Abstract
Although mammal vocalisations signal attributes about the caller that are important in a range of contexts, relatively few studies have investigated the transmission of specific types of information encoded in mammal calls. In this study we broadcast and re-recorded giant panda bleats in a bamboo plantation, to assess the stability of individuality and sex differences in these calls over distance, and determine how the acoustic structure of giant panda bleats degrades in this species’ typical environment. Our results indicate that vocal recognition of the caller’s identity and sex is not likely to be possible when the distance between the vocaliser and receiver exceeds 20 m and 10 m, respectively. Further analysis revealed that the F0 contour of bleats was subject to high structural degradation as it propagated through the bamboo canopy, making the measurement of mean F0 and F0 modulation characteristics highly unreliable at distances exceeding 10 m. The most stable acoustic features of bleats in the bamboo forest environment (lowest % variation) were the upper formants and overall formant spacing. The analysis of amplitude attenuation revealed that the fifth and sixth formant are more prone to decay than the other frequency components of bleats, however, the fifth formant still remained the most prominent and persistent frequency component over distance. Paired with previous studies, these results show that giant panda bleats have the potential to signal the caller’s identity at distances of up to 20 m and reliably transmit sex differences up to 10 m from the caller, and suggest that information encoded by F0 modulation in bleats could only be functionally relevant during close-range interactions in this species’ natural environment.
Highlights
The acoustic structure of mammal vocal signals encodes information about the caller that has functional relevance in a range of behavioural contexts
Hold-out-sample Discriminant Function Analyses (DFA)’s trained with bleats re-recorded at 1 m were able to classify 60% of bleats rerecorded at 10 m, 40% of those rerecorded at 20 m, 14% for those rerecorded at 30 m, and 16% of those rerecorded at 40 m (Table 2)
In the current study we have shown that the acoustic structure of giant panda bleats remains individually distinctive over distances of up to 20 m in a bamboo forest environment
Summary
The acoustic structure of mammal vocal signals encodes information about the caller that has functional relevance in a range of behavioural contexts (for a review see[1]). Females could use acoustic cues to the caller’s identity to become progressively familiarised to the vocalisations of high-quality males that are able to outcompete other rivals and maintain close contact with them during the lead up to mating, and subsequently go on to prefer these individuals in mate choice contexts[29] In support of this contention, free-ranging male giant pandas are known to compete for access to females during the breeding season, and will often associate with females for up to one month before copulation and subsequent separation[13,35,36]. Information about the caller’s sex could allow unreceptive (non-oestrous) females to better avoid potentially aggressive roaming males during the breeding season, and females could use the rate of F0 modulation in bleats to assess male androgen levels and sexual motivation in mate choice contexts. The wide-range of information encoded in bleats cannot, be functionally relevant unless it is reliably transmitted in the giant panda’s bamboo forest environment
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