It’s possible that their messages are very sophisticated because they can change colour very quickly, and control the hues of different body parts independently. Their bodies don’t just flip between two settings. They’re dynamic living displays. And Russell Ligon and Kevin McGraw from Arizona State University have now shown that chameleons can convey different information by changing the colours of different body parts.

The duo set up duels between male veiled chameleons—a large species that grows up to two feet long, and has a reputation for being aggressive. When males meet each other, they react aggressively. They hiss, rock and curl their tails. They turn sideways and change the shape of their bodies from a narrow tube into a flat panel, filled with bright stripes and fleckles of green, turquoise, orange, yellow, lilac and charcoal.

“The changes essentially, turn the chameleon’s entire body into a billboard advertisement,” says Ligon. “The situation can escalate rather quickly. If neither chameleon backs down, they fight with full-body lunges and bites.” This usually lasts for just a few seconds, before one combatant realises he’s outmatched and backs down. Ligon and McGraw only had to intervene in one of their staged bouts, when a smaller rival pushed his luck so far that his opponent drew blood.

As the lizards squared off, the duo photographed them every four seconds, and measured the brightness and colours of 28 body parts. They also converted their photos according to the technical specifications of chameleon eyes, to see the individuals as other chameleons would see them.

Veiled-chameleon2They found that the brightness of the chameleons’ stripes predicted how likely they were to approach their rivals. This factor alone accounted for 71 percent of the variation in their motivation. Meanwhile, the brightness of their heads predicted their odds of actually winning their fights, and accounted for 83 percent of the variation in their fighting ability. (To a lesser extent, the speed of their colour change also says something about their combat skills.)

So, if you were a veiled chameleon facing off against a rival, pay attention to their stripe. If it becomes much brighter, they’re fixing for a fight. If their head becomes really bright, and does so quickly, they’re one tough lizard, and they’ve got a good chance of winning. And ignore the actual colours—making big jumps from one hue to another doesn’t tell you very much.

Stuart-Fox, who was not involved in the study, praised it for being the first to look at colour change as it would be perceived by actual chameleons—something that no one has done before when assessing contests.“It shows for the first time that the speed of colour change can affect contest dynamics – a discovery only possible because of the sophisticated way they quantified colour change,” he says.

Why do different body parts convey different information? “If I had to guess, I would say that these links exist because selection favoured the display of signals which could be accentuated at different, appropriate stages of an aggressive interaction,” says Ligon.

Chameleons are slow-moving, and their fights progress through a series of gradual stages. When they’re threatening each other, they face side-on, so it makes sense for their stripes to communicate their motivation for a fight. When they actually come to blows, they face head-on; again, it makes sense that their heads should signal their prowess in combat.

So far, Ligon and McGraw have found some intriguing correlations. Next, they want to start doing experiments, by controlling the changing colours of a chameleon model or robot to see how a rival reacts. Ligon also wants to know how the males use their colour-changing abilities during courtship, rather than just combat. “There’s a lot left to be done,” he says.

Reference: Ligon & McGraw. 2013. Chameleons communicate with complex colour changes during contests: different body regions convey different information. Biology Letters http://dx.doi.org/10.1098/rsbl.2013.0892

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Lookalike Lizards and the Predictability of Evolution

When Luke Mahler started his PhD, he had one stipulation. “I wanted to work on anything but anoles,” he says. Anoles are small, colour-changing lizards that are abundant in the Caribbean, and especially in the Greater Antilles. Hundreds of species live on these six islands, and most evolutionary biologists would regard them as dream subjects.  That’s because the anoles are an extraordinary example of convergent evolution—where different living things independently acquire the same adaptations to the same challenges.

For example, each island has an anole that lives among twigs. They have prehensile tails, short legs, a lichen-like pattern on their back, and slow, creeping movements. But these aren’t the same lizard. They’re not even close relatives. Those on Cuba evolved their camouflaged colours, twig-like bodies and erratic movements independently from those on Puerto Rico, Jamaica or Hispaniola. But because they’ve adapted to the same ecological opportunities, these lizards have ended up looking so similar that even experienced biologists would have trouble telling them apart.

The same applies to the islands’ grass-dwelling anoles, or their trunk-hugging anoles. “If you weren’t an anole biologist and someone came down, blindfolded you and put you on a different island, you’d think: Oh yeah, those are the same lizards,” says Mahler.


