The Mystery of Smell Smell is one of the most important of the senses, perhaps the most important. It allows animals to find food; recognise harmful substances; detect predators or prey; navigate and communicate with other members of their group; and locate and choose a mate. Scientists call the sense of smell olfaction. Olfaction is believed to be one of the oldest and most universal senses used by living creatures, but science is only now beginning to understand it: Click here for access to images online.
All living organisms are able to detect chemicals in their environment. We call it the sense of smell. Most species rely heavily on their sense of smell. It s one of the most important of the senses, perhaps the most important. It allows them to find food; recognise harmful substances; detect predators or prey; navigate and communicate with other members of their group; and locate and choose a mate. Rice weevil antenna Elephant s trunk
Scientists call the sense of smell olfaction. Olfaction is believed to be one of the oldest and most universal senses used by living creatures, but science is only now beginning to understand it: This a general problem how animals, including ourselves, search for things, says Massimo Vergassola, a professor of physics at UC San Diego. For example, humans were thought to be able to detect about ten thousand different odours. This was accepted for decades until, in 2014, researchers found that we may be able to detect as many as a trillion. In other words, we re much better at sniffing out odours than was previously believed possible. Smell is not all about noses. There are a number of different methods that animals and insects use to smell. Flies have specialised glands on their rears; bees use antennae; ticks have organs on their legs; snakes use their tongue; elephants use their trunk. But they all have a single evolutionary precursor: a common chemical sense that allowed the earliest single-celled organisms to identify food and detect the presence of harmful substances. In scientific terms, olfaction is the ability to take and interpret a chemical sample of the environment. The detectors are called olfactory receptors ; specialised nerve cells that transmit smell information to the brain. They are called olfactory receptors because they were first found in the nose the name comes from olfactorius, Latin for to smell or sniff. The importance and sensitivity of smell varies amongst different organisms. Most mammals and insects have a good sense of smell; most birds don t. This is probably because mammal predators and prey always need to be aware of each other. For example, dogs the descendants of the predatory wolf have an olfactory sense up to a million times keener than that of humans. And bloodhounds, specifically bred to track humans, have noses that are 100 million times more sensitive than humans. And yes, fish can smell as well. Although they inhabit a marine environment, which is less able to rapidly disperse odour molecules than the atmosphere, their olfactory ability is quite well developed. Salmon, for example, use their sense of smell to identify and return to their home waters. The range of chemicals that animal olfactory systems can detect is remarkable, allowing them to discriminate between thousands of different odour molecules at incredible distances. Bears can detect the scent of food at a distance of over 25 kilometres; while a silkworm moth can detect a single molecule of the chemical bombykol. Researchers have shown that olfaction is very similar in almost all animals and insects humans, elephants and moths. But no one knows the underlying cause. It could be that the olfactory system has been inherited from a very ancient common ancestor. Or it could Human smell receptor Tick leg smell receptor
Ant antenna be an evolutionary adaption for similar tasks, for example, mating. But there s no theory yet that completely explains olfactory perception. Olfactory receptors operate as a lock-and-key system. If a chemical odour molecule can fit into a specific lock on the receptor, the nerve cell will respond, initiating a series of biochemical reactions that result in the brain determining the smell. But researchers admit they don t know exactly how this happens, resulting in a number of competing theories. Some suggest that receptors recognise some or all of the shape of the odour molecule. Perhaps the strangest claims that olfactory receptors actually detect the vibrations of odour molecules. Research published in 2014 ranked sharpness of smell in animals in terms of the number of genes involved in olfaction. Humans appear to have better smelling ability than other primates, probably because primates rely more on their welldeveloped vision. Rats have three times as many genes devoted to olfaction than humans, which accounts for their ability to locate nocturnal sources of food. But the king of animal sniffers is the African elephant, which has around 2,000 genes devoted to olfaction. Elephants can distinguish odour molecules that other mammals completely miss. The researchers can t account for this, but point out that the elephant s trunk is a marvel of dexterity which functions like a human hand. Couple this dexterity with a superior sense of smell, and you have a powerful feeding system necessary for such a large animal. In animals, the olfactory receptor is one of the largest known families of genes. This illustrates the importance of smell in the animal kingdom. For example, most moles are virtually blind. So they can t detect food sources by sight. Instead, they smell in stereo: each nostril takes in a different odour, which is interpreted by the brain, allowing the mole to locate food in the same way that stereo eyesight works in other mammals. Without procreation, a species cannot survive. This is where the sense of smell becomes vital. And it throws up some strange animal behaviours. Take the Flehman response. The male lion is one example. After smelling the urine or faeces of another lion or lioness, the big cat will lift his head and pull his lips back in a strong grimace for several seconds. It s not a snarl, but a way of testing the air. The snarl is, in Nasal cavity
