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.
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.”
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.
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
More on lizards:
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.
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
More on sex determination:
Humans have travelled all over the planet but many uncharted regions of the globe still hide unknown animal species waiting to be discovered. With some exceptions, these new finds are largely small creatures that are hard to spot amid the bustle of a tropical forest. So imagine Luke Welton’s surprise when he came across an entirely new species of giant monitor lizard in the forests of northern Philippines.
At two metres in length, it’s not quite as large as its close relative the Komodo dragon, but it’s hardly inconspicuous either. It’s also brightly and beautifully coloured with intricate golden spots running down its otherwise black back. As is often the case, the lizard may be new to science but the local tribespeople – the Agta and Ilongot – have known about it for centuries. It’s actually one of their main sources of protein. Their name for the monitor, bitatawa, is now part of its official species name – Varanus bitatawa
Rafe Brown, who leads Welton’s group, says, “Clues to its existence had filtered in over the last ten years.” Photos of the mysterious animal had been circulating since 2001, but the clincher came when Welton and another student, Cameron Siler, salvaged a specimen that had been brought to them by a hunter. “They knew it was something special, either a rare colour pattern or a new species,” says Brown.
The dead lizard went on a round-the world trip from the Philippines to Kansas. There, Brown’s team counted its scales, examined its internal organs and sequenced its DNA. Their meticulous examination revealed that the animal was closely related to the Gray’s monitor (Varanus olivaceus), which also lives on the same island. But it was distinct enough to count as a species in its own right. “The team in the field were very celebratory,” says Brown.
Not Exactly Pocket Science is a set of shorter write-ups on new stories with links to more detailed takes by the world’s best journalists and bloggers. It is meant to complement the usual fare of detailed pieces that are typical for this blog.
Cold-proof tongue allows early chameleon to catch early insect
Chameleons are some of the most versatile of lizards. They live in baking deserts and freezing mountaintops and part of their success hinges on a weapon that works just as well in the warmth as in the cold – its tongue. Relying on stored elastic power for its ballistic strike, the chameleon’s tongue is largely cold-proof. At temperatures that would flummox most reptile muscles, the tongue carries on snatching insects with great efficiency.
Chameleon tongues can reach twice the length of their body in less than a tenth of a second, latching onto prey with a sticky, grasping tip. Rather than pushing it forward with muscle power, like a spear-thrower, the chameleon behaves more like an archer. It ratchets the tongue backwards by slowly contracting its muscles, as if it was drawing an arrow on a bow. It fires by relaxing its muscles, and the whole sticky snare shoots forward on its own momentum. Once the prey is caught, long muscles pull the tongue back into the mouth.
Christopher Anderson and Stephen Deban from the University of South Florida filmed veiled chameleons with a high-speed camera as they shot their tongues at dangling crickets. Their performance certainly improved as the temperature increased from 15 to 35C, but not by much. Even at low temperatures, the tongue shot out with impressive acceleration, speed and power that fell by just 10-20% across a ten degree gradient. When it retracted under muscular control, the effects of the chill were more obvious and a similar gradient led to a 40-60% fall in performance.
By freeing their killer strike from the constraints of temperature, chameleons have been able to exploit chilly windows of opportunity denied to other lizards. They can hunt during the early morning hours when insects are very active and they can expand across a wide range of habitats. They also have to waste less energy on the simple business of keeping warm. After all, why bother with central heating when you can catch food at body temperatures of 3.5C, as some chameleons can?
Image by Christopher V Anderson
Zebrafish babies shut off their eyes at night
Many animals find it harder to see in the darkness of night, but the larvae of zebrafish must find it particularly difficult. Every night, they essentially shut down their eyes, losing the ability to see. Fairda Emran found that the retinas of the baby fish responded normally to light during the day, but they were almost totally impassive after 90 minutes of darkness. The fish themselves totally failed to follow a moving target.
