Fig. 2

Vigilance-related calls across Ruminantia. A Hiss and B snort of a giraffe; C hiss of a musk deer Moschus moschiferus; D snort of a female goitred gazelle Gazella subgutturosa; E snort of a waterbuck Kobus ellipsiprymnus; F snort of a male impala Aepyceros melampus; G snort of a male Western tur Capra caucasica cylindricornis; H two snorts of a male klipspringer Oreotragus oreotragus; I two snorts of a male springbok Antidorcas marsupialis; J bark of a female greater kudu Tragelaphus strepsiceros; K bark of a white-tailed gnu Connochaetes gnou; L bark of a male Indian muntjac Muntiacus vaginalis; M bark of a female sambar deer Rusa unicolor; N bark of a female sika deer Cervus nippon; O bark of a female Siberian red deer Cervus elaphus sibiricus; P bark of a female Bactrian red deer Cervus elaphus bactrianus. The illustrative spectrograms are based on calls recorded from adult wild-living animals (except the Western tur, recorded in captivity) that vocalized at the sudden appearance of a human. During their vocalizations, the callers did not flee but either froze or slowly passed by the human. Spectrograms were created with 24 kHz sampling frequency, Hamming window, Fast Fourier Transform (FFT) 1024 points, frame 50% and overlap 93.75%. Original wav-files are available in the electronic supporting information (Additional file 3: Audio S2)