Star-nosed moles breathe air bubbles back through their nostrils to smell underwater (Image: Kenneth Catania)

Some mammals are able to smell under water, a new study reveals. High speed video footage shows that the star-nosed mole and the water shrew sniff through water by quickly re-inhaling the air bubbles that leave their nostrils.

Based on these counter-intuitive findings, researchers speculate that other semi-aquatic mammals might also have the capacity to pick up on underwater scents.

Biologist Kenneth Catania of Vanderbilt University in Nashville, Tennessee, US, says he became intrigued when he observed star-nosed moles (Candylura cristata) blowing lots of bubbles while they swam. Curious, he measured the rate at which the animals push air out of and back into their nostrils while under water.


They manages to blow and sniff roughly five to 10 times per second – about the same rate that rats inhale and exhale while actively sniffing an odour. Take a look at one of the surprising, slow-motion videos, here (mp4 format).

Catania placed various objects at the bottom of a water tank, including pieces of earthworm and fish. Using a high-speed video camera, he found that the moles blew lots of bubbles that hit these targets when they neared them.

Some of the odour molecules from the objects enter the bubble, and reaches the animal’s nose when it re-inhales the air, Catania explains: “This type of diffusion happens very quickly.”

Through the wire

To be certain that the moles could smell while swimming, he tested their ability to follow a scent trail under water. The biologist smeared the scent of an earthworm across a sheet of Plexiglas and then placed this at the bottom of the water tank.

He also covered the Plexiglas with a wire mesh to prevent the mole from using the odd-shaped fleshy appendages around its nose to touch the trail. Watch a slow-motion video of a star-nosed mole sniffing an underwater scent trail through a wire mesh using air bubbles, here (mp4 format).

The five star-nosed moles followed the underwater scent trail with 85% accuracy, on average. When the mesh was replaced with one that had openings too small to allow air bubbles to pass through, the moles lost their ability to follow the trail.

Food discrimination

Catania says that the ability to smell underwater could give the moles, which have poor sight, a helpful advantage: “It would help the animals discriminate food under water.”

He repeated a similar experiment with two water shrews (Sorex palustris) and found that these animals achieved similar accuracy following underwater scent trails using air bubbles.

“I suspect most semi-aquatic mammals might be able to do this,” Catania says, referring to other animals such as rats.

He stresses, however, that classic marine mammals such as dolphins and whales are not thought to have the ability to smell under water. Previous research has even shown that some whales lack olfactory regions in their brains.

Catania also doubts that people can smell under water because we inhale and exhale so slowly compared with the star-nosed mole. He refuses to say whether he has tested this out himself: “I’ll take the fifth amendment on that.”

Journal: Nature (DOI: 10.1038/nature4441024a)