Every time you put on some music or listen to a speaker’s words, you are party to a miracle of biology – the ability to hear. Sounds are just waves of pressure, cascading through sparse molecules of air. Your ears can not only detect these oscillations, but decode them to reveal a Bach sonata, a laughing friend, or a honking car.
This happens in three steps. First: capture. The sound waves pass through the bits of your ear you can actually see, and vibrate a membrane, stretched taut across your ear canal. This is the tympanum, or more evocatively, the eardrum. On the other side, the eardrum connects to three tiny well-named bones—the hammer, anvil and stirrup—which link the air-filled outer ear with the fluid-filled inner ear.
The bones perform the second-step: convert and amplify. They transmit all the pressure from the relatively wide eardrum into the much tinier tip of the stirrup, transforming large but faint air-borne vibrations into small but strong fluid-borne ones.
These vibrations enter the inner ear, which looks like a French whisk poking out of a snail shell. Ignore the whisk for now – the shell is the cochlea, a rolled-up tube that’s filled with fluid and lined with sensitive hair cells. These perform the third step: frequency analysis. Each cell responds to different frequencies, and are neatly aligned so that the low-frequency ones are at one end of the tube and the high-frequency ones at another. They’re like a reverse piano keyboard that senses rather than plays. The signals from these cells are passed to the auditory nerve and decoded in the brain. And voila – we hear something.
All mammal ears work in the same way: capture sound; convert and amplify; and analyse frequencies. But good adaptation are rarely wasted on just one part of the tree of life. Different branches often evolve similar solutions to life’s problems. And that’s why, in the rainforests of South America, a katydid—a relative of crickets—hears using the same three-step method that we use, but with ears that are found on its knees.
Of all the adjectives you could use to describe a crocodile’s face, “sensitive” might not be an obvious one. But their huge jaws, pointed teeth and armoured scales belie a surprising secret. Their faces, and possibly their entire bodies, are covered with tiny bumps that are far more sensitive than our own fingertips.
The bumps are obvious if you look carefully. Each one is a small dome, barely a millimetre wide, surrounded by a groove. There are around 4,000 of them on an alligator’s jaws and inside its mouth. Crocodiles and gharials also have the bumps on virtually every scale of their bodies, giving a total of around 9,000. (All of these animals are called crocodilians.)
Here’s an amazing fact: Adult robins have a magnetic compass in their right eye that allows them to sense the direction of the Earth’s magnetic field, and navigate when all other landmarks are obscured. Here’s an even more amazing fact: Baby robins have two such compasses, one in each eye. They lose the left one as they grow up.
Robins kick-started the study of magnetic senses in the first place. In the 1950s, a German biologist called Hans Fromme showed that robins would always try to escape from a cage in the same direction when it came time to migrate. Even though they had no visual bearings, they headed south-west, as if sunny Spain lay just beyond their cages. In 1966, the husband and wife team of Wolfgang and Roswitha Wiltschko showed that a powerful magnet could disrupt this constant vector, sending them skittering in all sorts of directions.
The Wiltschkos have been studying the magnetic sense of robins ever since. In the 1980s and 1990s, they showed that their compass depends on light. They need some of it, and blue-green wavelengths in particular, to find their way. And in 2002, they showed that the compass lies in just one eye – the right one. If they wore a one-sided goggle that blocked their left eye, they could navigate just fine within their featureless cages. If their right eye was blocked, they headed in random directions. It’s not just robins. They right-eye compasses that the Wiltschkos discovered also exist in Australian silvereyes, homing pigeons and domestic chickens.
A migrating robin can keep a straight course even when it flies through a cloudy night sky, devoid of obvious landmarks. That’s because it can sense the Earth’s magnetic field. Something in its body acts as a living compass, giving it a sense of direction and position.
This ability – known as magnetoreception – isn’t unique to robins. It’s been found in many other birds, sharks and rays, salmon and trout, turtles, bats, ants and bees, and possibly cows, deer and foxes. But despite more than 50 years of research, the details of the magnetic sense are still elusive.
