Asymmetrical trunk use in elephants represents a distinctive example of motor lateralization. Previous studies have shown that trunk movements in the elephant behaviour associated with feeding is lateralized at the individual, but not the population level. The manifestation of lateralized behaviour depends on the nature of the behaviour and is usually more pronounced in social context. Therefore, we hypothesized that population-level lateralization of trunk use in elephants is manifested in social behaviour. One-sided biases in trunk movements were assessed in wild individually-identified Asian elephants during feeding (tearing off a tuft of grass) and social interactions (trunk-to-mouth contacts and trunk-to-genitals contacts between male initiator and female recipient). In feeding, lateralization at the individual and population levels was estimated based on 50 lateral trunk uses per individual. In social interactions, only the population-level bias was assessed using a single trunk contact from each individual. For trunk movements during feeding, elephants showed robust individual preferences, but no population-level lateralization. The distribution of right- and left-trunkers in the present study did not differ significantly from that obtained in previous studies of both the same elephant population and geographically distinct population. No population-level bias in trunk movements during trunk-to-mouth contacts was revealed. In trunk-to-genitals contacts, in contrast, a population-level lateralization was found. Right-sided trunk movements prevailed in males touching females. While individual preferences for feeding, combined with the absence of one-sided population bias, is obviously a species-typical characteristic of Asian elephants, social behaviour, such as male-female socio-sexual contacts, can elicit population-level lateralization of trunk use in this species. The right-sided bias in trunk-to-genitals contacts may reflect lateralization of olfactory perception. If this is the case, the revealed lateralization indicates a right-hemispheric advantage in the processing of social information which is consistent with the general tendency in mammals.
