We make up for our limited sensory abilities

Transcription

1 ANIMAL SUPERSENSES When it comes to sensing the world around us, human beings have it pretty poor compared to most of the animal kingdom. For example, our eyes can only detect a fraction of the colours that birds can see and our ears can t hear very high frequencies like bats or dogs. Similarly, our noses need to be bombarded with odours before detecting any scent, compared to moths, which can detect individual molecules. Feature Story 1180

2 We make up for our limited sensory abilities by inventing equipment like sensors, detectors, GPS, sonar, night vision goggles and thermographic cameras. But it seems evolution is one step ahead, endowing bats, birds, fish and even insects with far more efficient natural powers. Taking a closer look at these super senses may help us to build better technology and lead to discoveries about hidden super senses of our own. Touch In habitats where normal vision or hearing might be obscured, an enhanced sensitivity to touch is vital for survival. Most mammals, and some birds, have long hairs on their faces known as whiskers, or vibrissae. These hairs are extremely sensitive to touch. The smallest deflection of the hair shaft causes the nerves at the base to register movement. Some animals, such as mice, are so sensitive they can even detect tiny air movements. For fish, living life underwater mean vibrissae would not be much use. Instead, they have evolved specialised hair cells along the lateral line of their body that can detect water currents or motion. Sharks have evolved an even more sophisticated set of sensing tools. Located within the snout are electrically sensitive cells known as the ampullae of Lorenzini. These cells can detect weak electrical impulses in the water that are generated by the muscle contractions of other fish. The ampullae of Lorenzini are also thought to assist sharks with geomagnetic navigation and sensing water temperature and salinity. Human divers have learned to take advantage of the shark s electrosensitivity by developing electronic shark repellents. Hearing Dogs can hear sounds beyond the range that we humans can hear. Trainers utilise this ability by using dog whistles that emit a high-pitched sound. These whistles have a frequency range of between 16,000 to 22,000 hertz (Hz) - most sounds are inaudible to the human ear beyond 20,000 Hz. At the lower end of the sound spectrum, whales use very low frequency sounds to communicate with each other over hundreds of miles of ocean. Studies of Humpback whale songs reveal that during the mating season, males emit a complex series of repeating sounds with a frequency range from 20 Hz to 10 Hz. Since the human ear can only hear down to 20 Hz, researchers must use special underwater microphones that are sensitive to this range. Whales and dogs certainly possess supersensitive hearing but there are a group of animals that use sound in a completely different way. Dolphins, bats, shrews and oilbirds are able to emit a rapid series of high frequency clicks and listen for the echoes that bounce or reflect off a nearby surface. This amazing ability allows the animal to work out the location and size of objects and prey in complete darkness. The ability is known as biosonar, or echolocation. Bats can emit noises in ultrasound, up to 100,000 Hz, and at speeds of 200 clicks per second, enabli ng them to hunt prey as small and speedy as mosquitoes. Dolphins are also sophisticated users of biosonar. Experiments have shown that they can tell the difference between a golf ball and a table-tennis ball using biosonar to sense the different densities. Echolocation used to be thought of as unique to only a few animals, but the case of Ben Underwood, a blind teenager from the US, who is able to walk, run and even skateboard freely by using vocal clicks and listening to echoes, highlights an amazing example of the potential for hidden super-senses amongst humans. Vision Scientists believe birds have the most advanced visual system of any vertebrate. They see colours over four wavelengths of light, so-called tetrachromatic vision, compared to most human retinas, which can only detect three wavelengths. This fourth light detector is sensitive to the ultraviolet (UV) part of the spectrum, which humans can only detect with special equipment or photographic techniques. UV vision opens up an unimaginably rich visual world that has important implications for survival. To a female pigeon or peacock, the tetrachromatic display shown by the plumage of the male bird reveals subtle patterns and markings that allow her to select the best mate. UV sensitivity can also help when hunting for food. Kestrels hovering high in the air are able to spot the UV-reflecting urine trail of a small rodent and many songbirds are able to spot fruit berries, which reflect UV light strongly. Bees, wasps and other pollinating creatures also have UV vision. They can detect invisible honey guides on flowering plants. Recently, researchers have discovered a tiny minority of humans with a genetic mutation that allows them to see in tetrachromatic vision. Their fourth colour receptor is not UV sensitive like birds, but it still allows them to see colours more richly than the rest of us. Super-sharp vision is the hallmark of birds of prey such as eagles, vultures and owls. Their ability to spot small prey from long distances relies on several visual enhancements. These birds have eyes that face forward (binocular vision) to give overlapping fields of view and great depth. A large lens magnifies and processes light very efficiently. Finally, birds of prey have an extremely dense region of light-collecting cells on the retina. The buzzard, for example, has over a million light sensitive cells five times as many as humans. For humans to see in the dark, we need to rely on high-tech night vision goggles. Nocturnal animals have their own inbuilt night vision system. Owls, for example, have huge eyes in proportion to their head size. The eyes are tubular rather than round, giving telescopic vision, which allows more light to enter and provides a larger surface area on the retina. Owls can also completely open the pupils of their eyes, so that no iris is visible, which maximises the amount of available light hitting the densely packed retinas. Owl night-vision is 100 times more sensitive to low light than our own. But they do have their drawbacks. Owls are long-sighted and the eyes do not move in their sockets. To compensate, they have flexible necks that allow them to turn 270 degrees left or right, or up to 90 degrees vertically. Some owls have an additional eye structure, shared with many other nocturnal animals. This is a highly-reflective layer of tissue behind the retina known as the tapetum lucidum. A car driver at night who catches a rabbit, deer or