Unlike light, sounds or tastes, which come and go, the Earth’s magnetic field is always ‘on’. To study how animals sense it, scientists first have to cancel it out using magnetic coils, and set up their own artificial field. The field also pervades the entire body, so there’s no obvious opening, like an eye socket or ear canal, where a magnetic sensor would most likely lie.
In birds – the best-studied of the magnetic-sensing animals – scientists have narrowed down the location of a possible sensor to the eye (sometimes, just the right one), the beak, and possibly the inner ear. But it’s been far trickier to find the individual cells responsible for sensing magnetic fields. Now, Stephan Eder from the Ludwig-Maximilians-University in Munich has developed a way of doing that. It’s deceptively simple: look at cells under a microscope surrounded by a rotating magnetic field, and spot the ones that start to spin.
In a swarm of buzzing mosquitoes, every insect probably looks the same to you. You wouldn’t notice that some have swollen abdomens, engorged with red blood, while others are hungry and empty. You wouldn’t differentiate between the antennae of the males (fluffy) and the females (straight). But there is one animal that can spot all of these traits, using eyes that have lower resolution than yours and a nervous system that’s far simpler.
E.culicivora is an East African jumping spider that feeds on mammal blood. Don’t worry: it’s not going to bite you. This indirect vampire only attacks mosquitoes that have recently bitten mammals, and it’s an incredibly discerning diner.
Jumping spiders are famously fussy anyway. They sit and wait for just the right victim to come along, spotting them with large eyes and pouncing upon them with well-judged leaps. Some eat other spiders, but only eat certain species. E.culicivora stalks mosquitoes, but it only female malarial mosquitoes that have recently fed. It ignores: males; individuals that aren’t full of blood; and insects of the wrong species (including other mosquitoes).
“It is the pickiest predator that we know of,” says Ximena Nelson, who studies the spider at the University of Canterbury in New Zealand.
To be that choosy, the spider must have very keen senses. Smell clearly plays a role (the spider is drawn to the odour of both bloody mosquitoes and human feet, and they themselves smell sexier once they’ve drunk some blood). But vision is important too. Even if all scents are blocked, E.culicivora can still pounce on exactly the right kind of prey. Now, Nelson, together with Robert Jackson, has worked out the visual cues that it uses.
They confronted captive spiders with lures built from body parts of dead mosquitoes, which had been glued together in different combinations like miniature Frankenstein’s monsters. The spiders saw two lures at a time, and Nelson noted which they pounced upon. “They are easy-to-handle, patient spiders,” she says. “Being so picky, it means we can ask them questions and get answers regarding their preferences that makes it seem like they answered in English.”
Nelson and Jackson found that the spiders always went for mosquitoes with blood-filled abdomens, rather than empty or sugar-filled ones, no matter which head had been stuck on top. The head matters too, though. When given a choice between two lures with bloody abdomens, the spiders picked the one with a female head rather than the one with a male head.
To check that the spiders weren’t relying on the smell of the lures, Nelson also showed them virtual mosquitoes on a screen. Again, they were more likely to pounce on virtual prey with female antennae than identical ones with male antennae. Human eyes would find it hard to tell the difference. The spiders’ eyes (and it has four pairs) have no such problem.
Having worked out the cues it uses, Nelson and Jackson are working to build the spider’s “decision tree”: the mental steps it makes in order to decide whether to pounce or hold. For now, all we know is that these preferences are innate. No learning is required. The spider appears to be born with some mental template of the ideal mosquito.
This feat is all the more impressive because the spider’s eyes and brain are so simple. The front pair is the largest and most sensitive, but even they probably only have a thousand or so receptors. The young spiders, which are just as fussy as the adults, probably just have 300 receptors per eye.
