Naïve orangutans (Pongo abelii & Pongo pygmaeus) individually acquire nut-cracking using hammer tools

Nut-cracking using hammer tools has been argued to be one of the most complex tool-use behaviours observed in non-human animals (henceforth: animals). Recently, even the United Nations Convention on the Conservation of Migratory Species (CMS) recognised the unique nature of chimpanzee nut-cracking by making it the first animal behaviour to be awarded UN-protected status (Picheta, 2020). So far, only chimpanzees, capuchins and macaques have been observed using tools to crack nuts in the wild (Boesch & Boesch, 1990; Gumert, Kluck, & Malaivijitnond, 2009; Ottoni & Mannu, 2001). However, the learning mechanisms behind this behaviour, and the extent of nut-cracking in other primate species are still unknown. The aim of this study was two-fold. First, we aimed to examine whether other great ape species would develop nut-cracking when provided with all the tools and motivation to do so. Second, we wanted to examine the mechanisms behind the emergence of nut-cracking in a naïve sample. Orangutans ( Pongo abelii; pygmaeus ) have not been observed cracking nuts in the wild, despite having the second most extensive tool-use repertoire of the great apes (after chimpanzees), having the materials for the behaviour in the wild (albeit rarely) and possessing flexible problem-solving capacities. Therefore, orangutans are a valid candidate species for the investigation of the development of nut-cracking. Four nut-cracking-naïve orangutans at Leipzig zoo ( Pongo abelii ; M age =16; age range=10-19; 4F; at time of testing) were provided with nuts and hammers but were not demonstrated the nut-cracking behavioural form, in order to control for the role of copying social learning in the acquisition of this behaviour. Additionally, we report data from a previously unpublished study by one of the authors (MF) with eight orangutans housed at Zürich zoo (10 Pongo abelii and two Pongo pygmaeus; M age =14; age range =2-30; 5F; at time of testing) that followed a similar testing paradigm. Out of the twelve orangutans across both testing institutions, at least four individuals, one from Leipzig ( Pongo abelii ) and three from Zürich ( Pongo abelii ; pygmaeus ), spontaneously expressed nut-cracking with a wooden hammer. These results suggest that the behavioural form of nut-cracking using hammer tools can emerge in orangutans when required through individual learning combined, in some cases, with non-copying social learning mechanisms.