Newborn babies have a preference for the way living things move

Blogging on Peer-Reviewed ResearchFrom an animal’s point of view, the most important things in the world around it are arguably other animals. They provide mates, food, danger and companionship, so as an animal gazes upon its surroundings, it pays for it to be able to accurately discern the movements of other animals. Humans are no exception and new research shows that we are so attuned to biological motion that babies just two days old are drawn to extremely simple abstract animations of walking animals.

Running animal Animals move with a restrained fluidity that makes them stand out from inanimate objects. Compared to a speeding train or a falling pencil, animals show far greater flexibility of movement but most are nonetheless constrained by some form of rigid skeleton. That gives our visual system something to latch on to.

In 1973, Swedish scientist Gunnar Johansson demonstrated this to great effect by showing that a few points of light placed at the joints of a moving animal to simulate its gait. When we see these sparse animations, we see them for what they represent almost instantaneously.

Don’t believe me? Just look at this human walker from Nikolaus Troje’s BioMotion Lab website. With just fifteen white dots, you can not only simulate a walking adult, but you can also tell if it’s male or female, happy or sad, nervous or relaxed. Movement is the key to the illusion – any single static frame merely looks like a random collection of unconnected dots. But once they start to move in time, the brain performs an amazing feat of processing that extract the image of a human from the random dots.

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Argentavis, the largest flying bird, was a master glider

The largest ever flying bird, Argentavis, was a giant predator, as big as a light aircraft. But how did such a giant take to the skies and stay there?

An artist rendering of ArgentavisSix million years ago, the skies of Argentina were home to fearsome predator – Argentavis magnificens, the largest bird to ever take to the air. It weighed in at 70kg and had a wingspan of 7m, about the same size as a Cessna 152 light aircraft.

Argentavis was a member of an extinct group of predatory birds understandably called the teratorns – ‘monster birds’. They are related to storks and New World vultures such as turkey vultures and condors. But Argentavis completely dwarfed even the massive Andean condor, weighing six times more and with a wingspan over twice as long (in the picture below, its silhouette is placed next to a bald eagle for scale).

There is no question that Argentavis flew. It has all the characteristics of modern flyers including light, hollow bones and strong, sturdy wings. It’s how it flew that palaeontologists have puzzled over, given its massive size in relation to modern birds.

Take-off

For a start, how did it get its large bulk off the ground in the first place? The heaviest living flier, the Great Kori Bustard, is over three times lighter than Argentavis, and even it can only take off after arduously ‘taxiing’ like a airplane.

Sankar Chatterjee from the Museum of Texas Tech University decided to model the giant’s flying style by running simulations with known fossils. He found that Argentavis simply couldn’t have generated enough lift from a running-take-off.

It needed height to get airborne, but it could manage with surprisingly little. Even a gentle down-slope of 10° and a light headwind would have given it enough extra power to avoid an embarrassing crash. Albatrosses and hang-glider pilots use the same technique today.

In the air

Argentavis’s formidable skeleton, in comparison to a bald eagle.Once in the air, the flapping flight that small birds use was out of the question for the giant predator. By studying its skeleton, Chatterjee estimated the maximum amount of power that its flight muscles could have generated. And while substantial, it was still 3.5 times less than the minimum amount of power needed to fly.

Instead, Chatterjee believes that Argentavis was a master glider. It was capable of soaring for great distances at a shallow angle of 3°, continually re-shaping its wings to control its glide.

Unlike flapping, the efficiency of gliding doesn’t change very much with size, if a bird sticks to the standard body plan. So despite its enormity, Argentavis sailed through the air with as much grace as much smaller species like the buzzard or white stork.

Like modern soarers, Chatterjee believes that Argentavis used two techniques. By flying along the Andean ridges, it stayed aloft using upwards air currents produced by wind deflected up the cliffs. The several fossils found at the Andean foothills support his idea.

Because of its efficient gliding, it could stay aloft using relatively slow drafts of wind. Chatterjee calculated its top speed at about 70 km/h, allowing it scan vast tracts of land for prey. It’s a very energy-efficient style and today, eagles and vultures use it to great effect, sometimes covering hundreds of miles without a single wing flap.

Hot air

When the bird switched from the mountains to the wide, open spaces of the pampas, it switched to a different method – thermal soaring, where rising columns of hot air provided it with lift.

