Stereotypic hair manipulation during social grooming in zoo-housed western chimpanzees (Pan troglodytes verus): a Social Network Analysis

Stereotypies are abnormal repetitive behaviours expressed unvaryingly as a coping mechanism in relation to sub-optimal living conditions, typical of captive environments. Especially self-stimulating stereotypies may suggest past environmental paucities (e.g., improper early social environment) and/or present ones (e.g., adverse unavoidable stimuli/boredom). Here, in a group of zoo-housed chimpanzees coming from a long living history in a private house, we report evidence of self-directed stereotypies (i.e., hair-pulling; over-grooming) linked to social grooming, a primary prosocial behaviour in chimpanzees. By means of a Social Network Analysis, never applied to the study of stereotypies so far, we analysed the different social grooming contexts in which stereotypies were or were not expressed. We first confirmed the initial hypothesis, based on a pilot study, of stereotypy dependence on a social grooming context. More than 90% of the 189 bouts of recorded stereotypy occurred during or shortly after social grooming. Networks’ analysis showed grooming partners differently affecting the responses of stereotyping individuals, who performed most of the stereotypies when grooming with adults (especially with other stereotyping partners) as compared when grooming with the juvenile individual. Furthermore, asymmetry of grooming interactions showed stereotypies being more likely to be expressed when received grooming increased. We might envision three possible and non-exclusive causes to explain this pattern of stereotypy: (i) the strongly biased anthropic rearing and living experiences may have altered social competences (main original cause) and social stress may now play a key role in the observed behavioural abnormalities; (ii) grooming stimulations and/or the stereotypy of one grooming partner may function as facilitating factors, favouring the stereotypy (contingent cause); and (iii) actual conditions of unmanaged captive time/boredom (contingent cause). Encouragingly, a six-month enrichment program provided to the group, halved the time spent in stereotypies, which showed to be still sensitive to environmental corrections