Sir

In the opening sentence of his excellent Brief Communication “Underwater 'sniffing' by semi-aquatic mammals” (Nature 444, 1024; 2006), Kenneth C. Catania states that mammals cannot smell underwater because it is impossible to inspire air. It is true that there is no air underwater; however, there has been a long debate about whether air is actually necessary to smell.

Ernst Heinrich Weber, a German physician who pioneered experimental psychology in the nineteenth century, heroically filled his nostrils with eau-de-Cologne diluted in water, and reported that he could not perceive the distinct smell of the dilution. Weber concluded that odours can only be smelled in air.

This was the reigning wisdom for the next 40 years, until Eduard Aronsohn repeated Weber's experiment (Arch. Physiol. 321–357; 1886). Aronsohn reported a “horrible explosion of the most unpleasant and painful sensations in the nose” after filling it with diluted eau-de-Cologne. He learned from the experience, and from then on used a warm sodium chloride solution instead of cold water.

Aronsohn continued to do experiments on himself — and on colleagues and friends — with clove oil, camphor, eau-de-Cologne, coumarin and vanillin. He came to the conclusion that all odours could be smelled when he filled his nose with a dilution of each one in salt water.

Of course, this ability won't help humans to follow an earthworm scent trail in a river, as it does the ingenious star-nosed mole. But at least it allows us and other mammals to smell in the absence of air in the womb (B. Schaal, L. Marlier and R. Soussignan Chemical Senses 25, 729–737; 2000).