The babies’ body clocks drove this cycle of blindness. It kicked in every night and even if the fish were kept in darkness for several days, they always anticipated the arrival of daylight by restoring their sight. Only a flash of light at night managed to break this tidy cycle, restoring the zebrafishes’ vision at a time when they would normally be blind.
At five days of age, baby zebrafish have just used up all the yolk from their eggs and are starting to find their own food. For them, energy is a precious commodity and eyes are energy-guzzling appliances, even when they’re set to standby at night. It makes sense to just shut them off instead.
The amazing ways in which animals see the world
Whiptail lizards are a fairly ordinary-looking bunch, but some species are among the strangest animals around. You might not be able to work out why at first glance, but looking at their genes soon reveals their secret – they’re all female, every single one. A third of whiptails have done away with males completely, a trick that only a small minority of animals have accomplished without going extinct.
Some readers might rejoice at the prospect of a world without males but in general, this isn’t good news for a species. Sex has tremendous benefits. Every fling shuffles the genes of the two partners and deals them out to the next generation in new combinations. Sex creates genetic diversity and in doing so, it arms a population with new weapons against parasites and predators. These benefits are so big that sex is nigh universal among complex life. Only a few groups, like the incredible bdelloid rotifers, have found ways of becoming permanently asexual.
Doing away with sex is even rarer for vertebrates (back-boned animals). The whiptails of the genus Aspidocelis are a flagrant exception. Their forays into asexuality started when two closely related species mated. For some reason, these encounters produced asexual hybrids. For example, the New Mexico whiptail (Aspidocelis neomexicana) is a hybrid of the Western whiptail (A. Inornatus) and the little striped whiptail (A. tigris). In the hybrid species, the females (and there are only females) reproduce by laying eggs that have never encountered any sperm.
The problem is that this really shouldn’t work. Sperm and egg cells are created through a process called meiosis, where a cell’s chromosomes are duplicated before the cell divides twice. This produces four daughter cells, each with half the DNA of the original. This means that egg cells only contain half the total number of chromosomes that most other cells in the body do. It’s their union with sperm, which are also genetically half-cocked, that restores the full balance of chromosomes, ready for the next generation.
So how do the lizards get their full set? The answer is deceptively simple. They start off with twice as many.
In the White Sands National Park of New Mexico, there are three species of small lizard that all share white complexions. In the dark soil of the surrounding landscapes, all three lizards wear coloured coats with an array of hues, stripes and spots. Colours would make them stand out like a beacon among the white sands so natural selection has bleached their skins. Within the last few thousand years, the lesser earless lizard, the eastern fence lizard and the little striped whiptail have all evolved white forms that camouflage beautifully among the white dunes.
Erica Bree Rosenblum from the University of Idaho has found that their white coats are the result of changes to the same gene, Mc1r. All of these adaptations arose independently of one another and all of them reduce the amount of the dark pigment, melanin, in the lizards’ skin. It’s a wonderful example of convergent evolution, where the same environmental demands push different species along the same evolutionary paths. But Rosenblum has also found that there are many ways to break a gene.
Each of the three lizards has a different mutation in their Mc1r gene, that has crippled it in diverse ways. These differences may seem slight, but they affect how dominant and widespread the white varieties are, and how likely they are to branch off into new species of their own. Even when different species converge on the same results – in this case, whitened skin – and even when the same gene is responsible, their evolutionary paths can still be very different.
The Mc1r gene encodes a protein called the melanocortin 1 receptor (MC1R). It’s a messenger that sits astride the cell’s membrane and transmits messages across it. It triggers a sequence of events that stimulates the production of the dark pigment melanin. In this way, it affects the skin colour of many animals and faulty copies of the gene tend to result in lighter colours. In humans, for example, around 80% of redheads owe their hair colour to common faulty variant of Mc1r.
In each of the White Sands lizards, just one of the MC1R protein’s many amino acids has been swapped (red circles above), and it’s a different one in each species. All three amino acids lie within the part of the protein that straddles the cell membrane. These regions are important for keeping the protein together, and for channelling signals from one side of the membrane to another.