Yet another paper on panda vocalizations to add to the string of other papers that have appeared throughout the year. The very media-friendly Ben Charlton of Atlanta Zoo is doing very well out of panda sounds.

This time, he and his colleagues have built upon research from the 1980s that showed female pandas change their vocalizations when fertile. It makes sense that they should advertise this fact, as the window of peak panda fertility is terribly tight, just a couple of days a year.

Working with the “Nixon pandas” at the National Zoo in Washington D.C. in the 1970s, Devra Kleiman, an expert on the behaviour of mammals in captivity, quickly realised that vocalisations were going to be the quickest way to work out when the female Ling-Ling was entering her oestrus period. “It’s a really good clue to the behavioural reproductive condition of both the male and the female,” she told me in an interview earlier this year. Kleiman brought in German ethologist Gustav Peters to make recordings of panda calls and describe how they changed with fertility. In spite of this work, published in 1982, this very useful cue to female reproductive condition has not been exploited to the max, she says. “I think even now folks are not sensitive enough to the sounds and the changes in them in trying to make management decisions.”

Maybe this latest work from Charlton and friends, published online in the Journal of the Royal Society of London, Series B this week, will change all that. The researchers mapped changes in female vocalisations onto changes in female fertility (worked out by making regular check on hormone levels) and made a very detailed acoustic analysis of exactly how that change occurs. “In particular, female giant panda chirps signalling fertile callers were of longer duration and characterized by higher jitter and harshness,” they write.

They then made the important experimental verification of playing back different calls to male pandas to see what they made of them. When the calls were those of fertile females, males showed significantly greater movement, were more likely to approach the speaker and spent much more time near it than when the calls were from non-fertile females. “The results of this study indicate that female giant panda chirps have the potential to provide males with precise information about the timing of the caller’s fertile stage,” write Charlton and colleagues. Since they now know exactly what kinds of calls they are looking for when a female is fertile, it should help them to improve further still on the coordination of natural matings and artificial insemination of captive pandas.

I’ve been wanting to interview Charlton for my book and I feel now is the time to contact him. I’m hoping he’ll let me post some pre-fertile and fertile vocalizations here. It would be nice to know just how dramatic the change is.

More panda sounds