All images by D.L. Mahler, except for A.cuvieri by J.Losos and A.alumina by M.Landestoy
All images by D.L. Mahler, except for A.cuvieri by J.Losos, A.cybotes by Bryan Falk and A.alumina by M.Landestoy

Jonathan Losos has made a career from studying the Caribbean anoles—everything from their behaviour, to their genes, to their running abilities. So, when Mahler joined his lab as a student, he wanted to pave his own way. “I thought it was all played out,” he remembers. “It’s all been done. And the Caribbean’s not badass enough, and I want to go to Africa.”

But the anoles sucked him in. While Losos had found many impressive examples of convergence between various anole species, he hadn’t yet looked at all of them. If you measured every anole in the Antilles, exactly how convergent would they be? “Jonathan said: Why don’t you do that?” says Mahler. “I thought: Surely someone has done that. He said: No. I said: You’re kidding me.”

This matters because evolutionary biologists have long argued about how repeatable evolution is. “There’s always been a sense that the answer is not very much,” says Mahler. “Evolution is highly idiosyncratic. There are so many inputs that can influence the course of evolution at any moment that you’re unlikely to see repeated outcomes.”

Stephen Jay Gould, for example, famously imagined that if we could replay life’s tape from some point in the past, evolution would head down very different paths. Others challenged that view, and pointed at the many examples of convergent evolution. If living things repeatedly end up in the same destination, surely this implies that they only ever had a finite number of routes for getting there? By that logic, if you replay life’s tape, you’d get more or less the same result.

This is where the anoles come in. They’ve effectively carried out Gould’s thought experiment across the Antilles. These islands have much the same climate and very similar plants. If you put the same lizards on all of these islands and wait for them to evolve and diversify, do they produce the same forms or very different ones?

To find out, Mahler measured 100 of the 119 anole species in the Greater Antilles, including the length of their bodies, tails and legs, and the number of sticky pads on their toes. All of these are “battle-tested” traits that affect how well the anoles survive in the wild. And when he plotted all the data on a single graph, the results were very clear: most of the anoles were more similar to those on other islands that you’d expect by chance. Across the board, these lizards were converging on similar forms.

Together with Travis Ingram for Harvard University, Mahler developed several mathematical models to try and describe the pattern of anole evolution. The one that most closely matched the actual data relied on the concept of an “adaptive landscape”—a metaphorical terrain of peaks that represent opportunities for species to evolve towards.

That’s what the anoles seem to have done. Despite living on different islands, the anoles faced the same adaptive landscape with the same niches to exploit. As they diversified, they ended up converging on the same peaks. “There’s a substantial and undeniable element of repeatability,” says Mahler.

Walter Salzburger from the University of Basel praises the study, and says that Mahler’s team have effectively used statistical methods to prove what others had suspected based on their observations.  “The paper is extremely well done and one of the best examples we have that evolution does repeat itself, at least when beginning with similar ancestral material,” adds Dolph Schluter from the University of British Columbia. “Striking cases of convergence, not just of individual species but diverse sets of species like the anoles, tell us that evolution is much more predictable than we thought.”

Of course, this doesn’t mean that evolution is wholly deterministic. There are still surprises, especially on the larger islands which house a wider range of habitats. For example, the Hispaniolan hopping anole (Anolis barbouri) is unique—it’s a leaf litter specialist, and the only anole that lives entirely on the ground.

Still, the anoles show that evolution can be more deterministic and repetitive than what Gould had envisaged. We don’t want to overemphasise the similarities between these islands but there’s certainly the same themes cropping up again and again,” says Mahler. Replay the tape of life, and you’ll be greeted with familiarity as well as novelty.

Other diverse groups of animals provide more evidence for this. Salzburger, for example, studies the cichlid fish of Africa’s Lake Tanganyika and Lake Malawi. These fish have also diversified to a spectacular degree. Some graze on algae, others chase down small prey, and others pick the scales of larger fish. Cichlids in the two lakes have independently converged upon the same lifestyles, as have some cichlids within each lake.

“I know a lot of folks who are very interested in using our landscape approach to look at these cichlids,” says Mahler. There are many more species of these fish than there are anoles, and they are harder to measure, but Mahler adds, “The cichlid community is just on fire and I think they’ll probably get some answers pretty soon.”