3 cat in his headlights will see a pair of glowing eyes. Of course, the eyes do not actually glow. It is the layer of tapetum lucidum tissue behind their retinas that intensifies any available light and reflects it back to the retina. For the predator it enhances night vision to hunt for prey, and for those hunted, it gives them a chance to spot danger. Humans do not have a tapetum lucidum. Heat sense Snakes are one of the most successful animals on the planet. Endowed with poisonous venom and unencumbered by limbs, evolution has gifted some snakes with yet another super sense the ability to see in infrared. Pit vipers are so-called because of pit organs located just below the eye. Each pit contains thermoreceptors - infrared sensing cells which are able to detect prey due to the tiny temperature difference between the animal s body and the cooler environment. The pit organs are more than just simple heat sensors, as the snakes are also able to gauge depth, allowing them to accurately strike their target in complete darkness. The Melanophila acuminata beetle seeks out heat for a different reason to pit vipers. It prefers to lay eggs on the charred remains of wood from forest fires. These trees offer no resistance in the form of sap or sticky fluids that might hinder the beetle. Located on the beetle s underbody are specialised heat-sensing cells known as sensilla that excite nerve cells whenever infrared radiation from a nearby forest fire is detected. Scientists have revealed that the sensilla can locate fires up to 12 kilometres away. These remarkably sensitive natural heat receptors are now being researched by the military and industry to produce better fire detectors and heat-imaging systems. Smell & taste The senses of taste and smell are closely linked in humans - most food tastes are based on aromatic compounds initially detected by the nose. Animals are able to taste the air for subtle odours associated with food or mates by using very sensitive receptors on their vomeronasal organ. This organ, found within the mouth of most vertebrate animals, has to be exposed to the air for the animals to sense the odours. So when cats sneer and raise their lips or when snakes whip out their tongue, they are exposing their vomeronasal organ. Humans also possess a vomeronasal organ but scientists believe that it has lost nearly all of its functioning ability. An exquisite sense of smell is not restricted to vertebrates. Bloodsuckers such as mosquitoes and ticks are incredibly sensitive to the presence of carbon dioxide (CO2) breathed from the lungs of their quarry. Scientists have recently identified the specialised receptor proteins located on the surface of olfactory neurons that detect the minute concentrations of CO2. This discovery could one day lead to new agents to control the spread of disease-carrying insects such as the malaria mosquito. For male moths, love is in the air when their feathery antennas detect tiny amounts of a class of chemicals known as pheromones. Moths don t have noses but instead smell by using their antennas to trap minute quantities of pheromones. Female moths release pheromone compounds from special glands at the tip of their abdomen. Just a couple of molecules of these landing on specialised sensing cells on the male antenna is enough to provoke the male moth into a sexually charged frenzy. Some researchers believe that humans also produce pheromones to attract the opposite sex. Navigation The long distance migration of animals such as sea turtles, songbirds, Monarch butterflies and Atlantic salmon in the quest for new feeding, breeding or sheltering grounds is one of the most spectacular events in nature. Many animal species travel enormous distances. The arctic tern holds the long-distance record, covering 20,000 kilometres in one year from the Arctic to the Antarctic and back again. Young salmon swim hundreds of kilometres from their freshwater spawning grounds to reach the ocean, only to return to exactly the same spot as adults. And Monarch butterflies, although seemingly fragile, are actually confident flyers, able to migrate across almost the entire North American continent. These animals do not have access to compasses, satellite navigation, survey maps, radar or any of the multiple tools that we use. How they manage their amazing journeys without losing their way has baffled scientists until relatively recently. The answer lies with two super sensing abilities polarised light vision and internal magnetic receptors. Polarised light is light that vibrates in one plane relative to the direction of the source. When the sun is low on the horizon at dawn or dusk, there is a band of intense polarised light 90 degrees from the sun passing directly overhead through the zenith which intersects the horizon 90 degrees to the right and left of the sun. The human eye cannot see it, but birds and butterflies can and scientists believe that this intense band of polarised light acts as an accurate reference point for migrating animals. Migratory animals may also use an internal magnetic compass for navigation. Recent

