Enriched social environment affects the utterance of acoustic signals of captive François' langurs

Abstract

Vocal communication serves as an important mode of interaction among animals. The quantitative analysis of acoustic signals holds scientific value in highlighting the importance of understanding the social-ecological contexts and their effects on acoustic communication. The results can provide a direct scientific guidance for enhancing the welfare and management of captive animals. From January to February 2022 and from December 2022 to February 2023, we conducted a study involving 30 captive François' langurs (Trachypithecus francoisi) housed in nine enclosures. We collected acoustic signals along with their associated social-ecological contexts and recorded the social composition and number of François' langurs in each enclosure. We in total identified 23 call types based on human ear auditory, spectrogram visual inspection, and quantitative analysis of 37 acoustic parameters. Our findings revealed that adult males exhibited nine call types (five adult male-specific types), adult females 11 call types (seven adult female-specific types), juveniles eight call types (two juvenile-specific types), and infants five call types (two infant-specific types). The asymmetry in call types indicated distinct social roles among different sex-age classes of François' langurs. We found that a complex social composition or larger group size resulted in a more diverse range of call types and increased utterance frequency among François' langurs. The increased utterance frequency was linked to calls used in neutral (contact), affiliation, mating, and vigilance contexts, indicating that a diverse social setting encourages social interaction and boosts the occurrence of natural behaviors, and may also result in increased tension and anxiety. Implementing a one-male multi-female model with ample breeding space for captive François' langurs was identified as beneficial for enhancing vocal communication and improving animal welfare. This study established the first vocal repertoire of François' langurs, offering a novel scientific approach to evaluating the welfare of this endangered species through vocal behaviors.