Reference: Mahler, Ingram, Revell & Losos. 2013. Exceptional Convergence on the Macroevolutionary Landscape in Island Lizard Radiations. Science http://dx.doi.org/10.1126/science.1232392


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The Myth of the Komodo Dragon’s Dirty Mouth

In 1969, an American biologist named Walter Auffenberg moved to the Indonesia island of Komodo to study its most famous resident—the Komodo dragon. This huge lizard—the largest in the world—grows to lengths of 3 metres, and can take down large prey like deer and water buffalo. Auffenberg watched the dragons for a year and eventually published a book on their behaviour in 1981. It won him an award. It also enshrined a myth that took almost three decades to refute, and is still prevalent today.

Auffenberg noticed that when large animals like water buffalo were injured by the dragons, they would soon develop fatal infections. Based on this observation, and no actual evidence, he suggested that the dragons use bacteria as a form of venom. When they bite prey, they flood the wounds with the microbes in their mouths, which debilitate and kill the victim.

This explanation is found in textbooks, wildlife documentaries, zoo placards, and more. It’s also wrong. “It’s an enchanting fairy tale, which has been taken as gospel,” says Bryan Fry from the University of Queensland.

In 2009, Fry discovered the true culprit behind the dragon’s lethal bite, by putting one of them in a medical scanner. The dragon has venom glands, which are loaded with toxins that lower blood pressure, cause massive bleeding, prevent clotting and induce shock. Rather than using bacteria as venom, the dragons use, well, venom as venom. (The full set of experiments is breathtaking in their scope—read about them here.)

Based on a thorough analysis of the dragon’s skull, Fry thinks that they kill with a grip, rip and drip tactic. They bite down with serrated teeth and pull back with powerful neck muscles. The result: huge gaping wounds. The venom then quickens the loss of blood and sends the prey into shock.

That doesn’t discount the possibility that the dragons might also rely on oral microbes. To study these microbes, Fry contacted Ellie Goldstein from the UCLA School of Medicine—an expert on microbes an animal bites. Goldstein has advised people around the world on treating unusual bite wounds, including at least one from a Komodo dragon. “The bacteria-as-venom model seemed to be based on faulty and dated studies,” he says. “There was no really good data on the topic.”

Goldstein tried calling several zoos with captive dragons. “Many would not respond and sometimes actively tried to deter our research for reasons unclear to me,” he says. “The detractors [said] this study had already been done and no new info would result,” adds Kerin Tyrrell, who is part of Goldstein’s team. Fortunately, three zoos in Los Angeles, Honolulu and Houston were more cooperative, and the team managed to swab the mouths of 10 adults and 6 hatchlings.

They found… nothing special. All the microbes they found were common in the skin and guts of their recent meals. There were no virulent species at all, and certainly nothing capable of causing a quick, fatal infection. And the species that were there weren’t particularly abundant. “The levels of bacteria in the mouth are lower than you’d get for a captive mammalian carnivore, such as a lion or Tasmanian devil,” says Fry. “Komodos are actually remarkably clean animals. This is another nail in the coffin to the idea of them using bacteria as a weapon.”

The Komodo dragon: surprisingly clean. Photo by Bryan Fry
The Komodo dragon: surprisingly clean. Photo by Bryan Fry

Of course, you might argue that wild dragons might harbour deadlier bacteria. But the captive animals aren’t living in a sterile environment nor eating sterile food. If wild dragons are truly using bacteria as weapons, the captive ones should at the very least have some way of encouraging bacteria to grow in their mouths. “If they were facilitating the growth of bacteria in their mouths in the wild, they should be doing it in captivity,” says Fry. “They don’t. Their mouths were not dramatically different from the mouth of any other captive carnivore.”

Aside from Auffenberg’s book, the only other support for the bacteria-as-venom hypothesis comes from a team at the Universtiy of Texas at Arlington. In 2002, they found a wide range of bacteria in the saliva of 26 wild dragons and 13 captive ones, including 54 disease-causing pathogens. When they injected the saliva into mice, many of them died and their blood was rich in one particular microbe—Pasteurella multocida.

