Kenneth Catania, of the School of Biological Sciences at Vanderbilt University in Nashville, Tennessee, has discovered a previously unknown behaviour that enables two species of mammals to use their sense of smell underwater.

The star-nosed mole and the water shrew are semi-aquatic mammals; they are primarily terrestrial, but spend a great deal of time underwater foraging for food. Semi-aquatic mammals were believed to be unable to smell underwater, because their sense organs are adapted for land.

Catania first laid down underwater scent trails which led to food. The animals were then filmed with high-speed cameras as they followed the scent trails. It was found that both species exhale small bubbles of air onto objects or a scent trails they encounter while foraging underwater. The bubbles are then drawn back into the nose, so that odorant molecules in the air contained within the bubbles are wafted over the olfactory receptors.

When an earthworm scent was laid down, the five moles and two shrews tested followed it accurately 85% of the time. Fish scents were followed to a similar degree of accuracy. The bubbles exhaled by the mole and the shrew were 0.06-0.1 milliltres in volume. Both species exhaled and inhaled between 8 and 12 bubbles every second.

(Watch a film clip of a water shrew doing the same thing here)

The nose of the star-nosed mole is one of the most sensitive organs in the animal kingdom, and is used like an ultra-sensitive hand, to feel objects and obtain tactile information about them. When a fine mesh was used to prevent the mole’s exhaled bubbles from coming into contact with the scent trails, the accuracy with which the animals followed the scents dropped to about 50%, confirming that the mole can indeed smell by blowing bubbles, and suggesting that it has to come into contact with, or at least come into close proximity to, a scent trail in order to smell it while underwater.

These findings show that the assumtion that olfaction is useless underwater is incorrect, and raise the possibilty that other aquatic mammals also have the ability to smell underwater.