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Planet Earth

These Otters May Smell Prey as They Swim

Most mammals can’t smell underwater, but Eurasian otters may have a way of sniffing out prey even while swimming.

By bioGraphicDecember 24, 2019 4:00 PM
Otter scent
(Credit: Greg Lecouer)

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Although most Eurasian otters (Lutra lutra) spend their time scouring inland river bottoms for prey, those along the coast of Scotland’s Shetland Islands are able to take advantage of the kelp forest’s oceanic bounty. More than a thousand of the semi-aquatic mammals live here, making the Isles’ coastline home to one of the densest otter populations in Britain.

Plunging into the frigid waters of the kelp forest, this particular diver is likely browsing for a crab or fish to take back to its family — and it’s potentially employing an unexpected sense to track that prey down.

The ocean is a wonderfully smelly soup compared to dry air, because so many more substances can dissolve in water. It’s no surprise, then, that a sense of smell would be a useful tool for animals living in the sea.

Ancient fish were the first in evolutionary history to develop nostrils with olfactory receptors, which are proteins that match up, like puzzle pieces, with certain molecules in the environment. When the right molecules come into contact with these receptors, they trigger neural signals that are interpreted as particular smells, which can help an animal identify the scent trails of prey, the smell of nearby predators, even the chemical cues of potential mates. Early mammals developed the sense of smell some 700 million years ago — and that trait, which has its origins in the ocean and has been passed down and modified over countless generations, persists in many mammalian noses today.

The rather inconvenient link between the nose and the lungs, however, means that most mammals have lost the ability to smell underwater, since inhaling a breath full of water has obvious drawbacks. Indeed, even mammals that spend their lives in the ocean, such as toothed whales, have entirely lost the sense of smell. But there are exceptions.

A 2006 study found that some semi-aquatic mammals can sample odors underwater by exhaling tiny bubbles from their nostrils, which capture smells from the surface of an object, and then quickly sniffing them back in again. The initial research focused on star-nosed moles (Condylura cristata) and water shrews (Sorex palustris), but an informal experiment with river otters suggests they might use the same technique, especially since they generally hunt at night when sight is of little use.

After capturing a surprise portrait of this particular otter, photographer Greg Lecouer noticed another otter flopping out of the waves with a squirming octopus clenched in its teeth. The skilled predator may or may not have used its nose to source this delicacy, but it almost certainly enjoyed a feast for its nostrils once the meal commenced on land.

This story originally appeared in bioGraphic, an online magazine featuring beautiful and surprising stories about nature and sustainability.

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