How do we remember smells for so long if olfactory sensory neurons only survive for about 60 days? —A. A. Bozorgi, Irvine, Calif.

Donald A. Wilson, a zoology professor at the University of Oklahoma and co-author of Learning to Smell (Johns Hopkins University Press, 2006), replies:

We recognize an old scent, despite having replaced at least a subset of the olfactory sensory neurons that first interacted with that odor, because the overall pattern of activity within the olfactory system remains relatively constant over time.

Olfactory sensory neurons, which sit in the mucus in the back of the nose and relay data to the brain via axons (fingerlike projections that transmit information out from the cell body), are one of an increasingly large number of neuron types that are known to die and be replaced throughout life. Fortunately, they do not all die at the same time, and there are many thousands of olfactory sensory neurons that respond to any given scent.

The 2004 Nobel Prize in Physiology or Medicine went to Linda B. Buck and Richard Axel for their 1991 research showing that there is a huge family of genes that encode proteins called olfactory receptors. One of their important observations was that individual olfactory sensory neurons typically express just one of those genes. That is, signals from a given neuron provide information about odors that activate the specific receptor protein expressed by that cell. In fact, when an olfactory sensory neuron expressing a particular receptor gene dies and a new neuron expressing that same gene matures, the new neuron's axons plug into the same group of olfactory bulb neurons that its predecessor did. This phenomenon results in remarkable pattern stability over years, despite continual rewiring.

A single receptor protein, however, appears to bind (or recognize) many different odors. Thus, rather than having neurons that respond selectively to coffee or vanilla or Bordeaux, most individual cells respond (via their receptors) to submolecular features of the volatile chemicals coming from those objects. For example, an olfactory sensory receptor neuron may respond to a hydrocarbon chain of a particular length or to a specific functional group such as an alcohol or aldehyde.

Therefore, any given sensory neuron will respond to many different odors as long as they share a common feature. The brain (specifically, the olfactory bulb and the olfactory cortex) then looks at the combination of sensory neurons activated at any given time and interprets that pattern; the brain's interpretation is what you perceive as smell. With so many inputs contributing to the formation of a scent pattern, the absence of a small number of constituents does not appreciably change the pattern or the brain's perception.

Why do migratory birds fly in a V formation? —J. F. Bowman, Corte Madera, Calif.

Bruce Batt, who retired in 2007 as chief biologist for Ducks Unlimited, a wetlands conservation group based in Memphis, Tenn., offers this explanation:

There are two well-supported and complementary explanations for why birds fly in formation. (Both V and J structures are typical and readily recognizable flight formations for migratory birds; studies have shown that a J formation is, in fact, more common than a true V-shaped structure.) One way to account for the phenomenon is that followers benefit from a supportive upwash of air created by the lead birds. The other is that regimented flight formation facilitates proper spacing, directional orientation and group communication.

The relative importance of each benefit undoubtedly shifts along with changes in various factors, such as the season of the year or the purpose of an individual flight. During local feeding flights, for example, energy conservation is probably much less important than careful orientation and collision avoidance are. During long-distance migration, on the other hand, each member of the flock gains a great deal by optimizing its position to conserve energy.

HAVE A QUESTION?... Send it to experts@SciAm.com or go to www.SciAm.com/asktheexperts