Their highly sensitive noses not only enable creatures to survive in the wild, but can be used to help humans too.
AS THE SEPTEMBER sun sets, a hedgehog emerges from the safety of the hedgerow and begins to move out into the long grass nearby; shuffling along in its characteristic manner at the start of a nocturnal foraging expedition. It is early autumn, and the late evening air is awash with scent: the last flowers of summer, the hawthorn and elderberries, and the honeyed smell of one of the latest flowering plants, the ivy; a magnet for honeybees and hoverflies at this time of year and throughout the coming season. Like many mammals, the sense the hedgehog uses more than any other is smell.
With their small, beady eyes, hedgehogs have relatively poor sight, especially colour vision. Although their hearing is acute, they mainly rely on their sense of smell to find food and avoid danger. To enhance their ability to pick up different scents, hedgehogs walk with their head up and snout held in the air; sniffing regularly to detect the specific smells around them. Nearby, a fox has left its earth to hunt for food.
Like hedgehogs, foxes also have an acute sense of smell and regularly mark their territory with urine, which sends a clear message to rivals to keep their distance. A rabbit has picked up the presence of the fox by twitching its nose. Helped by special folds inside their nostrils that magnify and enhance scents from outside, this nose-twitching improves the rabbit’s ability to detect any imminent danger, giving it enough time to escape down into its burrow before the fox can get too close. At this time of autumnal plenty, a squirrel is hoarding nuts in the woodland close by; burying them under the leaf litter to create a cache.
This will be an insurance policy against a dearth of food later on. During the winter, when a fall of snow may mean that food is hard to come by, the squirrel will relocate the nuts using both its excellent spatial memory and also its acute sense of smell.
Heightened for survival
Smell is in many ways the primary sense for mammals, due partly to the fact that a large number of species are primarily nocturnal in habits, and unless there is a full moon illuminating the scene, sight is not terribly useful after dark. Scientists have recently discovered that this heightened sense of smell evolved approximately 200 million years ago, during the dinosaur age, when the earliest mammals used it to exploit a new ecological niche. Hunting by smell enabled these tiny creatures, which were very similar to modern-day shrews, to find insects and other invertebrates by night.
By avoiding being out and about in the day, they ensured that they were not preyed upon and eaten by much larger carnivorous dinosaur species. By contrast, birds, which evolved from a common ancestor with the dinosaurs, mostly have a rather poor sense of smell. Instead, they rely on their excellent sight to find food and avoid danger. While birds of prey in particular have very advanced sight that is up to 10 times as acute as that of humans, they generally have a poor sense of smell. However, there are exceptions to this general rule. The woodcock is a very unusual wading bird in that it does not wade, lives in woods and forests, and is mainly active by night. Woodcocks find their food by probing deep into the soft earth on the forest floor with their long bills, mainly after dark. Yet this is not something they do randomly, but instead are able to locate worms using their excellent sense of smell, even before they insert their bill into the ground. Another group of birds that also uses the sense of smell to find food is the seabirds.
On the open ocean, there are no regular, reliable sources of food: instead, it can be found in random concentrations almost anywhere, making it very hard to locate. So, for oceanic species, such as the storm petrel and Manx shearwater, a good sense of smell is essential, allowing them to find food from up to 12 miles away, simply by picking up its scent drifting on the wind. Birdwatchers hoping to see these elusive species have learned to take advantage of this ability by using a technique known as ‘chumming’, which involves pouring a smelly mixture of fish guts over the back of the boat to attract seabirds for close-up views. Seabirds also use their sense of smell to find their way across these huge, often featureless, bodies of water. They do so by creating what scientists call an ‘olfactory map’ of the ocean, which allows them to navigate the thousands of miles between their feeding and breeding grounds.
Repulsing and attracting
Insects, too, use smell in a wide range of different and often unexpected ways. Ladybirds secrete a yellow-coloured liquid, which smells horrible, through the joints of their exoskeleton, to put off any potential predators. But other insects use the sense in a more positive manner.
