Scientists have confirmed what poets have long known: Absence makes the heart grow fonder.

The scientists found that after four days away from their mates, male voles experienced changes in the emotional center of their brains, causing them to become unresponsive and lethargic. When given a drug that blocked the changes, however, lonely voles emerged from their funk.

The same chemical is found in human brains, and scientists said the research could provide insight into treating human grief and separation.

"Whenever you form a pair bond, it changes your neurochemistry," said Larry J. Young, a neuroscientist at Emory University in Atlanta and an author of the study. "If you lose that partner, it has a dramatic impact on the brain."

Experts noted that human relationships are more complex than animal bonds and involve culture, socialization and rational thought. Thus, there may be little to learn from the depressed voles.