Thermals are hot rising columns of air.Popcorn-like cumulus clouds betray the location of thermals, and by circling around one, Argentavis could have risen through the air, giving itself enough height to soar to the next thermal. Despite its large size, Chatterjee calculated that Argentavis was manoeuvrable enough to manage the tight circular turns needed to stay within a thermal column.

Even with this reliance of thermals, Argentavis was pushing the limits of even gliding flight. Any heavier and it would have exceeded the maximum weight for safe gliding. So why are there no equally sized giants today?

Chatterjee thinks that the late Miocene’s climate provided the answer. Six million years ago, Argentina was much hotter and drier than it is today – just the weather needed for generating the powerful thermals needed to lift such a large bird.

Argentavis was beautifully adapted to take advantage of this large, open habitat, where it could travel across large distances in search of prey. And unlike modern condors, it was no mere scavengers. Its skull was as long as my forearm and ended in a formidable hooked beak – it was an active hunter, possibly taking prey on the wing. .

Reference: Chatterjee, Templin & Campbell. The aerodynamics of Argentavis, the world’s largest flying bird from the Miocene of Argentina. PNAS doi.10.1073/pnas.0702040104.

Images from PNAS paper and Apokryltaros

Related posts on extinct animals:
Bone-crushing super-wolf went extinct during last Ice Age
Microraptor – the dinosaur that flew like a biplane
How many types of dinosaur were there?
Tracks provide evidence of swimming dinosaurs

Inner ear size can predict a mammal’s agility

The semicircular canals of an animal’s inner ear controls its sense of balance. Their size can tell us whether an animal is slow and ponderous or fast and agile. They can even help us to reconstruct the behaviour of extinct species.

Studying the way an animal moves by looking at its ears might seem like a poorly thought-out strategy. After all, short of watching it directly, most biologists would choose to look at more obvious traits like tracks, or limb bones.

But while an animal’s limbs may drive it forward, its inner ear makes sure that it doesn’t immediately fall over. By controlling balance, it plays a key role in movement, and its relative size can tell us about how agile an animal is.

Organs of balanceA 3-D reconstruction of a baboon’s skull and its semicircular canals.

When we walk, the image that forms on our retinas changes quite considerably. But no matter how fast or erratically we move, our view of the world neither jerks nor judders. It’s all stable images and smooth transitions, and the inner ear plays a large role in that.

In the inner ear, three semicircular canals control our balance by acting like small gyroscopes. The canals are bony, fluid-filled tubes arranged at right angles to each other and send information to the brain about the body’s orientation.

When the body moves, so does the fluid and this sloshing is sensed by hairs in the canals and relayed to the brain. The muscles of the neck and eye tense reflexively in response to these signals, and these help to stabilise our view of the world.

In humans, the inner ear doesn’t really have to work too hard – we’re limited to moving on the ground, and not very quickly at that. It’s a whole different story for a fast and agile animal like a bat, twisting and turning in three-dimensional airspace while avoiding obstacles and predators.

Acrobatics vs. stealth

Fred Spoor from University College London and colleagues from around the world reasoned that these different movement styles must be reflected in the size of a species’ balance organs. There is some evidence for this already – the practically immobile sloths have small semicircular canals, while manoeuvrable birds have relatively large ones.

But these findings seem almost anecdotal compared to the massive amount of data that Spoor collected. His group looked at the canals of 91 different species of primates, representing all the major families.

Primates have a wide range of movement styles from the fast siamang (top) to the slow loris (bottom).The primates are an ideal group for this type of analysis – despite being closely related, they have a vast range of different movement styles.

Acrobatic gibbons swing through jungle canopies at high speed using ball-and-socket-jointed wrists (top). At the other end of the spectrum, the appropriately named slow loris is a ponderous and stealthy climber (bottom).

The group used a special CT scanner, a hundred times more sensitive than those used by hospitals, to build detailed 3-D reconstructions of the skull of each species, and the three canals inside. As well as the primates, they also looked at 119 other mammals, from mouse to elephant, and gave each one a score from one to six, based on how swift or agile they were.

Canal size predicts agility

As predicted, they found that the canals of agile animals with fast, jerky movements like tarsiers (image below, left) are larger for their body size and more strongly curved. Slower species like lorises have relatively small and less curved canals.

