Am J Primatol. 2021;83:e23251. wileyonlinelibrary.com/journal/ajp © 2021 Wiley Periodicals LLC | 1 of 10 https://doi.org/10.1002/ajp.23251 Received: 10 June 2020 | Revised: 1 February 2021 | Accepted: 20 February 2021 DOI: 10.1002/ajp.23251 RESEARCH ARTICLE Ontogeny and sex differences in object manipulation and probe tool use by wild tufted capuchin monkeys (Sapajus libidinosus) Tiago Falótico 1,2,3 | Carolina Q. Bueno 1 | Eduardo B. Ottoni 1 1 Institute of Psychology, University of São Paulo, São Paulo, Brazil 2 School of Arts, Sciences and Humanities, University of São Paulo, São Paulo, Brazil 3 Neotropical Primates Research Group, São Paulo, Brazil Correspondence Tiago Falótico, School of Arts, Sciences and Humanities (EACH), University of São Paulo, Av. Arlindo Bettio, 1000—Bloco CMP1 Sala T06, São Paulo 03828‐000, Brazil. Email: tfalotico@gmail.com Funding information Fundação de Amparo à Pesquisa do Estado de São Paulo, Grant/Award Numbers: 2013/ 05219‐0, 2014/04818‐0; Conselho Nacional de Desenvolvimento Científico e Tecnológico, Grant/Award Numbers: 308746/2017‐1, 376637/2015‐3 Abstract Tufted capuchin monkeys (Sapajus spp.) are the only Neotropical Primates that regularly use tools in the wild, but only one population of bearded capuchin monkeys (Sapajus libidinosus) is known to habitually use sticks as probes. In this population, males are typically the only sex to use stick tools, something un- expected, since there are no obvious physical constraints, and females do use stone tools in the wild and sticks in experimental conditions. We investigated the development of probe tool use in eight infants to clarify whether social influ- ences on learning varied between the sexes, as tool use learning by capuchin monkeys is a socially biased process. We found that in the first 10 months of age, females manipulate sticks as much as males, but after 10–12 months of age, males begin to manipulate them at higher frequencies. We examined if social connections—as opportunities for social learning—could explain this difference and verified that, on close distance social networks, infant males and females have similar connections with older males. However, males observe probe tool use events more often than females when close to such events. The higher fre- quency of manipulation of sticks, as well as the higher rates of probe tool use observation, appear to be the key to understand why only males are probe tool users in this population. Since there are only male potential models of probe use, a sex motivational bias could explain the sex difference in observation; a bias in observation could explain the differences in manipulation—and manipulation rates would certainly influence the chances of individual, trial‐and‐error learning (a case of “local/stimulus enhancement”). KEYWORDS object manipulation, ontogeny, probe tool, sex differences, stone tools, tool use 1 | INTRODUCTION Probe tools are known to be used by a few primate species, but this behavior is not equally distributed within populations, even when raw material is abundantly available in the environment. Some chimpanzee communities use sticks to fish for termites (Boesch et al., 2020; McGrew & Collins, 1985; Sanz et al., 2004) or algae (Boesch et al., 2016), and to dip for ants (Humle & Matsuzawa, 2002). A few communities even use a stick tool set to first perforate the nest and then dip for termites (Sanz et al., 2004) or honey (Boesch et al., 2009). The chimpanzees from the Fongoli community hunt for galagos using wooden “spears” to hurt the prey (Pruetz &