Camel whistling vocalisations: male and female call structure and context in <i>Camelus bactrianus</i> and <i>Camelus dromedarius</i>

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 <i>Camelus bactrianus</i> and one male and one female one-humped <i>C. dromedarius</i>. 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 <i>C. bactrianus</i>) or when protesting against preventing locomotion over enclosure (female <i>C. dromedarius</i>). 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 <i>C. bactrianus</i> than in <i>C. dromedarius</i> (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 <i>C. dromedarius</i> are the highest-frequency vocalisations in Artiodactyla.