Male bats primp daily for odor display.

For the first time, scientists have described the male bat's equivalent of slapping on aftershave before hovering around his harem.

About the same time every afternoon, male sac-winged bats in a colony all settle down to refill pouches on their wings with urine and other bodily secretions, report Christian C. Voigt of Boston University and Otto von Helversen of the University of Erlangen-Nurnberg in Germany.

In the December 1999 BEHAVIORAL ECOLOGY AND SOCIOBIOLOGY, they describe the seduction secrets of Saccopteryx bilineata, a small bat of the New World tropics. Keeping body odor at its most alluring makes sense, they say, for males that fan their fragrance in daily displays for up to eight females.

Males "hover in front of a female like a hummingbird in front of a flower," recounts Jack W. Bradbury of the Cornell University Laboratory of Ornithology. He and his colleagues first described the displays in the tropics in the 1970s but didn't learn the scent's origin. He had suspected that the sacs secreted the perfume but was troubled by a German study finding no secretory cells in sac tissue.

Bradbury welcomes the new paper warmly. "That solves the riddle," he says.

The odor struck Bradbury as "very sweet and spicy--actually a nice smell," he says. Hovering males fan it at females emphatically enough to rustle leaves.

As males hover for up to a quarter of a minute, a pink sac, roughly a half-inch across, opens and closes on each wing. The male chirps, and the female responds, "Screeee."

The odor sacs also seem to play a role in expressive wing shakes directed toward bats outside the harem territory. Bradbury called this gesture salting because it reminded him of someone using a salt shaker.

Voigt and von Helversen recognized the link between odor displays and a grooming routine that they saw in a bat colony in Costa Rica. Starting around 4:30 p.m., a male roosting on a wall licks his wing sacs. Several times, he curls down to take a drop or so of urine in his mouth and then licks the sacs again.

In a second phase, the bat curls down again, pressing his throat against his penis and quivering for a second. Then, he reaches back to transfer a white droplet hanging from his chin into a wing sac. Next, he opens his mouth and squeezes out droplets from what is called a gular gland, rubbing these into the wing sac. He repeats this sequence several times.

Getting the wing odor up to standard takes more than half an hour. "My impression is that it's exhausting," Voigt says.

Males seem to face a considerable competition for females. With DNA fingerprinting, Voigt and von Helversen found ample extra-harem paternity among the offspring.

Dustin Penn, who studies mouse odors at the University of Utah in Salt Lake City, says he'd love to know what female bats learn from sniffing the male brew. Urine holds the potential to reveal a lot to a discerning female, including the male's health. Penn found that female mice respond to a sexy male's urine odor before and after he has flu, but not while he's ill.

Voigt agrees that odor has great potential for communication among mammals. "It's also common in humans, but we don't recognize it," he says.