Gauging age by the smell of urine

00:01 12 March 03

NewScientist.com news service


Mice can recognise their elders by the smell of their urine, a new study shows. This unusual skill could turn out to be very important to mice, as it suggests a mechanism behind one theory of mate choice.

The "good genes" hypothesis proposes that females who prefer older mates do so because longevity may indicate hardy genetic stock. The preference for older mates has been documented in several species.

For example, both sexes of meadow voles are attracted to the smell of older members of the opposite sex.

"There's lots written about the genetic theory behind this," says research leader Gary Beauchamp, at the Monell Chemical Senses Center in Philadephia, Pennsylvania. "But there's a lack of data showing how the animals might detect age or the molecules involved."


Smelly creatures

Beauchamp and his colleagues chose to study age-related smells in mice, because while they have not yet been shown to prefer older mates, they do rely heavily on smells for a variety of social interactions.

The researchers compared urine from reproductively active mice from two age groups: three to 10 months and greater than 17 months. "That would be like comparing men in their 20s and men in their 50s," he says.

Next they were able to train "sensor" mice to find the older or younger urine at the end of a Y-shaped tunnel, suggesting some odorous compound did distinguish old from young urine.

Some chemical detective work identified phenylacetamide and indole as possible suspects, both of which were found to be elevated in the urine of the older mice. And when young urine was spiked with these two chemicals, the sensor mice misidentified it as old urine.

Hard to fake

The physiological significance of the chemicals is not clear, although phenylacetamide is associated with the function of immune cells called granulocytes that proliferate with age.

Intriguingly, since aging is also associated with a decline of the immune system, this would be a change that would be hard for younger animals to fake.

"But I suspect it would have to be sensed in the context of other chemicals as well," notes Beauchamp. "Otherwise animals would also be attracted to sick mates."

Beauchamp now plans to track down the biological origin of the chemicals and test whether their levels also change with aging in other animals. These will include humans, in which young women have been noted to prefer successful older men.

Journal reference: Proceedings of the Royal Society of London B (DOI: 10.1098/rspb.2002.2308)


Philip Cohen