The semicircular canals of a tarsier (left) are relatively larger than those of the Palaeopropithecus (right).Spoor’s data suggests that the size of the semicircular canals are an important adaptation to give fast-moving animals greater stability.

It explains why some primates can gracefully race through dense treetops at speeds where humans, with out relatively smaller canals, would embarrassingly collide with a branch. Just look at this amazing video from the Life of Mammals, of various lemurs (and their predators) moving through the trees.

This method can also be used forensically, to recreate the movement styles of extinct mammals. To prove this principle, Spoor looked at the canals of several species of extinct lemur, and found that their canals gave important clues about their behaviour.

Of the species he looked at, Palaeopropithecus (image above, right) had by far the smallest canals for its size. Accordingly, palaeontologists believed it was the lemur equivalent of a sloth; its hands and feet are curved for hanging from branches, and its wrists and ankles have lost the flexibility needed for effective walking.

Reference: Spoor, Garland, Krovitz, Ryan, Silcox & Walker. 2007. The primate semicircular canal system and locomotion. PNAS doi/10.1073/pnas.0704250104

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Images: Top and bottom images from Alan Walker lab, Penn State, siamang by William H Calvin, loris by Sandilya Theuerkauf

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Death of dinosaurs did not lead to rise of modern mammals
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Orang-utan study suggests that upright walking may have started in the trees

A common theory of human evolution says that after our ancestors descended from the trees, they went form walking on four legs to two. But a new study in orang-utans could overturn that theory, by suggesting that our ancestors evolved a bipedal walk while they were still in the trees.

Did and upright posture evolve in a tree-dwelling ancestor?Walking on two legs, or bipedalism, immediately sets us apart form other apes. It frees our arms for using tools and weapons and is a key part of our evolutionary success. Scientists have put forward a few theories to explain how our upright gait evolved, but the ‘savannah theory’ is by far the most prolific.

It’s nicely illustrated by this misleading image that has become a mainstay of popular culture. It suggests that our ancestors went from four legs to two via the four-legged knuckle-walking gait of gorillas and chimps. Dwindling forests eventually pushed them from knuckle-walking to a full upright posture. This stance is more efficient over long distances and allowed our ancestors to travel across open savannahs.

But this theory fails in the light of new fossils which push back the first appearance of bipedalism to a time before the forests thinned, and even before our ancestors split from those of chimpanzees. Very early hominins, including Lucy (Australopithecus afarensis) and Millennium Man (Orrorin) certainly ambled along on two legs, but they did so through woodland not plains.

Our arms provide a further clue. Even though our ancestors’ back legs quickly picked up adaptations for bipedalism, they steadfastly kept long, grasping arms, an adaptation more suited to moving through branches. To Susannah Thorpe at the University of Birmingham, these are signs that bipedalism evolved while our ancestors were still living in trees.

Two legs good, four legs bad?

Orang-utans can go bipedal and our ancestors may well have done the same in the trees.But there is a snag – an adaptation must provide some sort of benefit. And, as many children painfully discover, it is hard to imagine how walking on two legs could benefit sometime in a tree.

But Thorpe has an answer to this too. She spent a year in the Sumatran jungle, studying the orang-utan – the only great ape to spend the majority of its life in the trees.

She carefully documented over 3,000 sightings of wild orang-utans moving through the treetops. On large sturdy branches, they walk on all fours (below right), and on medium-sized ones, they start to use their arms to support their weight.

But on the thinnest and most unstable branches, the apes use a posture that Thorpe calls ‘assisted bipedalism’ (below left). They grip multiple branches with their long, prehensile toes and use their arms to balance and transfer their weight. And unlike chimps which bend their knees while standing up, bipedal orang-utans keep their legs straight, just like humans do.

An orang-utan used both two-legged and four-legged postures.

It’s a win-win posture – the hands provide extra safety, while the two-legged stance frees at least one hand to grab food or extra support. With it, the apes can venture onto the furthest and thinnest branches, which provides them with several advantages.

As Thorpe says, “Bipedalism is used to navigate the smallest branches where the tastiest fruits are, and also to reach further to help cross gaps between trees.” That saves them energy because they don’t have to circle around any gaps, and it saves their lives because they don’t have to descend to the ground. “The Sumatran tiger is down there licking its lips”, she said.

