When Malcolm Burrows first heard the sound of a pygmy mole cricket leaping from water, he was enjoying a sandwich. Burrows, a zoologist from the University of Cambridge, was visiting Cape Town and had snuck out the back of the local zoology department to eat his lunch by a pond. “I heard sporadic thwacking noises coming from the water,” he says. “When I looked more closely I could see small black insects jumping repeatedly from the water and heading towards the bank.”
They were pygmy mole crickets, a group of tiny insects just a few millimetres long. Despite their name, they’re more grasshoppers than crickets, and are some of the most primitive members of this group. They’re found on every continent except Antarctica.
Pygmy mole crickets cannot fly, but they can certainly jump. Burrows collected some of the individuals from the pond, and took them back to the lab to film them with high-speed cameras. When they take off, they often spin head-over-tail, but what they lack in elegance they make up for in distance. They can jump over 1.4 metres, more than 280 times their own body length.
Doing this on land is one thing, but as Burrows saw at the pond, these insects can also jump from water. This ability serves them well—they live in burrows near to fresh water, which frequently flood. Their leaps send them back to terra firma, saving their lives.
Burrows found that these insects jump from water in a completely new way. Animals like pond-skaters and the basilisk lizard can walk on water by relying on surface tension—the tendency of the surface of water to resist an external force. But the mole cricket extends its hind legs so quickly that they break right through the surface.
As the legs move through the water, three pairs of flat paddles and two pairs of long spurs flare out from each one. These structures have a concave shape, much like an oar. As they flare out, they increase the surface area of the mole cricket’s leg by around 2.4 times, allowing it to push down on a much larger volume of water. And once the legs are fully extended, the paddles retract to reduce the drag on the airborne insect. From water, the mole crickets can only jump for 3 centimetres or so. That’s pathetic compared to their land-based attempts, but still more than 5 times their body length, and enough to save them from drowning.
When Burrows shone ultraviolet light onto the paddles, they glowed with a bright blue colour at their bases. That’s the signature of resilin, an incredibly elastic protein that powers the jumps and wingbeats of many insects. Its presence on the mole cricket suggests that the paddles and spurs are spring-loaded.
“It just shows what amazing things can be found close to where we live and work,” says Burrows. “Instead of spending time exploring the more exotic parts of South Africa, I spent most of my visit there essentially looking outside my back door.”
Reference: Burrows & Sutton. 2012. Pygmy mole crickets jump from water. Current Biology 22: R990
All photos and video by Malcolm Burrows
Icelandic horses can move in an odd way. All horses have three natural gaits: the standard walk; the two-beat trot, where diagonally opposite pairs of legs hit the ground together; and the four-beat gallop, where the four feet hit the ground in turn. To those, Icelandic horses add the tölt. It has four beats, like the gallop, but a tölting horse always has at least one foot on the ground, while a galloping one is essentially flying for part of its stride. This constant contact makes for a smoother ride. It also looks… weird, like watching a horse power-walk straight into the uncanny valley.
The tölt is just one of several special ambling gaits that some horses can pull off, but others cannot. These abilities can be heritable, to about the same extent that height is in humans. Indeed, some horses like the Tennessee Walking horse have been bred to specialise in certain gaits.
Now, a team of Swedish, Icelandic and American scientists has shown that these special moves require a single change in a gene called DMRT3. It creates a protein used in neurons of a horse’s spine, those which coordinate the movements of its limbs. It’s a gait-keeper.
Since as long as I can remember, nature documentaries and textbooks have said that flocking birds and shoaling fish gather in large coordinated groups to protect themselves from predators. That explanation makes complete sense. After all, many eyes can spot danger more easily, and many bodies can confuse the senses of hunters. But common sense often leads us astray in biology, and very few people have checked to see if collective motion does offer safety from predators.
Christos Ioannou is one of the first. By allowing a predatory fish to hunt virtual prey, whose movements he could precisely control, Ioannou showed that coordinated groups are, indeed, less likely to be attacked. So far, so obvious, but Ioannou’s results have a fascinating implication: predators can trigger the evolution of collective movements, even if their prey can’t see them. While we often think of flocking and shoaling as a response to a hunter’s advances, that response doesn’t have to be a deliberate one. Threatened prey could evolve to move as one even without any knowledge of the threat they face.
Sticking to surfaces and walking up walls are so commonplace among insects that they risk becoming boring. But the green dock beetle has a fresh twist on this tired trick: it can stick to surfaces underwater. The secret to its aquatic stride is a set of small bubbles trapped beneath its feet. This insect can plod along underwater by literally walking on air.
The green dock beetle (Gastrophysa viridula) is a gorgeous European resident with a metallic green shell, occasionally streaked with rainbow hues. It can walk on flat surfaces thanks to thousands of hairs on the claws of their feet, which fit into the microscopic nooks and crannies of whatever’s underfoot. Most beetles have the same ability, and some boost the adhesive power of their hairs by secreting a sticky oil onto them.