But Fry thinks the study is laughable. Sure, they studied wild dragons, but the microbes in Fry’s captive animals were actually closer to those from the wild ones in the Texan study. The so-called pathogens they discovered are just normal non-virulent members of an animal’s microbial entourage. And despite making a big deal of Pasteurella, they only found it in 2 of their 39 dragons. Goldstein never saw it in his captive ones.

And worst of all, no single species of microbe has ever been consistently identified in all dragons. How could these lizards rely on a strategy that’s so variable? “It’s evolutionary implausible,” says Fry.

The only remaining lifeline for the bacteria-as-venom hypothesis, says Tyrrell, is that the team only identified the bacteria that they could grow in laboratory cultures. Some species can’t be identified in this way, so one of these might contribute to the dragon’s killing bite.

Fry thinks that bacteria do help to kill the largest of the dragon’s victims, but not in the way that Auffenberg suggested. When the dragons tackle natural prey—medium-sized mammals like deer or pigs—the victims die very quickly from blood loss. The venom helps, but it’s the wounds that are important. But water buffalos are a different story.

These creatures were introduced to Komodo by humans. They’re too big to kill outright and always escape the initial attack. In their natural environment, they’d disappear into wide marshlands, but there’s nothing like that in Komodo. Instead, the buffalos seek refuge in rank water holes, stagnant and contaminated with their own faeces. In this microbial wonderland, their wounds soon become infected. “It’s the same as if you dumped a whole bunch of cow dung in your pool during the peak heat of summer, shaved your legs with a very old razor, and then went and stood in the water for a day,” says Fry. “You’d end up with some very tasty infections!”

Reference: Golstein, Tyrrell, Citron, Cox, Recchio, Okimoto, Bryja & Fry. 2013. ANAEROBIC AND AEROBIC BACTERIOLOGY OF THE SALIVA AND GINGIVA FROM 16 CAPTIVE KOMODO DRAGONS (VARANUS KOMODOENSIS): NEW IMPLICATIONS FOR THE ‘‘BACTERIA AS VENOM’’ MODEL. Journal of Zoo and Wildlife Medicine http://dx.doi.org/10.1638/2012-0022R.1

PS – It’s amazing how many enshrined “facts” about natural history are based on very little evidence. The cheetah’s status as the world’s fastest land animal as based on a single measurement taken in the 1960s. This month, scientists published the first proper measurements of running speed in wild cheetahs… and showed that they really are that fast. Another longstanding fact—that the honeyguide bird leads honey badgers to beehives—turns out to be a lie perpetuated by deceitful documentary-makers.

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What Do Lizard Tails Have In Common With Toilet Paper?

Toilet paper comes with lines of small cuts between the individual sheets, so it is easy to tear one off at pre-determined places.  A gecko’s tail works in the same way.

Geckos, skinks, and many other small lizards are known for their ability to amputate their own tails when threatened by predators. The tails don’t break off at random places. Instead, they have sets of “score lines”, where the tissue on either side is loosely stuck together and can be easily separated.  The gecko’s tail effectively comes pre-severed along several easy-to-tear lines.

But shedding a tail is more complex than it might seem. It’s not that a biting predator just pulls it off. The lizard helps the process along by contracting its muscles, which is why it takes more force to break the tail of an unconscious or dead lizard. Typically, the animal jettisons the tail just before the place where it was grabbed. After all, a tail is useful for communication, balance, storing fat, and even aerobatics—it’s not a thing to be casually lost, and the lizard benefits by detaching as little as possible.

Scientists have studied tail-shedding, or “caudal autotomy”, for several decades (there’s a good review here), focusing on when, why and how it happens. It’s the last one that interested Kristian Sanggaard from Aarhus University, who wanted to understand how the tail’s microscopic structures helped it to break off. To do that, he studied the Tokay gecko from south-east Asia, one of the largest of the 1,500 gecko species.

Here’s a slice through one of the gecko’s tail segments, stained with different dyes to highlight the various tissues. You can see the scales in dark blue running along the top and bottom, muscle fibres in red, and a huge core of white fat.

Section through a gecko's tail, showing clear "score lines" between segments

The segments are immediately obvious, with clear lines running through the fat and muscle. These divisions become less clear near the scales, but Sanggaard noticed dense clusters of collagen fibres at the points where the segments separate. You can see these in the image below (and the insets in the image above)—they’re the even patches blue in the midst of more marbled areas. The yellow arrows show the score line where the two segments break away from one another.