4 research on homing pigeons has discovered that their beaks contain tiny magnetic crystals, known as magnetite, linked directly to nerves. This magnetic receptor acts as a simple but elegant compass that is sensitive to the Earth s magnetic field. It seems that migratory animals use a combination of normal visual cues, such as the sun, stars and landmarks, together with their polarising light and magnetic senses to form a mental map of their route and guide them on their amazing journeys. Conclusion The animal kingdom has evolved some extraordinary sensing abilities. Compared to our own senses, the ability to see in the dark like an owl, sniff the air like a moth or hear long distance like a whale seem like amazing powers. But these abilities have only evolved thanks to the immense pressures exerted by natural selection in the wild. As soon as a predator evolves something useful, an equally useful trait in their prey will be selected for and eventually become the norm leading to a never ending evolutionary cat and mouse arms race. The best we can hope for is that our one and truly unique super-ability our brains can help discover new and exciting forms of technology that can match, or even improve on nature s best senses. ENDS 2150 WORDS Written by Seymour Yang 2007 SCIENCE PHOTO LIBRARY BOX: CAN TECHNOLOGY MATCH NATURE S SUPER SENSES? Animals have evolved an amazing variety of solutions to enhance their senses of touch, taste, hearing, vision and smell. For humans, without these enhancements, could technology provide the answer? 1. Night vision goggles. Nocturnal animals rely on their tapetum lucidum to intensify available light. Technology offers the image-intensifier tube. This tube collects and amplifies infrared and ambient visible light. It converts photons of light into electrons which are multiplied many thousand fold. The electrons hit a screen coated with phosphor, which excites and releases photons to create the green image seen by the wearer. But the goggles are unwieldy and the image quality variable to poor, especially if a sudden bright light hits the tube. Night vision in animals is much superior. 2.Sonar Sonar stands for SOund Navigation And Ranging. A transmitter sends out a low frequency pulse of sound that can travel through air or water. The sound wave reflects off a surface and a receiver listens to the echo. If the speed of the sound is known, the distance to the object can be calculated by timing the echo. It is the same principle used by echolocating dolphins and bats, although technology has a long way to catch up with these animals use of sound. 3.Satellite imaging. A buzzard can spot small prey from a height of 4.5 kilometres. Compared to today s top commercial satellites, which offer images at up to 60 centimetres resolution taken 450 kilometres above the Earth, the buzzard comes off poorly. But imaging satellites cannot see through cloud cover and are very limited in range and coverage by their orbit. The high resolution data also takes time to process. Although both are very different methods of long distance vision, buzzard vision versus satellite vision probably comes out even. 4.Chromatography Compounds such as pigments, petrochemicals, drugs or odours are mixed in a solvent of either gas or liquid. This mixture is passed through a chromatograph and the various components separate due to their differing physical properties. The techniques range from simple paper chromatography, to more complex, such as high performance liquid chromatography (HPLC), but the result is that the separated compounds can be analysed in more detail and identified. It is technology s answer to super sensitive taste and smell but even the best chromatographs have yet to identify much of the mysterious compounds that make up the many pheromones in existence. The tiny moth and mosquito easily beat chromatographs. 5.Voltmeter. Man-made voltmeters respond to electrical stimuli by measurng potential differences in volts. So do the Ampulla of Lorenzini in sharks, only the shark electroreceptors are highly sensitive, being able to respond to just a few microvolts. This, coupled with salinity, temperature and geomagnetic sensitivity makes this a win to the shark. 6.Thermal imaging. Pit vipers and Melanophila beetles can sense their targets using infrared sensing organs. Thermographic cameras operate in the same manner by converting infrared radiation into colour-coded images. The colours indicate temperature variations. For example. a cool 23 degrees Celsius is shown as purple and a warmer 35 degrees Celsius as red. Thermal imaging has a wide range of uses, from finding people in fires to building inspection and surveillance. However, even the best cameras have low resolutions and limited range, making the humble beetle and viper much better at seeing in the infrared. 7.GPS. Global Positioning System (GPS) satellites offer travellers with appropriate radio receivers an accurate pinpoint location of almost anywhere in the world. For mapmakers, land surveyors, polar explorers and transatlantic yachting crew, GPS for navigation is indispensable.