These adaptations work well enough in dry conditions, but they ought to fail on wet surfaces. Water molecules should interfere with the hairs’ close contact, and disrupt the adhesive power of the oil. “People believed that beetles have no ability to walk under water,” says Naoe Hosoda from the National Institute for Material Science in Tuskuba, Japan.
They were clearly wrong. Together with Stanislav Gorb from the Zoological Institute at the University of Kiel, Germany, she clearly showed that the green dock beetle has no problems walking underwater. The duo captured 29 wild beetles, and allowed them to walk off a stick onto the bottom of a water bath. Once there, they kept on walking. Read More
Here’s the ninth piece from my BBC column
In 2008, at the Beijing Olympic Games, Jamaican sprinter Usain Bolt ran the 100m in just 9.69 seconds, setting a new world record. A year later, Bolt surpassed his own feat with an astonishing 9.58-second run at the 2009 Berlin World Championships. With the 2012 Olympic Games set to begin in London, the sporting world hopes Bolt will overcome his recent hamstring problems and lead yet another victorious attack on the sprinting record. He is arguably the fastest man in history, but just how fast could be possibly go?
That’s a surprisingly difficult question to answer, and ploughing through the record books is of little help. “People have played with the statistical data so much and made so many predictions. I don’t think people who work on mechanics take them very seriously,” says John Hutchinson, who studies how animals move at the Royal Veterinary College in London, UK.
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.”
Anyone who has tried to pull a razor clam from a sandy beach knows that they can dig fast. These edible animals can bury themselves at around one centimetre per second, and they go deep. A clam the length of a hand can create a burrow up to 70 centimetres down.
Like all molluscs, the clam has a muscular foot, but it’s not that muscular. Based on measurements of the foot’s strength, Amos Winter from Massachussetts Institute of Technology calculated that it should only be able to dig a couple of centimetres into the mud. It shouldn’t be able to submerge its body, much less create a burrow five times longer.
But Winter knows the razor clam’s secret: it doesn’t just rely on raw power. The clam adds water to the soil just below it, making it softer and easier to penetrate. It digs by turning part of a beach into quicksand.
The clam’s digging equipment couldn’t be simpler: a pair of long valves that run the length of its body and open or close its shell; and a foot that sits beneath the them. It extends the foot downwards and pushes against it to lift the shell up slightly. Then, it contracts the valves, sending blood into the foot and inflating it. This foot becomes an anchor. By pulling against it, the clam can drag its shell downwards.
To understand how these motions create a burrow despite the foot’s weedy nature, Winter had to capture several clams. And to do that, he had to become a licensed clam digger. Back in the lab, studying the clams wasn’t easy. As Winter writes in a wonderfully deadpan academic way: “The adage ‘clear as mud’ is used to describe the difficulty of visually investigating burrowing animals.” He finally saw what the clams were doing when he created a homemade “visualiser”— a repurposed ant farm. The animal was trapped between two transparent plates, filmed with high-speed cameras, and surrounded by a ‘beach’ of glass beads.
Winter’s videos revealed that when the clam contracts its valves, it does more than just pump the foot with blood. The contraction closes its shell, which relieves the pressure on the surrounding soil. The soil starts to crumble, and mixes with water pulled into the gap from above. The water “fluidises” the soil, making it soft and loose like quicksand. It offers far less resistance, and the clam can move through it with around ten times less energy. It does so quickly, before the soil has a chance to solidify again.
Winter isn’t just studying clams for the sake of it. The list of sponsors for his study is telling: Battelle Memorial Institute, a science and technology development company; Bluefin Robotics; and the Chevron Corporation, an energy company that explores for oil and gas.
Winter used his newfound knowledge to create RoboClam: a robot that duplicates the clam’s burrowing technique. It’s about the size of a lighter, but it comes with a much larger supportive frame of pistons and regulating elements. After further development, RoboClam could act as a lightweight anchor that could be easily set and unset. It could tether small robotic submarines for studying the ocean floor; help to install undersea cables or deep-water oil rigs; or even detonate buried underwater mines.
Reference: Winter, Deits & Hosoi. 2012. Localized fluidization burrowing mechanics of Ensis directus. Journal of Experimental Biology http://dx.doi.org/10.1242/jeb.058172
Photo by Arne Huckelheim
In a small office north of London, Stephanie Pierce from the Royal Veterinary College is watching a movement that hasn’t been seen for 360 million years. On her computer, she has resurrected the long-extinct Ichthyostega – one of the earliest four-legged animals to creep about on land. By recreating this iconic beast as a virtual skeleton, Pierce has shown that while it looked like a giant salamander, it couldn’t possibly have walked like one. It had some of the planet’s earliest bony legs, but they weren’t very good at taking steps.
Ichthyostega hails from the Devonian period, a time in Earth’s history when swimming transformed into walking. Fish invaded the land and evolved into the first tetrapods—four-limbed animals that include amphibians, reptiles, birds and mammals. Muscular fins used for steering and balance evolved into legs for walking.