Along this line are dark blue dots. These are cells, and Sanggaard likens them to a zipper. It’s possible that when the lizard wants to shed its tail, the cells secrete substances that weaken the collagen, allowing the tissue to split apart more easily.

Left: yellow arrows show a ready-made score line in a gecko's tail. Right: a line of cells forming a zipper in the tail
Gecko tail segment showing muscle wedges

The tips of the broken tail segments end in wedge-shaped ‘fingers’ of white muscle. In an intact tail, these wedges fit into grooves within the preceding segment, like the finger jointsyou see on furniture.

Sanggaard looked at them under a powerful electron microscope, and saw that the muscle fibres end in mushroom-shaped tips. When the tail is intact, these fibres have flat heads that meet one another and stick together. When it’s time to detach the tail, Sanggaard thinks that the muscles contract and the ends expand into the rounded mushroom shapes. This reduces the adhesive forces between them, and allows the segments to disconnect.

An MRI scan of an unbroken tail confirmed his suspicions. There are clean gaps between adjoining segments with no structures running through them. This means that they’re held together by sticky forces, rather than by any physical anchors. In this way, the gecko gets the best of both worlds – a tail that holds together under normal circumstances, but that can be easily broken off at pre-determined points when its life is in danger.

Muscle fibres at the end of gecko tail segements end in mushroom shapes

That’s not the end of its defence, though. The severed tail will dance, writhe and wriggle for up to half an hour, probably to distract the predator’s attention from the escaping lizard or to put it off entirely. And if the tail isn’t eaten, the lizard will often return to it later to gulp it down itself. After all, why waste so much valuable fat?

Reference: Sanggaard, Danielsen, Wogensen, Vinding, Rydtoft, Mortensen, Karring, Nielsen, Wang, Thogersen & Enghild. 2012. Unique Structural Features Facilitate Lizard Tail Autotomy. PLoS ONE http://dx.doi.org/10.1371/journal.pone.0051803

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Geckos evolved sticky feet many times

Geckos are superb wall-crawlers. These lizards can scuttle up sheer surfaces and cling to ceilings with effortless grace, thanks to toes that are covered in microscopic hairs. Each of these hairs, known as setae, finishes in hundreds of even finer spatula-shaped split-ends. These ends make intimate contact with the microscopic bumps and troughs of a given surface, and stick using the same forces that bind individual molecules together. These forces are weak, but summed up over millions of hairs, they’re enough to latch a lizard to a wall.

Many geckos have these super-toes, but not all of them. There are around 1,450 species of geckos, and around 40 per cent have non-stick feet. A small number are legless, and have no feet at all. Initially, scientists assumed that the sticky toes evolved once in the common ancestor of all the wall-crawling species. That’s a reasonable assumption given that the toes look superficially similar. It’s also wrong.

Tony Gamble from the University of Minnesota has traced the evolutionary relationships of almost all gecko groups, and shown that these lizards have evolved their wall-crawling acumen many times over. In the gecko family tree, eleven branches evolved sticky toes independently of each other, while nine branches lost these innovations.


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Cockroaches and geckos disappear by swinging under ledges… and inspire robots

One minute, a cockroach is running headfirst off a ledge. The next minute, it’s gone, apparently having plummeted to its doom. But wait! It’s actually clinging to the underside of the ledge! This cockroach has watched one too many action movies.

The roach executes its death-defying manoeuvre by turning its hind legs into grappling hooks and its body into a pendulum. Just as it is about to fall, it grabs the edge of the ledge with the claws of its hind legs, swings onto the underneath the ledge and hangs upside-down. In the wild, this disappearing act allows it to avoid falls and escape from predators. And in Robert Full’s lab at University of California, Berkeley, the roach’s trick is inspiring the design of agile robots.

Full studies how animals move, but his team discovered the cockroach’s behaviour by accident. “We were testing the animal’s athleticism in crossing gaps using their antennae, and were surprised to find the insect gone,” says Full. “After searching, we discovered it upside-down under the ledge. To our knowledge, this is a new behavior, and certainly the first time it has been quantified.”


