Lun Lun, a female panda at Zoo Atlanta, rests against a tree. Researchers have just discovered the meanings of the bleat calls giant pandas make during mating season.Adam K. Thompson/Zoo Atlanta
Giant panda bleat calls, often emitted during the panda's mating season, have just been deciphered, according to new research.
The study reveals that males broadcast information about their size, while females advertise how old they are.
The study, which has been accepted for publication in the journal Animal Behavior, is the first to examine the informational content of any giant panda vocalization.
Lead author Benjamin Charlton explained to Discovery News that bleats are just one type of call made by the distinctive black and white members of the bear family.
"Giant pandas (also) produce barks, moans, honks, growls, roars and squeals," said Charlton, a postdoctoral research biologist at Zoo Atlanta. "In addition, females produce chirps when they are approaching and entering estrus, and some non-vocal sounds, such as the snort and chomp, which are sometimes heard."
For the new research, however, Charlton and colleagues Zhang Zhihe and Rebecca Snyder focused only on bleats, which the giant pandas emit year round, but do so more frequently and prominently during the breeding season.
The scientists recorded 14 adult giant pandas at a research center in Chengdu, China, as well as four adult giant pandas at Zoo Atlanta, Memphis Zoo and the National Zoological Park. Nine females and nine males were included in the mix. They ranged in age from 6 to 21 years old.
In addition to age, data on size and body weight were also noted for each of the pandas.
The researchers next conducted an acoustical analysis of the recordings using a special computer application and automated programs.
They determined that both males and females had distinctive masculine and feminine voices, so panda listeners would have no trouble identifying the sex of the caller, which is obviously useful during the mating season.
Beyond that, however, only male bleats contained reliable information about the individual's body size, so females can actually hear from a certain distance whether or not the caller is big and husky, or small and puny.
Charlton explained that formants, or vocal tract resonances, are the key, since there is "a close relationship between formant spacing, vocal tract length and overall body size." Male hormones affect all of these as the individual matures.
Females, on the other hand, do not include reliable information about their body size in their bleats, but they do "tell" their age, while males do not.
"It's not that females don't mind revealing their age," Charlton said. "They don't have a choice. Information on female age is likely to be present in bleats due to age-related changes in vocal production anatomy."
He and his colleagues further explained that older females "are more likely to be experienced breeders," so males likely seek them out and "avoid pursuing those that are too young to breed."
He doesn't think females would need to know the age of other adult females, but males may benefit by knowing each other's sizes, which "could help them avoid risky encounters with larger rivals."
One giant panda without such concerns at present is the 3.9-pound male cub recently born at the San Diego Zoo.
"He vocalized a little today," Geoff Pyle, the zoo's senior veterinarian, said Thursday evening. "It was nothing dramatic. He was just letting us know he was aware of what we were doing."
Mother Bai Yun and the rare cub are being shielded from potentially risky public view at present, but may be observed via the zoo's live Panda Cam.
In the future, Charlton hopes studies will be conducted on the vocal communication of giant pandas, with findings that may also apply to other bear species.