Camel whistling vocalisations: male and female call structure and context in Camelus bactrianus and Camelus dromedarius

Among ruminants, some species of cervids, bovids and camelids are capable of producing very high-frequency (HF) calls potentially produced by the aerodynamic whistle mechanism. We analysed the HF calls of six individual adult captive camels: three male and one female two-humped Camelus bactrianus and one male and one female one-humped C. dromedarius. Context of emission differed between sexes and individuals. Males of both species vocalised when guarding females during the rut. Females of both species vocalised towards their mates, postpartum (female C. bactrianus) or when protesting against preventing locomotion over enclosure (female C. dromedarius). In either species or sex, the HF calls were faint tonal vocalisations slightly modulated in fundamental frequency (f0). Between species, the calls were significantly lower-frequency (1.7 +/- 0.16 kHz) and longer (0.23 +/- 0.08 s) in C. bactrianus than in C. dromedarius (3.12 +/- 0.11 kHz; 0.16 +/- 0.05 s). Nonlinear vocal phenomena (subharmonics and sidebands) occurred in both species but not in all individuals. We discuss the relationship of the f0 of the HF calls with body size and vocal fold length in ruminants. We conclude that the 'whistling' HF calls of C. dromedarius are the highest-frequency vocalisations in Artiodactyla.

Automated summary

This study found that captive camels produced high-frequency calls in different contexts, with males typically using them for guarding females or competing for mating rights, while females used them for communication or protest. There were differences in frequency and duration of high-frequency calls between the two camel species, and the presence of nonlinear vocal phenomena was observed.