Love Chirps. Female pandas make a high-pitched chirping call, which lets males know they are ovulating.

Adam K Thompson

How to Woo a Panda

There's one sound certain to make a male panda swoon: the high-pitched chirp of a female panda. Although the noise may not be appealing to us, researchers have determined that it's music to the ears of male pandas, as females make it only once a year when they are ready to mate. The discovery is the first instance of a nonhuman mammal changing its voice to advertise her peak of fertility, and it may help with panda conservation efforts.

Although giant pandas (Ailuropoda melanoleuca) are relatively solitary, males and females keep track of each other via smells and sounds. Pandas have a rich repertoire of vocalizations, from growls and moans to bleats and chirps, the latter made primarily by females. "It's been known for some time that around estrous, females begin to chirp," says Benjamin Charlton, an ethologist at Zoo Atlanta in Georgia and the lead author of the study. Scientists have recently shown that women's voices become more high-pitched during their most fertile periods, and Charlton wondered whether female pandas did something similar with their chirps.

He and colleagues recorded the chirps of 14 female pandas at the Wolong National Nature Reserve in Sichuan, China. They also monitored the female pandas' hormone levels to determine the date of ovulation. After analyzing the chirps, Charlton and his team found a distinct difference between the chirps females produced during the prefertile and the fertile phases of their reproductive cycles. The latter calls are longer, harsher, and more numerous, says Charlton.

And they drive male pandas wild. In experiments at the China Research and Conservation Center for the Giant Panda in Sichuan, Charlton and colleagues played the two types of chirps for seven adult male pandas and videotaped the males' responses. "They spent significantly more time near the speakers pacing" when the chirps of fertile females were played, says Charlton. "In the wild, such chirps may increase male-male competition," he says.

The find, reported online today in the Proceedings of the Royal Society B, is the first time that vocal changes indicating the precise period of ovulation have been detected outside of humans, says Charlton. He suspects that the females of other species may also make such calls.

The results make sense, says Devra Kleiman, an ethologist at the University of Maryland, College Park, who did some of the first work in the wild on panda calls. The females may use their chirps "to attract one male," she says, "while they moan, growl, or bark" at others they're not interested in as mates.

Animal behaviorist Donald Lindburg, the former leader of the San Diego Zoo's panda program in California, adds that he is "delighted to see a highly sophisticated analysis that leaves no room for doubt" about what the fertile panda chirp conveys.

Charlton says the work could aid in panda-conservation efforts. For example, playing female fertility chirps may help get male pandas in the mood.