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Animal supersenses: See like a bee
COVER STORY
Sense and sense ability The human body gives us a small glimpse of the world, compared with the extraordinary senses that have evolved elsewhere in the animal kingdom. Caroline Williams peeks inside the animal mind to see what we’re missing
32 | NewScientist | 20 August 2011
See like a bee When a bee flies into your garden, it doesn’t see what you and I see. Flowers leap out from much darker-looking leafy backgrounds, and they have ultraviolet-reflecting landing strips that show the way to the nectar. Some spiders might even have evolved to exploit these displays, spinning UV patterns into their webs that could work to fool a bee into thinking that it was making a beeline for a tasty treat. If the bee manages to resist the spider’s trap, she finds her way back home by checking the pattern of polarised light in the sky. All this is seen through the
Michael Durham/minden/flpa BJORN RORSLETT/spl
Ultraviolet patterns on a flower’s petals help attract bees’ attention
pixellated window of mosaic vision, with each unit of the insect’s compound eye providing one of the 5000 dots that make up an image. It’s a world of vision that it is difficult to imagine, but we might get some clues from people with aphakia: a condition in which the lens of the eye – which normally absorbs UV light before it can reach the retina – has been removed in surgery or lost in an accident. Bill Stark, an insect-vision researcher at Saint Louis University in Missouri, lost the lens in his left eye after an accident when he was 10 years old. He says he can see UV light as a kind of “whitish blue”, which he would see washing the scenery at a funfair, for example. Because the sight in his left eye is not great, however, he cannot see the subtle patterns in flowers that bees do. Mind you, even if Stark’s vision was corrected his experience of UV could not match that of a bee, says Lars Chittka, a sensory and behavioural ecologist at Queen Mary, University of London. “Bees have a specific UV receptor – humans don’t,” he explains. “These people see UV with their blue receptors, because the sensitivity of our blue receptors extends weakly into the ultraviolet. But humans can’t perceive UV as a separate colour.” The complexity of the bee’s colour system is nevertheless comparable to human vision, since, like humans, they only have three colour receptors – for UV, blue and green, compared with the human > 20 August 2011 | NewScientist | 33
NICK CALOYIANIS/ngs
set-up of blue, green and red. This means that false-colour photographs, in which red has been filtered out and UV has been added in a colour visible to human eyes, gives us a close approximation of the patterns a bee sees. Besides their UV vision, bees can also detect the polarisation of light. “Just like you see red from blue they see one polarity from another,” says Stark. Air molecules in the atmosphere scatter photons to create a pattern of polarised light arranged around the sun, for example (see diagram, right). This helps bees to navigate by the position of the sun even when the sky is cloudy. Since polarised light is measurable using relatively simple detectors, we can again create images of the kind of information they can pick up. If a bee’s-eye view of the world seems alien to our own, it is nothing compared with that of some insects and birds, which have four, five or even six colour receptors, allowing them to perceive colours that it is impossible for us to experience or even imagine. For them, the three-colour world of human vision would be as dull as greyscale.
A polarised view Air molecules in the atmosphere scatter photons to create a circle of strongly polarised light at 90° to the sun. This band moves with the sun throughout the day, allowing bees to use this information to navigate, even when the sun is obscured Polarised light
SUN 90°
90° SUN
THE HORIZON FROM THE BEE’S PERSPECTIVE
Animal magnetism
Sea turtles use the Earth’s magnetic field as a kind of GPS
34 | NewScientist | 20 August 2011
The idea that animals can navigate using their own internal compass is so startling it was once dismissed as pure fantasy. Now there is good evidence that many species – including pigeons, sea turtles, chickens, naked mole rats and possibly cattle – can detect the Earth’s geomagnetic field, sometimes with astonishing accuracy. Young loggerhead turtles, for example, read the Earth’s magnetic field to adjust the direction in which they swim. They seem to hatch with a set of directions, which, with the help of their magnetic sense, ensures that they always stay in warm waters during their first migration around the rim of the North Atlantic. Over time they build a more detailed magnetic map by learning to recognise variations in the strength and direction of the field lines, which are angled more steeply towards the poles and flatter at the magnetic equator. What isn’t known, however, is how they sense magnetism. Part of the problem is that magnetic fields can pass through biological tissues without being altered, so the sensors could, in theory, be located in any part of the body. What’s more, the detection might not need specialised structures at all, but may instead be based
on a series of chemical reactions. Even so, many researchers think that magnetic receptors probably exist in the head of turtles and perhaps other animals. These might be based on crystals of magnetite, which align with the Earth’s magnetic field and could pull on some kind of stretch receptor or hair-like cell as it changes polarity. The mineral has already been found in some bacteria, and in the noses of fish like salmon and rainbow trout, which also seem to track the Earth’s magnetic field as they migrate.