A new view of ape & human evolution

With these strong adaptive benefits, it becomes reasonable to suggest that bipedalism evolved among the branches. Based on this theory, Thorpe, along with Roger Holder and Robin Crompton from the University of Liverpool, have painted an intriguing new picture of ape evolution.

It begins in the same way as many others – with the rainforests of the Miocene epoch (24 to 5 million years ago) becoming increasingly patchy. For tree-dwelling apes, the gaps in the canopy started becoming too big to cross. But in Thorpe’s view, these ancestral apes were already using a bipedal stance, and different groups took it in separate directions.

Our ancestors were bipedal long before they came down from the trees.The ancestors of orang-utans remained in the increasingly fragmented canopy and became specialised and restricted there. The ancestors of chimps and those of gorillas specialised in climbing up and down trees to make use of food both in the canopy and on the ground. The postures used in vertical climbing are actually very similar to those used in four-legged knuckle-walking and this became their walk of choice on the ground.

The ancestors of humans abandoned the trees altogether. They used the bipedal stance that served them well on thin branches to exploit the potential of the stable land environment. Over time, they brought in further adaptations for efficient walking, culminating in the human walking style that we now neglect by sitting at a computer all day.

Thorpe’s reconstruction is delightfully non-human-centric. It suggests that in the evolution of movement, we were conservatives who relied on a walk that had been around for millions of years. Chimps and gorillas with their fancy new knuckle-dragging gait were the true innovators.

Reference: Thorpe, Holder & Crompton. 2007. Origin of human bipedalism as an adaptation for locomotion on flexible branches. Science 316: 1328-1331.

Image: Black and white image from Science magazine.

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Related stuff:
On ape and human evolution:
Chimps show that actions spoke louder than words in language evolution
Hidden ‘junk’ gene separates human brains from chimpanzees
Chimps have more adaptive genetic changes than humans

On the evolution of movement:
Salamander robot walks, swims and sheds light on evolutionary step from sea to land
Microraptor – the dinosaur that flew like a biplane

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Bats: internal compasses and record-breaking tongues

Bats are masterfully adapted to life in the night skies. Now, two new studies demonstrate previously unknown abilities in bat species – an inbuilt magnetic compass, and a tongue so big it has to be stored in the rib cage.

If you were a biologist looking for astounding innovations in nature, you could do much worse than to study bats. Bats are like showcases of nature’s ingenuity, possessing a massive variety of incredible adaptations that allow them to exploit the skies of the night.

Bats navigate their way at night with echolocation and a magnetic sense. They are the only mammal group capable of true flight and are one of only four groups of animals to have ever evolved the ability. As a result, they have spread across the globe with tremendous success. Today, one in every five species of mammal is a bat.

They are most famous for their incredible echo-location. By listening for the echoes of sound waves rebounding off solid objects, bats have been finding their way in the dark using a radar that humans have only managed to duplicate millions of years later.

Magnetism

Echolocation is a short-range skill. Over longer distances, the bats’ ability to control their signals and perceive their echoes weakens, and other navigational skills must be used.

Until now, it was unclear what these might be. Richard Holland and colleagues from Princeton University have changed all that by showing that a large North American species, the big brown bat (Eptesicus fuscus), finds its way home by using the Earth’s magnetic field as a compass.image-big-eared-townsend-fledermaus.jpg

Holland took several big brown bats 20km away from their roosts and tracked their way home with radio telemetry. Before they were released, the bats were acclimatised at sunset to a magnetic field that was rotated to face either east or west.

For 5km, the group exposed to the easterly oriented field flew east, and those exposed to the westerly oriented field flew west. They were clearly using a magnetic compass which had been calibrated during sunset.

After initially losing their bearings, many of the confused bats corrected themselves and found the right way home. Holland believes that they recognised that the direction they were flying in did not match their magnetic map, possibly through other cues such as the position of the stars.

Other species including turtles and homing pigeons are known to navigate with a magnetic sense, but this is the first time the ability has been shown in bats. It adds to the already impressive array of senses that these animals possess.

