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The nose knows: Olfactory perception in animals

Our golden retriever Jack is always testing the air to make sense of scents. Photo by Dan Wilcox

The great jazzman and comedian Jimmy Durante was known for his schnozzola, a huge zucchini of a proboscis. He said "Da nose knows." Despite his big honker, Durante couldn't detect odors anywhere near as well as a common yard dog.

With my normal-sized human nose, I can smell flowers, coffee, good food, wood smoke and autumn leaves. When my nose isn't plugged up, my sense of smell gives pleasure, evokes long-forgotten memories and signals warnings of pungent things like spoiled food and skunks. Adult humans produce and are attracted to pheromones, odor molecules that attract members of the opposite sex. A huge industry has developed in concoctions that mask our body odors in attractive perfumes. Few people have noses trained enough to distinguish between many different kinds of flowers, coffee or wine. We rely on sight and hearing for most of the information that we get from our environment.

Our golden retriever Jack lives in a world dominated by scents. As our chief of homeland security, he checks out the perimeter of his outdoor territory with his nose and marks it for other dogs to know who lives there. Dogs and their wolf ancestors have a phenomenal ability to detect odors at low concentrations. Dogs have about 200 million scent cells in their noses, compared to about 5 million in human noses. Dogs can detect odors at concentrations as low as several molecules per liter of air. We are often impressed when a hunting dog, running at full tilt through a field, suddenly snaps his or her head around, does a sharp turn and points or flushes a bird hidden in the grass. We are amused or disgusted when dogs greet each other nose to tail.

Most insects rely on chemical scent detection. Ants leave trails of formic acid to guide the nearly blind workers between food sources and the nest. Moths find their mates in the dark by detecting pheromones in the air. We are often amused by the zig-zag paths of flying moths and hunting dogs in the field on the scent of birds. They detect odors, move in the direction of the greater concentration, compare points successively and move in a zig-zag path toward the source.

Fish live in water, a great solvent for chemicals. In addition to sensory receptors that detect acceleration, sound and pressure changes in their hydraulic environment, fish are covered with chemical scent receptors. The late Arthur Hasler of the University of Wisconsin-Madison demonstrated that many species of fish become imprinted as juveniles on the chemical signature of their nursery habitat and migrate back as adults to the same streams to spawn, following minute concentrations of scent in the water.

Fish also produce and respond to pheromones. Sea lampreys are ancient parasites that invaded the Great Lakes through the Welland Canal and devastated the native lake trout populations. Sea lampreys were found in Lake Superior by 1946. The Lake Superior lake trout harvest dropped from an average 4.5 million pounds annually to just 368,000 pounds.

Sea lampreys spawn in streams and the larvae live in the gravel. After screening thousands of chemicals, researchers at the former U.S. Fish and Wildlife Service Fish Laboratory in La Crosse discovered TFM, a chemical effective in killing larval sea lampreys. Application of the lamprey larvicide in 120 spawning streams around the Great Lakes has been effective in controlling sea lampreys, allowing lake trout populations to recover. Application of lamprey larvicide in all those streams costs over $20 million each year.

Researchers at University of Minnesota and Michigan State University have been able to find and recently synthesize a pheromone given off by larval sea lampreys. Sensing the presence of larvae in the tributary streams, adult sea lampreys are attracted there to spawn. The researchers found that they could attract and trap large numbers of sea lampreys with small quantities of the synthetic pheromone, sterilize the males, and then release them to spawn unsuccessfully. This scent-based lamprey control method shows promise to be more ecological and cost-effective than the larvicide.

White-tailed deer are really tuned into their environment with excellent vision, hearing and noses that can detect predators and other deer from far away. Hunters go to extremes to stay downwind of deer and some try to mask or eliminate their own scent. Deer produce and respond to sex pheromones. Bucks throw caution to the wind following their noses after does during rutting season in the fall. Many hunters use concoctions to mask their own scent and to attract bucks.

Enjoy the crisp smells of autumn, but remember that our noses don't begin to detect the myriad of scents that other critters do. Unlike Jimmy Durante, their noses let them really know what's out there.