Experimental evidence for nasty neighbour effect in western black crested gibbons ( Nomascus concolor )

Neighbour-stranger discrimination plays a crucial role in territorial animals and is expressed as either the dear enemy phenomenon or the nasty neighbour effect. Previous evidence showed that the size of intruding groups affected the expression of neighbour-stranger discrimination. However, few studies have compared neighbour-stranger discrimination between individual-level and group-level intrusions in conspecifics. In this study, we used playback experiments to investigate whether the western black crested gibbon ( Nomascus concolor ) discriminates intruding neighbours from strangers via vocal signal and explore the effect of intruder type (individual versus group) on neighbour-stranger discrimination. We found that the focal gibbons responded more intensively to neighbours versus strange intruders with shorter movement delay time and longer counter-singing duration. Furthermore, the intruder type had a pronounced influence on subjects’ response intensity. The focal gibbons reacted with faster movement velocity and shorter locomotor duration to individual intruders than to group intruders. Our results provide the first empirical evidence for the nasty neighbour effect in Hylobatidae. These results also suggest that group intruders are not necessarily more threatening than individual intruders, and that the perceived threat posed by intruders is affected by ecological factors and social contexts. Significance statement Neighbour-stranger discrimination (NSD) is widespread in territorial animals. However, few studies have compared the differences in NSD between individual-level and group-level intrusions in conspecifics. We studied the effect of intruder type (individual versus group) on NSD dynamics using acoustic playback experiments in free-ranging western black crested gibbons ( Nomascus concolor ). We provide the first experimental evidence for the nasty neighbour effect in Hylobatidae. Our findings support the threat level hypothesis and suggest that the threat level of the intruder depends on ecological factors and social contexts. We also suggest that group intruders do not necessarily have a higher threat level than individual intruders, at least among the western black crested gibbons.