Female giant panda (Ailuropoda melanoleuca) chirps advertise the caller's fertile phase.

Although female mammal vocal behaviour is known to advertise fertility, to date, no non-human mammal study has shown that the acoustic structure of female calls varies significantly around their fertile period. Here, we used a combination of hormone measurements and acoustic analyses to determine whether female giant panda chirps have the potential to signal the caller's precise oestrous stage (fertile versus pre-fertile). We then used playback experiments to examine the response of male giant pandas to female chirps produced during fertile versus pre-fertile phases of the caller's reproductive cycle. Our results show that the acoustic structure of female giant panda chirps differs between fertile and pre-fertile callers and that male giant pandas can perceive differences in female chirps that allow them to determine the exact timing of the female's fertile phase. These findings indicate that male giant pandas could use vocal cues to preferentially associate and copulate with females at the optimum time for insemination and reveal the likely importance of female vocal signals for coordinating reproductive efforts in this critically endangered species.