fact, increasing the amount of air and the chemicals it contains to a specialised smell organ above the lion s palate. The Flehman response plays an important role in enabling male lions and other mammals to determine whether a female is entering her oestrous cycle and is therefore ready to breed. How do we smell? Humans have about 350 kinds of ORs. Until a decade ago, these were believed to be located in a region of the nasal cavity called the olfactory epithelium. This would seem obvious: we smell through our noses. But researchers have now found that they re all over our bodies: in the heart, liver and skin. The latter appears to be bristling with ORs. Little is known about what these do, or how they came to be scattered around the body in the first place. But clues are gradually emerging: receptors found in the human testes could act as a kind of chemical guidance system that enables sperm cells to lock onto an unfertilised egg. The discovery of chemical compounds called pheremones launched a scent industry devoted to selling a modern version of love potions. Pheremones are small chemical compounds that transmit signals between individuals of the same species in search of a mate. These specific scent molecules were first identified about fifty years ago in both insects and animals. But, despite popular belief, they have not been conclusively found in humans. Instead, we appear to use odour prints, which are a collection of scents derived from sources as diverse as genetics and diet. A human odour print is more of a chemical cocktail than a simple pheromone. But pheromones are immensely important to creatures like insects. For example, how do male moths detect females at distances of up to a kilometre? This has puzzled researchers for a century, and was only solved recently by sophisticated techniques involving physics, numerical simulations and lab experiments. Female moths give off random puffs of pheremones every ten seconds or so. These are dispersed by the wind, so can only be located by males who are downwind and within a particular area the researchers call the cone of detection. Even males within the cone of detection are hard pressed to detect the signals, which last just a few milliseconds and are often near to the limits of the males sensitivity threshold. In other words, insects have amazing detection abilities. Research at the University of California, Riverside in 2013 has led to a breakthrough in understanding how the insect detection system works in practice. It concerns the female mosquito, and how it hunts down humans for a blood meal. It s been known for years that the female, which is a vector for deadly diseases like malaria and West Nile virus, initially locates us by detecting the carbon dioxide in our breath. But what happens next has baffled researchers for years. How does the mosquito then zero in on human skin, especially vulnerable exposed areas like ankles and feet? Since human skin doesn t emit carbon dioxide, there s obviously another detection system at work. And the researchers have found it: on an unexpected area of the mosquito called the maxillary palp. This houses a remarkably complex olfactory receptor that not only detects CO2 but also skin odours including those left smelly socks, worn clothing and bedding: It was a real surprise when we found that the mosquito s CO2 receptor neuron, designated cpa, is an extremely sensitive detector of several skin odourants as well, and is, in fact, far more sensitive to some of these odour molecules as compared to CO2, says Anandasankar Ray, an associate professor in the Department of Entomology and the project s principal investigator. For many years we had primarily focused on the complex antennae of mosqui- Python tongue
toes for our search for odour receptors, and ignored the simpler maxillary palp organs. According to the researchers, since cpa detects all the chemicals that attract mosquitoes, it s a key target. Disrupt or close down cpa, and the chemically-guided precision weapon that is the female mosquito will find it difficult or even impossible to locate a human. We take the sense of smell for granted. But science is only now beginning to discover how it actually works. This is not just curiosity; it is already yielding practical benefits. For example, the amazing chemical detection abilities of insects are being turned against them. By modifying or disrupting their pheromone systems, researchers could limit the ability of disease-carrying pests to mate. Flehman response in lions ENDS 1700 WDS SCIENCE PHOTO LIBRARY 2015 Rat nose blood vessels Click here for access to images online.