How keen is an elephant's sense of smell?

That was the question addressed by C. Claiborne Ray in her "Q&A" column in the December 4, 2007 New York Times Science section; the exchange follows.

Long story short: I knew it was good, but not that good.

Trunk Line

Q. How keen is an elephant’s sense of smell, and where in the trunk is it handled?

A. The sense of smell is very keen indeed, said Pat Thomas, general curator of the Bronx Zoo. “In fact it has been reported that elephants can smell water several miles away,” he said.

Mr. Thomas distinguished between elephantine olfaction and a similarly keen but separate system of chemical communication. A male elephant will, for example, touch its trunk tip to a trace of female urine and then carry it to the vomeronasal organ at the roof of its mouth to check for mating hormones.

In simple smelling, the handy trunk is waved in the air to collect air samples with odorant molecules and convey them to an extensive sensory system, contained in the elephant’s upper nasal cavity in a complex of scrolled, spongy bones, called turbinates or turbinals. These hold both chemical and olfactory sensors in the form of millions of receptor cells.

“The trunk is used for more than smelling,” Mr. Thomas said. “It is sensitive enough to pick up things as small as a single blade of grass. It is also used in tactile communication, in which it runs its trunk over another elephant’s body or puts the tip into the other elephant’s mouth.”