Detecting chemicals in the environment enables an individual to identify molecules secreted by predators or prey, potential poisons and sexual partners.
This benefit to survival and reproduction helps explain why smell – olfaction – is probably the oldest sense and evolved 500 million years ago, around the origin of animals.
- Which animal has the biggest nose?
- How do butterflies smell? And what about moths?
- Which animal has the best sense of smell?
What’s the difference between taste and smell?
The difference between taste and smell is down to whether a chemical is already dissolved in water or if it is airborne. Your tongue’s tastebuds identify five flavours (bitter, salty, sweet, sour and savoury/umami) in liquids, while your nose’s sensors can detect numerous odour molecules – odourants – that are airborne before they dissolve in the moist lining of your nasal cavity. But that’s for terrestrial species. For aquatic creatures such as fish, all molecules are in water so there’s no real distinction between gustation (taste) and olfaction (smell).
How does olfaction work?
The characteristic that we call a ‘smell’ or scent can be based on either a specific molecule (such as menthol or vanillin) or a combination of multiple odourants. An odour is detected when an odourant binds to its matching olfactory receptor on a sensory neurone (nerve cell). These nerve endings send signals via a neural circuit so only useful information is relayed to the brain. A high concentration of odourants will stimulate plenty of receptors, for example, signalling that an animal should avoid or be attracted to the source (such as a predator or prey).
What happens in invertebrates?
Arthropods use sensory appendages on their heads. Winged insects, such as fruit flies, have thousands of olfactory receptors in their mouthparts and antennae, while decapod crustaceans, such as lobsters, detect odours via short antennules. Invertebrates and vertebrates use distinct systems but the overall circuitry is similar: numerous sensory neurones lead to a hub (the insect ‘mushroom body’ or vertebrate ‘piriform cortex’) that pre-filters signals for the brain. Just as insects have a compound eye for vision whereas vertebrates use a single lens, olfaction has arisen independently in the two groups by convergent evolution.
How do vertebrates smell?
Fish have nostrils leading to olfactory ‘rosettes’ covered in odour receptors. They must force water toward those rosettes by pumping (using muscles or tiny oar-like cilia) or swimming because odourants travel too slowly via water currents. Amphibians mimic fish and flood their nasal cavity while in water, but on land they act like reptiles, swallowing air to detect odours. Mammals have a dedicated organ for smell, the nose, which contains scroll-shaped bones called turbinates that direct inhaled air towards millions of receptors on the nasal cavity’s surface. Most vertebrates also have an accessory olfactory system with a vomeronasal organ (or Jacobson’s organ) to detect chemicals such as pheromones, typically via the mouth – as seen when a snake flicks its tongue in and out.
Which animals have the best sense of smell?
Smell is usually measured by the number of genes for making olfactory receptors. Fish have about 100 but their genes are diverse so they can sense a wider range of odours. Mammals have more genes, which suggests their smell is more sensitive. Elephants hold the record with almost 2,000 olfactory genes. Humans have about 400 and dogs 800 genes, but contrary to popular belief, our olfaction isn’t so bad: we can detect geosmin, a molecule that gives soil its earthy odour, at a concentration of 5 parts per trillion (equivalent to one drop in four Olympic swimming pools).
And which animals have the worst sense of smell?
Toothed whales, such as the bottlenose dolphin, possibly have the worst sense of smell, as they have only a dozen or so olfactory genes and little neural circuitry for smell. These features were lost after the ancestors of cetaceans (hoofed mammals) evolved from living on land. Primates like us have poor olfaction compared to other terrestrial mammals in general because we map our vibrant surroundings using colour vision. Smell is useful but, to adapt, it sometimes makes sense to invest in the other senses.
Main image: African bush elephant/Getty
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