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How leaping lizards, dinosaurs and robots use their tails

What do a leaping lizard, a Velociraptor and a tiny robot at Bob Full’s laboratory have in common? They all use their tails to correct the angle of their bodies when they jump.

Thomas Libby filmed rainbow agamas – a beautiful species with the no-frills scientific name of Agama agama – as they leapt from a horizontal platform onto a vertical wall. Before they jumped, they first had to vault onto a small platform. If the platform was covered in sandpaper, which provided a good grip, the agama could angle its body perfectly. In slow motion, it looks like an arrow, launching from platform to wall in a smooth arc (below, left)

If the platform was covered in a slippery piece of card, the agama lost its footing and it leapt at the wrong angle. It ought to have face-planted into the wall, but Libby found that it used its long, slender tail to correct itself (below, right). If its nose was pointing down, the agama could tilt it back up by swinging its tail upwards.


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Cryptolacerta and the rise of the worm-lizards

This animal is not an earthworm. It is long and sinuous, it lives underground, and its flanks look like they’re lined with rings. But it is not an earthworm – after all, it has a skeleton, jaws, scales, and two stubby legs. It is a “worm lizard” or amphisbaenian.

Amphisbaenians are a group of burrowing lizards, and one of the most mysterious groups of reptiles. They’re named after Amphisbaena, a Greek serpent with a second head on its tail – indeed, amphisbaeneans do have tails that look a bit like their heads. They are meat-eaters, and they search for their prey underground, burrowing through the soil with strong, reinforced skulls. Most species are completely legless, but four of them – the ajolotes (including the one in the photo above) – have bizarre, stunted arms.

Their origins are mysterious. Their bones suggest that they are close relatives of snakes and obviously, neither group has any legs. But their genes tell a different story – they say that the amphisbaenians are most closely related to the lacertids, a common group of lizards. Now, Johannes Muller from Berlin’s Natural History Museum has found a fossil lizard whose features might settle the debate in favour of the lacertid camp.

Muller named his animal Cryptolacerta hassiaca, which means “hidden lizard from Hesse”. He found it in the Messel Pit, a disused quarry near the town of Hesse. The quarry has no shortage of famous former residents, including the over-hyped Darwinius, the giant bird Gastornis, and leaves that were scarred by fungus-infected ants. Cryptolacerta is the latest addition to this treasure trove of famous fossils

Muller used a CT scanner to get a glimpse of Cryptolacerta’s body, which was fully preserved except for the tip of its tail. Its huge skull has many features that are characteristic of amphisbaenians, including small eye sockets, indicating tiny eyes, and heavy thickened bone, making it strong and inflexible. That’s a far cry from the light, bendy skulls of snakes. Its body, however, looks far more lizard-like – it obviously has four legs, albeit small ones.

Muller compared Cryptolacerta’s features with those of other modern reptiles, and produced a family tree that linked them together. Cryptolacerta itself sat at the base of the amphisbaenean branch – it was an early member of the group. Meanwhile, the amphisbaenians and lacertids sat on adjacent branches, far away from the snakes.

This supports the genetic view: amphisbaenians are closely related to lacertids, and their superficial similarity to snakes is a great example of convergent evolution. They both evolved long legless bodies in independent ways.

With its legs and squat body, Cryptolacerta clearly wasn’t the specialist burrower that the amphisbaenians have become. By comparing its shape to other lizards, Muller thinks that it spent its days hidden among the leaf litter, burrowing from time to time when the opportunity arose. This concealed lifestyle may have been an intermediate step between open-air scurrying and fulltime burrowing.

Many burrowing animals, from worms to legless lizards (and there are at least 8 groups of those), have long bodies and no limbs, so it’s tempting to think that these features are a prerequisite for an underground life. But Cryptolacerta, with its reinforced skull, tells a different story – it suggests that early amphisbaenians adapted to a digging lifestyle headfirst. Only after they thickened their skulls did they lose their legs and lengthen their body.

Reference: Muller, Hipsley, Head, Kardjilov, Hilger, Wuttke & Reisz. 2011. Eocene lizard from Germany reveals amphisbaenian origins. Nature http://dx.doi.org/10.1038/nature09919

Image by Gary Navis and Robert Reisz

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One generation, new species – all-female lizard bred in a lab

In a lab in Kansas, Aracely Lutes has created a new species of all-female lizard that reproduces by cloning itself. There wasn’t any genetic engineering involved; Lutes did it with just a single round of breeding.

