Giant pandas perceive and attend to formant frequency variation in male bleats

Animals often use acoustic signals to assess the physical characteristics of conspecifics in reproductive contexts. Here, we manipulated two components of male giant panda bleats, the formant frequencies (an acoustic cue to size) and the fundamental frequency, to examine male and female responses to bleats characterized by different combinations of these acoustic components. Our results revealed that male giant pandas had greater looking responses and tended to respond faster to bleats with higher formants simulating small adult males. In contrast, females had greater looking responses to bleats with lower formants simulating large adult males. In addition, there was no interaction between the value of the fundamental frequency and the observed response of male and female giant pandas to formant frequency variation in male bleats. Taken together these findings indicate that formants are functionally relevant to male and female giant pandas, and suggest that the level of the fundamental frequency in male bleats does not significantly affect how receivers perceive formant frequency variation. Furthermore, the sex differences in response direction are consistent with the notion that giant pandas could be using formants as cues to the caller's sex, through the correlation with body size, during the breeding season.