Loud calls, startle behaviour, social organisation and predator avoidance in arboreal langurs (Cercopithecidae:...

Linking predator avoidance and social organisation predicts that large groups are favoured under heavy predation pressure but that small, inconspicuous groups may do equally well by avoiding detection altogether. We explored the relationships between antipredator behaviour (vocalisation, concealment, fleeing), detectability (colouration, group size) and social organisation in arboreal langurs. Three clear antipredation conditions emerged: (1) exemplified by Presbytis melalophos--brightly-coloured species (red, yellow) with contrasting colours, living in large unimale-multifemale groups (>10 individuals), that vocalised frequently and that flee loudly through the canopy; (2) exemplified by P. comata--greyish species (some contrasting colours), living in intermediate, unimale-multifemale or 1-male-1-female groups (approx. 7 individuals), that vocalise infrequently and that flee through the middle forest layers; (3) exemplified by P. frontata--dull-coloured species without contrast, living in small unimale-multifemale or 1-male-1-female groups (<5 individuals), that may freeze upon detection, and that may vocalise mainly during the night. Crypsis as an antipredator strategy is restricted to taxa that occur in 1-male-1-female groups. This wide range of antipredator strategies within a monophyletic taxon living in the same general area with a similar suite of predators facing similar predator pressures appears to be unique among the order Primates.