It seems hard to believe that with so few receptors these spiders can achieve that level of visual detail,” says Nelson. She says that the spider’s receptors are packed tightly in the central part of its eye, so it might be possible for it to see in extreme detail for a small part of its visual field. It probably also processes the information from its eyes in sophisticated way, but no one yet knows how it, or other jumping spiders, do this.
Reference: Nelson & Jackson. 2012. The discerning predator: decision rules underlying prey classification by a mosquito-eating jumping spider. Journal of Experimental Biology http://dx.doi.org/10.1242/jeb.069609
Images all by Robert Jackson
More on amazing spiders:
In the 1940s, visitors watching football games at Berkeley’s Californian Memorial Stadium would often be plagued by beetles. The insects swarmed their clothes and bit them on the necks and hands. The cause: cigarettes. The crowds smoked so heavily that a cloud of smoke hung over the stadium. And where there’s smoke, there’s fire. And where there’s fire, there are fire-chaser beetles.
While most animals flee from fires, fire-chaser beetles (Melanophila) head towards a blaze. They can only lay their eggs in freshly burnt trees, whose defences have been scorched away. Fire is such an essential part of the beetles’ life cycle that they’ll travel over 60 kilometres to find it. They’re not fussy about the source, either. Forest fires will obviously do, but so will industrial plants, kilns, burning oil barrels, vats of hot sugar syrup, and even cigarette-puffing sports fans.
The beetles find fire with a pair of pits below their middle pair of legs. Each is only as wide as a few human hairs, and consists of 70 dome-shaped sensors. They look a bit like insect eyes. In the 1960s, scientists showed that the sensors detect the infrared radiation given off by hot objects. Each one is filled with liquid, which expands when it absorbs infrared radiation. This motion stimulates sensory cells and tells the beetle that there’s heat afoot.
The world’s largest animals have been hiding something. The bodies of the giant rorqual whales—including the blue, fin and humpback—have been regularly displayed in museums, filmed by documentary makers, and harpooned by hunters. Despite this attention, no one noticed the volleyball-sized sense organ at the tips of their lower jaws. Nicholas Pyenson from the Smithsonian Institution is the first, and he thinks that the whales use this structure to coordinate the planet’s biggest mouthfuls.
Of all the super-senses that animals possess, the ability to sense the Earth’s magnetic field must be the most puzzling. We’ve known that birds can do it since the 1960s, but every new attempt to understand this ability – known as magnetoreception – just seems to complicate matters even further.
Take the latest discovery. Le-Qing Wu and David Dickman from the Baylor College of Medicine have found neurons in a pigeon’s brain that encode the properties of a magnetic field. They buzz in different ways depending on how strong the field is, and which direction it’s pointing in.
This is a big step. Scientists have identified parts of the brain that are important for magnetoreception, but no one has managed to nail down the actual neurons responsible for the sense. Miriam Liedvogel, who studies magnetic senses, calls it “a milestone in the field”. It’s a key puzzle piece that has been unavailable for a very long time.
But Wu and Dickman’s discovery doesn’t solve the magnetoreception puzzle. If anything, it makes it more complex. Until recently, scientists thought that birds had two separate magnetic detectors – one in the eye and one in the beak. And it looks like the new magnetic neurons don’t hook up to either of these. “We can’t say where the signals come from,” says Dickman.
If these neurons are responding to magnetic fields, which part of the bird is feeding them their information? Is there a third sensor?
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.
Vultures have among the sharpest eyes of any animal.
Vultures are among the birds most likely to crash into wind turbines and power lines.
If their eyes can spot a tiny carcass from high up in the air, why can’t they see a massive metallic structure looming in front of them? Because they can’t. Vultures, it turns out, have large blind spots above and below their heads. And because they hold their heads at a downwards angle when they fly, they are blind to everything directly in front of them.
I covered this story for Nature News. Head over there to find out why these blind spots exist, and what we can do to prevent vultures crashing into wind farms (featuring “vulture restaurants”).
Photo by M. Mirinha/STRIX