The tug of the south If this is the case, what might a migrating turtle feel as it set off on its 14,000 kilometre jaunt around the North Atlantic? One analogy, says Kenneth Lohmann at the University of North Carolina at Chapel Hill, might be to “imagine swimming while paying attention to two tufts of hair, one on the right side of your head and one on the left. When you go north, neither tuft is pulled. When you go east, the sensation is one of someone gently pulling on the tuft of hair on the left side of your head; when you go west, you feel a tug on the tuft on the right side. And when you go south, both tufts of hair are pulled.” Holding a steady course would be a matter of making
Sense and sense ability The human body gives us a small glimpse of the world, compared with the extraordinary senses that have evolved elsewhere in the animal kingdom. Caroline Williams peeks inside the animal mind to see what we’re missing
32 | NewScientist | 20 August 2011
See like a bee When a bee flies into your garden, it doesn’t see what you and I see. Flowers leap out from much darker-looking leafy backgrounds, and they have ultraviolet-reflecting landing strips that show the way to the nectar. Some spiders might even have evolved to exploit these displays, spinning UV patterns into their webs that could work to fool a bee into thinking that it was making a beeline for a tasty treat. If the bee manages to resist the spider’s trap, she finds her way back home by checking the pattern of polarised light in the sky. All this is seen through the
Michael Durham/minden/flpa BJORN RORSLETT/spl
Ultraviolet patterns on a flower’s petals help attract bees’ attention
pixellated window of mosaic vision, with each unit of the insect’s compound eye providing one of the 5000 dots that make up an image. It’s a world of vision that it is difficult to imagine, but we might get some clues from people with aphakia: a condition in which the lens of the eye – which normally absorbs UV light before it can reach the retina – has been removed in surgery or lost in an accident. Bill Stark, an insect-vision researcher at Saint Louis University in Missouri, lost the lens in his left eye after an accident when he was 10 years old. He says he can see UV light as a kind of “whitish blue”, which he would see washing the scenery at a funfair, for example. Because the sight in his left eye is not great, however, he cannot see the subtle patterns in flowers that bees do. Mind you, even if Stark’s vision was corrected his experience of UV could not match that of a bee, says Lars Chittka, a sensory and behavioural ecologist at Queen Mary, University of London. “Bees have a specific UV receptor – humans don’t,” he explains. “These people see UV with their blue receptors, because the sensitivity of our blue receptors extends weakly into the ultraviolet. But humans can’t perceive UV as a separate colour.” The complexity of the bee’s colour system is nevertheless comparable to human vision, since, like humans, they only have three colour receptors – for UV, blue and green, compared with the human > 20 August 2011 | NewScientist | 33
NICK CALOYIANIS/ngs
set-up of blue, green and red. This means that false-colour photographs, in which red has been filtered out and UV has been added in a colour visible to human eyes, gives us a close approximation of the patterns a bee sees. Besides their UV vision, bees can also detect the polarisation of light. “Just like you see red from blue they see one polarity from another,” says Stark. Air molecules in the atmosphere scatter photons to create a pattern of polarised light arranged around the sun, for example (see diagram, right). This helps bees to navigate by the position of the sun even when the sky is cloudy. Since polarised light is measurable using relatively simple detectors, we can again create images of the kind of information they can pick up. If a bee’s-eye view of the world seems alien to our own, it is nothing compared with that of some insects and birds, which have four, five or even six colour receptors, allowing them to perceive colours that it is impossible for us to experience or even imagine. For them, the three-colour world of human vision would be as dull as greyscale.
A polarised view Air molecules in the atmosphere scatter photons to create a circle of strongly polarised light at 90° to the sun. This band moves with the sun throughout the day, allowing bees to use this information to navigate, even when the sun is obscured Polarised light
SUN 90°
90° SUN
THE HORIZON FROM THE BEE’S PERSPECTIVE
Animal magnetism
Sea turtles use the Earth’s magnetic field as a kind of GPS
34 | NewScientist | 20 August 2011
The idea that animals can navigate using their own internal compass is so startling it was once dismissed as pure fantasy. Now there is good evidence that many species – including pigeons, sea turtles, chickens, naked mole rats and possibly cattle – can detect the Earth’s geomagnetic field, sometimes with astonishing accuracy. Young loggerhead turtles, for example, read the Earth’s magnetic field to adjust the direction in which they swim. They seem to hatch with a set of directions, which, with the help of their magnetic sense, ensures that they always stay in warm waters during their first migration around the rim of the North Atlantic. Over time they build a more detailed magnetic map by learning to recognise variations in the strength and direction of the field lines, which are angled more steeply towards the poles and flatter at the magnetic equator. What isn’t known, however, is how they sense magnetism. Part of the problem is that magnetic fields can pass through biological tissues without being altered, so the sensors could, in theory, be located in any part of the body. What’s more, the detection might not need specialised structures at all, but may instead be based
on a series of chemical reactions. Even so, many researchers think that magnetic receptors probably exist in the head of turtles and perhaps other animals. These might be based on crystals of magnetite, which align with the Earth’s magnetic field and could pull on some kind of stretch receptor or hair-like cell as it changes polarity. The mineral has already been found in some bacteria, and in the noses of fish like salmon and rainbow trout, which also seem to track the Earth’s magnetic field as they migrate.
The tug of the south If this is the case, what might a migrating turtle feel as it set off on its 14,000 kilometre jaunt around the North Atlantic? One analogy, says Kenneth Lohmann at the University of North Carolina at Chapel Hill, might be to “imagine swimming while paying attention to two tufts of hair, one on the right side of your head and one on the left. When you go north, neither tuft is pulled. When you go east, the sensation is one of someone gently pulling on the tuft of hair on the left side of your head; when you go west, you feel a tug on the tuft on the right side. And when you go south, both tufts of hair are pulled.” Holding a steady course would be a matter of making