Neighbouring chimpanzee communities show different preferences in social grooming behaviour

Abstract

Grooming handclasp (GHC) behaviour was originally advocated as the first evidence of social culture in chimpanzees owing to the finding that some populations engaged in the behaviour and others do not. To date, however, the validity of this claim and the extent to which this social behaviour varies between groups is unclear. Here, we measured (i) variation, (ii) durability and (iii) expansion of the GHC behaviour in four chimpanzee communities that do not systematically differ in their genetic backgrounds and live in similar ecological environments. Ninety chimpanzees were studied for a total of 1029 h; 1394 GHC bouts were observed between 2010 and 2012. Critically, GHC style (defined by points of bodily contact) could be systematically linked to the chimpanzee's group identity, showed temporal consistency both within and between groups, and could not be accounted for by the arm-length differential between partners. GHC has been part of the behavioural repertoire of the chimpanzees under study for more than 9 years (surpassing durability criterion) and spread across generations (surpassing expansion criterion). These results strongly indicate that chimpanzees' social behaviour is not only motivated by innate predispositions and individual inclinations, but may also be partly cultural in nature.

Figure 1.

Grooming handclasp style examples: (a) palm-to-palm and (b) wrist-to-wrist.

Figure 2.

Proportion by which (a) palm-to-palm and (b) wrist-to-wrist GHC styles were used, separately per year and group. n refers to the number of dyads (only dyads with at least five GHC bouts included). Sample sizes for this analysis were 619 GHC bouts and 42 subjects. Shown are medians (thick horizontal lines), quartiles (boxes), percentiles (2.5 and 97.5%, vertical lines) and minimum and maximum (laying crosses) of the proportions per dyad.