By Jody Bourton
Earth News reporter
Call of the wild
For female giant pandas who can only conceive on a few days once a year, being able to say "when" is vital.
Now a report reveals that female giant pandas use chirp calls to inform male pandas exactly how fertile they are.
The discovery suggests that panda vocal signals are more important than thought, and will aid conservation of the endangered animal, scientists say.
The researchers from the US and China publish their research in the journal Proceedings of the Royal Society B.
During their short breeding season, female giant pandas (Ailuropoda melanoleuca) make high pitched calls that are thought to solicit male attention.
Given the brief window of opportunity for mating, selection should favour female giant pandas who are able to advertise their fertility and for males who accurately read the female calls.
However, the information content and detailed function of the 'chirp' and 'bleat' vocalisations has remained a mystery.
To investigate, the research team recorded vocalisations of captive giant pandas in China and the US.
Using these audio recordings and a knowledge of individuals' reproductive cycles, they reveal that panda calls signal the precise timing of female fertility.
Chirp calls were observed to differ depending on whether they were in a pre-fertile or fertile stage of the reproductive cycle.
Female giant pandas in a fertile stage would give longer calls that were characterised by a higher jitter and harshness, the researchers write.
The increased harshness of the chirps could indicate greater arousal levels, they say.
By playing recorded female vocalisations to male giant pandas, the researchers also found that males use calls to preferentially mate with females who signal they are at the optimum time for mating.
This is the first experimental evidence to show that giant panda vocalisations can signal a female's exact fertile phase, says Dr Benjamin Charlton from Zoo Atlanta, Georgia in the US, who led the research team.
He completed the research along with researchers from San Diego Zoo's Institute for Conservation Research in the US and the China Research and Conservation Centre for the Giant Panda, Sichuan Province, China.
"Several nonhuman mammal studies have shown that female vocal behaviour can advertise fertility," Dr Charlton says.
And it is not just in the animal world this may occur.
"Recent work on humans has shown that that female vocalisations varies significantly around their fertile period," Dr Charlton says.
He explains that rising oestrogen levels around the time of ovulation in females has been suggested to change vocal structures and vocalisation.
With the knowledge that other species, perhaps including our own, use sounds to signal fertility, he not surprised to find that pandas do similar.
"I was excited to find acoustic cues to female fertility in giant pandas because it gives us a better understanding of this critically endangered species's reproductive behaviour," Dr Charlton says.
Research on communication and reproductive behaviour has been instrumental in recent improvements in conservation and breeding programmes for giant pandas.
The researchers hope this study will provide valuable information that will help the long term future of one of nature's most secretive and charismatic animals.
"By identifying key aspects of reproductive behaviour in giant pandas we can hope to provide the ideal environments and stimuli required for them to reproduce", Dr Charlton says.
"In doing so we can increase the success of captive breeding programmes."