Social network analysis offers new tools to study the social structure of primate groups. We used social network analysis to investigate the cohesiveness of a grooming network in a captive chimpanzee group (N 5 17) and the role that individuals may play in it. Using data from a year-long observation, we constructed an unweighted social network of preferred grooming interactions by retaining only those dyads that groomed above the group mean. This choice of criterion was validated by the finding that the properties of the unweighted network correlated with the properties of a weighted network (i.e. a network representing the frequency of grooming interactions) constructed from the same data. To investigate group cohesion, we tested the resilience of the unweighted grooming network to the removal of central individuals (i.e. individuals with high betweenness centrality). The network fragmented more after the removal of individuals with high betweenness centrality than after the removal of random individuals. Central individuals played a pivotal role in maintaining the network's cohesiveness, and we suggest that this may be a typical property of affiliative networks like grooming networks. We found that the grooming network correlated with kinship and age, and that individuals with higher social status occupied more central positions in the network. Overall, the grooming network showed a heterogeneous structure, yet did not exhibit scale-free properties similar to many other primate networks. We discuss our results in light of recent findings on animal social networks and chimpanzee grooming. Am. J. Primatol. 73:758-767, 2011.

28Several species of non-human apes have been suggested to rely on copying to acquire some of 29 their behavioural forms. One of the most cited examples -and UN-protected -is nut-cracking 30 in chimpanzees. However, copying might not be the most parsimonious explanation for nut-31 cracking, considering the lack of evidence for spontaneous copying in this species. The zone 32 of latent solutions (ZLS) hypothesis argues instead that the behavioural form of nut-cracking 33 is individually learnt, whilst non-copying social learning fosters frequency differences across 34 populations. In order to differentiate between the copying and the ZLS hypothesis, four nut-35 cracking-naïve orangutans (Mage=16; age range=10-19; 4F; at time of testing) were provided 36 with nuts and hammers but were not demonstrated the behaviour. Whilst the adults in the 37 group were able to open nuts with their teeth, one juvenile spontaneously expressed nut-38 cracking with a wooden hammer. We therefore show that the behavioural form of nut-39 cracking does not necessarily rely on copying in orangutans. 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56Overall, then, we may surmise that wild apes, alongside untrained/unenculturated captive 89 apes, most likely lack the ability to copy novel actions. 90Despite these data, action copying is still often cited as the main mechanism behind ape, and 91 especially chimpanzee, behavioural forms. Some have even further claimed that (certain) ape 92 behaviours (such as tool-use behaviours) depend on copying social learning to be acquired by 93

Nut‐cracking with hammer tools (henceforth: nut‐cracking) has been argued to be one of the most complex tool‐use behaviors observed in nonhuman animals. So far, only chimpanzees, capuchins, and macaques have been observed using tools to crack nuts in the wild (Boesch and Boesch, 1990; Gumert et al., 2009; Mannu and Ottoni, 2009). However, the learning mechanisms behind this behavior, and the extent of nut‐cracking in other primate species are still unknown. The aim of this study was two‐fold. First, we investigated whether another great ape species would develop nut‐cracking when provided with all the tools and appropriate conditions to do so. Second, we examined the mechanisms behind the emergence of nut‐cracking by testing a naïve sample. Orangutans (Pongo abelii and Pongo pygmaeus) have the second most extensive tool‐use repertoire among the great apes (after chimpanzees) and show flexible problem‐solving capacities. Orangutans have not been observed cracking nuts in the wild, however, perhaps because their arboreal habits provide limited opportunities for nut‐cracking. Therefore, orangutans are a valid candidate species for the investigation of the development of this behavior. Four nut‐cracking‐naïve orangutans at Leipzig zoo (P. abelii; Mage = 16; age range = 10–19; 4F; at the time of testing) were provided with nuts and hammers but were not demonstrated the nut‐cracking behavioral form. Additionally, we report data from a previously unpublished study by one of the authors (Martina Funk) with eight orangutans housed at Zürich zoo (six P. abelii and two P. pygmaeus; Mage = 14; age range = 2–30; 5F; at the time of testing) that followed a similar testing paradigm. Out of the twelve orangutans tested, at least four individuals, one from Leipzig (P. abelii) and three from Zürich (P. abelii and P. pygmaeus), spontaneously expressed nut‐cracking using wooden hammers. These results demonstrate that nut‐cracking can emerge in orangutans through individual learning and certain types of non‐copying social learning.