Which animals have the best sense of smell?
Be they big, small, pointy, flat, pendulous or upturned, noses are funny things. These holey protuberances are prone to drip, twitch, sniff, snort, whistle and sprout hair. And it doesn’t help that they are slap-bang in the middle of the face for all to see. They get poked into things – literally and figuratively – and wherever an animal goes, its nose almost always gets there first. But a sense of smell – a system for detecting and distinguishing airborne molecules – is about more than just noses. Air drawn into the nostrils passes through the nasal cavity, the walls of which are dotted with a multitude of nerve endings. Each of these is equipped with a complex folded protein called a receptor that is configured to fit precisely with molecules of one particular shape, like a lock and key. A perfect fit triggers the sending of a nerve impulse to a region of the brain called the olfactory bulb, which registers the signal, integrates it with others and communicates with other brain centres, so that an animal can use the information to find food, choose mates, distinguish friends from enemies and avoid predators.
As for the best sense of smell, it depends on how you measure it. Is it the one that can distinguish the greatest variety of smells? If so, a study published in 2014 points to elephants. Given all the other uses of an elephant’s trunk – feeding, drinking, showering, shelling peanuts and carrying logs – it’s easy to forget that it is also an organ of smell. The study scoured the genomes of a range of mammals for genes that code for nasal receptor proteins. With 2,000 such genes, each coding for a different receptor, elephants have more than other contenders such as dogs (800) and rats (1,200), though other likely candidates, such as bears, were not assessed.
Another possibility is that the best sense of smell can detect the faintest whiff. By this measure, it’s hard to beat a male silkmoth, which can sniff out a female’s sexual pheromone at a concentration of one molecule per 100 quadrillion (that’s a one followed by 17 zeros) air molecules. And a moth doesn’t even have a nose. How does it smell then? To ruin a perfectly good joke, it has receptor proteins on its antennae.
The size of the olfactory bulb might also be a good measure of an animal’s sense of smell. Among birds, they don’t get any bigger than those of turkey vultures, which rely on scent to detect carcasses from high in the air. Bears, too, are well endowed in this respect, and there is evidence that polar bears can detect seal breathing holes from 3km away. No clear winner then. Perhaps it’s just a case of picking your nose.
“The nasal cavity has a multitude of nerve endings”