A wicked tongue

Bats are also known for the variety of different food sources they exploit. Some species specialise in snatching spiders from their webs using their peerless radar, others take fish from lakes and perhaps the most famous representative drinks the blood of other mammals.

And many species make a living by drinking the nectar of the bountiful flowers found throughout the tropics, like leather-winged equivalents of hummingbirds.

Hummingbirds have evolved highly specialised relationships with flowers, so that some flowers are only accessible to a single species with the correctly shaped bill. Until recently, no such partnerships were seen between flowers and bats, mainly because the bat’s soft facial tissues are less easily moulded by natural selection than the bird’s hard bill.

The tube-lipped nectar bat (Anoura fistula) from the Andean forests of Ecuador is a striking exception (see left; photo by Nathan Muchhala).

The tube-lipped nectar batThese cloud forests are home to a plant called Centropogon nigricans that has flowers 8-9cm long. No ordinary bat can feed from these.

Nathan Muchhala from the University of Miami discovered that the tube-lipped nectar bat manages it with a tongue that is 50% longer than its body. In terms of relative size, its tongue is second only to the chameleon’s.

But where does a 5.5cm long bat store an 8.5cm long tongue? In most mammals, the base of the tongue is attached at the back of the mouth. But in the nectar bat, it is stored in its rib cage and its base lies between its heart and its sternum. Muchhala believes that the bat’s tongue and the Centropogon’s flower co-evolved to extreme lengths over time.

He captured various local bats over four months and found pollen from Centropogon nigricans only on the fur of the tube-lipped nectar bats and not related species.

Due to their exclusive relationship, the bat gets a permanently reserved dining spot and the flower gets a dedicated pollinating service.

Holland, Thorup, Vonhof, Cochran & Wikelski. 2006. Nature 444: 702.

Muchhala. 2006. Nature 444: 701.

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Tarantula climbs walls by spinning silk from its feet

All spiders spin silk from their abdomens and most use it to catch prey. But for the first time, scientists have discovered a type of spider – the zebra tarantula – that produces silk from its legs, and uses it for climbing.

There is an old joke that if Spider-Man has the powers of a spider, he really ought to be shooting webs from somewhere less salubrious than his hands. In the films and comic books, Peter Parker is empowered with the powers of a human-sized arachnid through a spider bite. He effortlessly scales walls and ceilings and shoots sticky webs from his wrists. Now, scientists have found a type of spider that does just that.

aphonopelma_spp_2.jpgLike Spider-Man, most spiders can climb sheer surfaces and they do so with two techniques. The most obvious are small claws, called tarsi, that grip onto rough surfaces. Going down in scale, their feet also end in thousands of tiny hairs. These hairs make such close contacts with the microscopic troughs and crests of seemingly smooth surfaces that they stick using the same forces that hold individual molecules together. .

But Stanislav Gorb and colleagues from the Max Planck Institute, Stuttgart, have found that one species of spider uses a third method, which exploits that most characteristic of spider traits – silk. All spider species spin silk from appropriately named organs on their rear ends called spinnerets. But uniquely to spiders, the Costa Rican zebra tarantula (Aphonopelma seemanni) from spins silk from its feet as well.

Gorb watched the tarantulas climbing up glass plates, and saw that they left behind silken footprints – dozens of fibres, just a thousandth of a millimetre wide. As the spider climbs, four of its legs leave the glass plate at any one time. As the legs land, they begin to slip but small nozzles secrete a viscous silken fluid that rapidly hardens and adheres to the surface. The silk acts as a tether, firmly holding the spider to the pane.

Spider silk is fantastically versatile and webs alone consist of several different types. As well as catching food, spiders use silk to create egg sacs, protect themselves and even travel by catching the wind. But this is the first time that silk has been documented as a rappelling aid and it throws up some questions about how it came to be used this way.

The tarantula group includes the largest of all spiders, some the size of a human hand. Gorb suggests that they may have evolved an extra source of traction to support their large bulk and prevent harmful falls. Alternatively, rather than being a new innovation, the zebra tarantula’s silken footsteps might reflect how the first spiders spun silk, with the specialised spinnerets only evolving later.

Finding how many species spin silk from their feet and the genes that control this will give the scientists further insights into how this incredible ability evolved.

Gorb, Niederegger, Hayashi, Summers, Votsch and Walther. 2006. Nature 443: 407.