Female moths emit strong-smelling chemicals, known as pheromones, from a gland in their abdomen, which they use to attract a mate. The amount used is absolutely tiny: just one nanogram; equivalent to one-billionth of a gram, but it is enough for the male to sense it from several hundred metres away. Many nectar-bearing flowers have a very strong scent: they use this to attract insect pollinators, such as bumblebees and butterflies; the latter using the club-shaped sensors on the end of their antennae to detect scents. Butterflies then feed on the sweet nectar and inadvertently pick up pollen on their bodies, which they deposit when they move to feed on other flowers, enabling the plants to reproduce. Some plants, including the familiar honeysuckle, emit a more concentrated scent at dusk, in order to attract night-flying moths.
Flies have sensory hairs on their feet, which ‘smell’ whatever object they land on, so that they can decide whether or not it is something they can eat.
‘Tasting’ the smell
When a snake flicks its long, forked tongue in and out of its mouth, it is using it to capture tiny molecules of scent. When the snake withdraws its tongue back inside, it uses the receptors of its Jacobson’s organ, in the roof of its mouth, to allow it to determine the meaning of each scent. Lizards can do the same. Amphibians, such as frogs and toads, have similar tiny openings, also on the roof of their mouth, which are used for breathing and to detect different smells; both to find food and avoid danger. Sea-going creatures, such as sharks, have a highly developed and sophisticated sense of smell, allowing them to detect food from some distance away. Smell is also crucial for fish that live in the deepest parts of the ocean, where light is almost non-existent. Deep-sea fishes use it to find food, avoid predators and detect the presence of a mate.
Oddly, however, marine mammals, such as whales, dolphins and porpoises, have a very poor or even non-existent sense of smell, as they mostly lack olfactory organs in their brain.
Rainy day scents
Another way in which animals use their sense of smell to great effect is during changes in the weather. When it rains, the soil gives off a substance known as geosmin, which has a distinctive earthy smell, detectable by many creatures. Again, this helps them to find new sources of food, especially when rain comes after a long period of drought, and signals new growth and so a glut of food.
It is often assumed that because other wild creatures use their sense of smell in so many varied and useful ways, that this sense is somehow superior to our own. And indeed, it is true that we tend to rely on our sight and hearing in much of our lives. But our sense of smell allows us to detect harmful scents, such as airborne chemicals or rotten food. On a more positive note, it is also essential in enhancing our ability to taste, as anyone who has tried eating while suffering from a badly blocked nose will be aware of. Pleasurable aromas, such as freshly baked bread, newly mown grass or the heady fragrance of flowers, would also be lost without our sense of smell, and its importance in our lives is something that should not be overlooked.
A NOSE FOR THE JOB
Like their wild counterparts, domestic dogs have a very advanced sense of smell; indeed, they use this throughout their waking hours to make sense of the world around them. Compared with the human brain, the part of the dog’s brain devoted to smell is 40 times larger, enabling them to detect specific smells more than 1,000 times better than us. Their nose contains more than 200 million olfactory receptors, compared with our mere 5 million.
This means they can detect differences in scent between not just other dogs, but people too. That is why dogs are used to detect drugs and explosives, and to find people; and why when a dog meets either another dog or a new person, it immediately sniffs them. They can even detect human emotions, such as fear or anxiety, by picking up on the scent produced by adrenaline. Their ability to recognise even subtle changes in their owner’s scent has led to them being trained as alert and support animals for people with conditions such as diabetes and epilepsy.
Of all the many breeds of domestic dog, some have a better sense of smell than others: notably the bloodhound, German shepherd and beagle. It has recently been found that when they come across a new smell, dogs initially sniff using their right nostril, but once the smell becomes more familiar, they switch to using their left nostril. However, when the smell is an unpleasant one, it was found that they continued to use their right nostril to sniff it.
While cats also have an acute sense of smell, they can go one step further due to their vomeronasal, or Jacobson’s organ, located in the roof of the mouth. The cat uses this organ to analyse pheromones, which provide it with important mating, social and territorial messages. When these scents collect in its mouth, the cat uses its tongue to flick the pheromones upwards to its vomeronasal organ. As it does so, its upper lip curls, and its mouth slightly opens in a facial expression known as the flehmen response. Cat owners may have witnessed their own pets doing this.