Since its discovery over 50 years ago, Ichthyostega has been an icon of this pivotal transition. Some 300 specimens have been found but many are incomplete, flattened or distorted. Pierce’s new model provides the best look yet at the animal’s skeleton. “It makes Ichthyostega a bit more tangible,” she says. “It’s not just a fossil laying in a rock now. It’s an animal that’s coming to life.”
Pierce built her virtual skeleton by putting dozens of Ichthyostega specimens in powerful CT-scanners, choosing only the best preserved ones out of the 300 or so in existence. “The front end of the animal was mainly composed from one beautifully preserved specimen called ‘Mr Magic’,” she says.
It was painstaking work. These fossils are so old that chemically, they are almost identical to the rocks around them. By eye, the bones stand out. To the scanners, they blend in. Pierce spent over two years scanning the specimens and building her model, but the results were worth it. “This has been on the wish-list for years,” says Michael Coates, who studies tetrapod evolution at the University of Chicago.
Those boots weren’t made for walking…
The model showed that Ichthyostega’s shoulders and hips were oddly restricted. They could move back and forth, and up and down, but they couldn’t rotate about their long axis. Hold your arm out and rotate your palm so it faces up then down—Ichthyostega’s shoulder couldn’t do that.
Most modern tetrapods need long-axis rotation in order to walk. Without it, their legs can’t be thrown forward or pulled backward. Ichthyostega’s limitations meant that despite having four limbs, it probably couldn’t have taken a step. It hind feet would never have been planted flat against the ground or supported its weight. It had invaded the land, but it wasn’t striding across it.
“It highlights the fact that the earliest tetrapods are not just ‘gigantic salamanders’, despite a vague similarity in outline,” says Per Ahlberg from Uppsala University. “The limbs and girdles are very different from anything now living.”
Pierce thinks that Ichthyostega moved by paddle with its front limbs, using powerful muscles and flexible elbows to make rowing motions. The closest living analogue is probably the mudskipper – a fish that drags itself along muddy land with its front fins (as in the video below).
Pierce also compared Ichthyostega’s joints and limbs to those of other living animals with sinuous bodies and interesting gaits, including a salamander, crocodile, seal, otter and platypus. Compared to these modern species, Ichthyostega’s hips and shoulders were similarly flexible in most planes of movements, but along their long axis, they could barely rotate.
Some scientists think that the tetrapods evolved limbs before they could walk, and their first members lived in shallow water. Others think that it’s the other way round, and that muscular limbs, hips and shoulders evolved while fish still had fins. The virtual Ichthyostega supports the former idea, since it had limbs but couldn’t walk. But Coates cautions against “fitting a smooth transition from swimmers to walkers.” He says, “Evolutionary transitions needn’t follow linear routes. Ichthyostega probably represents one of multiple experiments among the first tetrapods with limbs, trying-out life in the shallows.”
So… what made those tracks?
Other early tetrapods had similar shoulders and hips, so they probably had the same limitations too. John Hutchinson, who led the new study, plans to find out. His lab is busy reconstructing other early tetrapods including Acanthostega, one of Ichthyostega’s contemporaries, and Pederpes, a later model.
But Ahlberg notes that Ichthyostega had a very unusual and rigid spine, and may not have been representative of other early tetrapods. “Other tetrapods are known to have had more flexible spines” he says, “and this probably allowed them to overcome the limitations of their shoulders and hips”.
This might explain why Ahlberg and others have discovered tracks that pre-date Ichthyostega by around 20 million years, and had become fairly common by the time it evolved. Many of these tracks showed precisely the kind of salamander-like movements that Ichthyostega was apparently incapable of making. They were clearly made by early four-legged tetrapods, and to this date, we don’t know what made them.
Pierce agrees that the final word on Ichthyostega’s movements will have to wait until she can animate its entire skeleton. “The ultimate goal would be to try and create some sort of dynamic movement,” she says. She has applied for a grant to do just that, to model the motions of the entire animal, and compare them to salamanders or crocodiles. “That’s going to take so much time, but it’ll be very interesting,” she says.
PS: I want to point out that in researching this story, I spent a good minute on my living room floor trying to walk without long-axis rotation. It was really hard, and I looked like an idiot. I did a similar thing when I was writing about hummingbird wing movements for Nature. I’m going to christen this Method Science Journalism.
Reference: Pierce, Clack & Hutchinson. 2012. Three-dimensional limb joint mobility in the early tetrapod Ichthyostega. Nature http://dx.doi.org/10.1038/nature11124
Image by Julie Molnar
More on tetrapods:
On an uneventful day, five passers-by in busy Oxford shopping street suddenly stop and look upwards. They have spotted a camera mounted on a nearby roof, pointed straight at them. But these aren’t strangers who have suddenly realised that Big Brother is watching them. They are actors, who are taking part in a natural experiment that looks at how information spreads through crowds of people.
Andrew Gallup from Princeton University is behind the camera. Using its lens, and technology based on the video-gaming graphics cards, he can track the movement of each pedestrian, and calculate where they’re looking. With this set-up confirmed that people have a natural tendency to look where others are looking. But this contagion of glancing is much weaker than popular psychology books would have us believe.