This feat stands in stark contrast to the slow pace at which species usually arise. Here’s the typical story: different populations become separated in some way, whether by space, time, predators, sexual preferences, or an inability to understand one another. Differences gradually build up between them, until they can no longer produce fit and fertile offspring. Voila – where there was once one species, there are now two.


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Sex runs hot and cold – why does temperature control the gender of Jacky dragons?


This is an old article, reposted from the original WordPress incarnation of Not Exactly Rocket Science. I’m travelling around at the moment so the next few weeks will have some classic pieces and a few new ones I prepared earlier.

Among Jacky dragons, females are both hot and cool, while males are merely luke-warm. For this small Australian lizard, sex is a question of temperature. If its eggs are incubated at low temperatures (23-26ºC) or high ones (30-33ºC), they all hatch as females; anywhere in the middle, and both sexes are born.

This strategy – known as ‘temperature-dependent sex determination (TSD) – seems unusual to us, with our neat gender-assigning X and Y chromosomes, but it’s a fairly common one for reptiles. Crocodiles are all-male at high temperatures and all-female at low ones, while turtles flip the rules around and produce more males in cooler climes. Assigning gender based on temperature is not uncommon but it is nonetheless puzzling.

Gender seems like an incredibly fundamental physical trait to leave to something as variable as the temperature of your surroundings. How has such a system evolved? What possible benefits could a species receive by switching control of from chromosomes to the environment? Now, a thirty-year old explanation for this puzzling system has finally been confirmed.

The most widely accepted hypothesis was put forward by Eric Charnov and James Bull over thirty years ago. They suggested that TSD occurs when the temperature of the environment affects the success of males and females strongly but differently. Parents can then use local temperatures as a sort of crystal ball, producing more males in conditions that are suited to males, and more females in conditions where they have the edge.

The idea is sound, but testing it has been remarkably difficult. The ideal experiment would involve hatching both males and females at the entire range of incubation temperatures and comparing their success over the course of their lives. Obviously, the very nature of TSD rules out that approach; how do you hatch males at low temperatures if those same conditions, by definition, beget females?

If that weren’t enough, most species that use TSD are large and long-lived. Imagine following a turtle for its entire 60 year lifespan and you begin to see the problem. All that changed this decade when TSD was found in the small and short-lived Jacky dragon (Amphibolorus muricatus). With a lifespan of 3-4 years, here was an animal that could be reasonably studied in experimental conditions.

With one problem down, Daniel Warner and Rick Shine from the University of Sydney solved the other by using hormonal treatments to sunder the link between temperature and sex. Temperature may decide gender but it does so through hormones. The key event is the conversion of testosterone to oestradiol (a relation of oestrogen) by an enzyme called aromatase. This happens at low temperatures and tells developing dragons to become females.

Warner and Shine overrode this process with a chemical that blocks aromatase. With the enzyme disabled, the duo managed to hatch male babies at temperatures that are exclusively female. The hormonally nudged Jackies were physically similar to their male siblings who developed in the normal way; that was essential if they were going to be compared fairly. The duo raised the babies in enclosures that mimicked their natural environments, and waited.

After three consecutive breeding seasons, Warner and Shine found (as predicted) that males sired more offspring on average if they were hatched at an intermediate 27ºC, a normal temperature for them in natural conditions. Males hatched at temperatures that are usually the province of females produced almost three times fewer young. The reverse was true for females; they enjoyed greater reproductive triumphs if they were hatched at a cooler 23ºC or a warmer 33ºC.

Although these results don’t explain why males and females should fare better at different incubation temperatures, they do fully vindicate the Charnov-Bull model. Exactly as predicted, male Jacky dragons produce more young if they hatch at temperatures that usually produce males, and likewise for females.

Such careful fine-tuning has done the lizards well over the course of evolution but it may put them in danger as the globe continues to warm. Like crocodiles, turtles and other reptiles that use TSD, the Jacky dragon may become a casualty of climate change, as rising temperatures lead to an all-female population and no way of producing a new generation.

Reference: Warner, D.A., Shine, R. (2008). The adaptive significance of temperature-dependent sex determination in a reptile. Nature DOI: 10.1038/nature06519

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