Making scents of mother-infant bonding.

Each person has a smell of his or her own. Such characteristic odors are detectable even before birth, according to a study published in the March 28 Proceedings of the National Academy of Sciences. Trained rats can distinguish between samples of a woman's urine taken before and after she gives birth, Gary K. Beauchamp of the Monell Chemical Senses Center in Philadelphia found.

But Beauchamp and his colleagues were unsure how the rats made this distinction. So the researchers tested the rats with samples of the mother's urine during pregnancy and postpartum urine mixed with urine from her child. In fact, the rats could not distinguish between these samples 73 percent of the time.

Yet the rats could simply have been picking up on a general odor

characteristic of any fetus, says Beauchamp. So the researchers also tested the rats using a mixture of urine from a woman and an infant that was not her own. The rats responded to this mixture about 44 percent of the time.

This indicates that the odor type and pertinent genotype of the fetus is present in the mother's urine, says Beauchamp, although a more general fetal smell may also be detectable.

A genetic basis for individual odor types was first discovered in mice, Beauchamp says. Such odors -- often transmitted through urine -- are important in coordinating the animals' reproductive, social, and maternal behavior. The smells seem to come mainly from normal metabolites that happen to be odorous and whose outputs are subject to genetic variation.

"The nose, even the human nose, is a remarkable instrument," says Beauchamp. He suggests that humans detect odors in sweat, saliva, or milk rather than urine.

How much information humans gather from such odors remains unclear, says Richard H. Porter of Vanderbilt University in Nashville. "This is the first evidence [in humans] that odors produced by the fetus, secreted in the mother's urine, alter the odor of the mother herself.... This is theoretically interesting simply because it provides a new channel of information about the fetus itself."

Porter's research shows that human mothers can identify their infants by smell just a few hours after giving birth. Beauchamp's work may offer an explanation: "Perhaps," he says, "such identification has already been learned before birth, by means of odor types exchanged during gestation, representing possibly an important early factor in parent-infant bonding."