5 For drivers, in-car navigation means wrong directions are much less likely. In terms of accuracy, convenience and accessibility, GPS can definitely be said to match the most impressive navigation skills of long distance migrating animals. 8.Ultrasound. The human ear can hear sound in frequencies ranging from a low 20 Hz to an ear piercing high of 20,000 Hz. Bats easily beat that by hearing ultrasounds up to 150,000 khz. Simple bat detectors can convert bat ultrasound noises into sounds audible to the human ear. But very high frequency sound waves can also produce images that we can see ultrasound images of the unborn foetus is one example. Ultrasound is also used for a variety of medical and industrial diagnostic imaging and even for cleaning complex components. Given the degree of success with using ultrasound frequencies, technology beats nature in this category. 9.UV vision. Ultraviolet radiation can help and hinder human lives. UV rays from the Sun causes skin damage, but UV light is used to help identify fraud, harden new tooth fillings, spot fluorescing proteins in biomedical research and aid forensic examination of crime scenes. Ultraviolet sensors can also look down on Earth to see ozone holes and dust storms or far into space to see cosmic structures that visible light does not show. The diverse array of technology utilising UV light means technology beats nature but wouldn t it be amazing to see the world in tetrachromatic colour as birds do? 10.Touch. The human sense of touch is actually very sensitive to texture and temperature. Witness the fluency of a braille reader to see how human touch can adapt and become very sensitive. But for remote sensing, we need to rely on mechanical technology such as seismographs to detect earthquakes or laparoscopes for delicate medical techniques and robotic arms to handle dangerous materials. The cat s whisker is far more finely attuned to subtle movements and so must beat technology s attempts at enhanced touch. ENDS 915 WORDS 2007 SCIENCE PHOTO LIBRARY

6 FULL PICTURE SET For captions and credits, please refer to the captions.txt file Science Photo Library Feature Stories - Animal supersenses For further information, please contact: seymour@sciencephoto.com All images are copyright, please credit images as stated on the captions

7 For further information, please contact: All images are copyright, please credit images as stated on the captions