Uso di strumenti in una popolazione di Cebus libidinosus allo stato selvatico in Piauí, Brasile

This study investigates the tool use behaviour in two wild groups of bearded capuchin monkeys Cebus libidinosus) living in the Fazenda Boa Vista (Piauí, Brazil) by determining the frequency of tool use in relation to seasonal changes in food availability and by investigating the use of stone-hammers and anvils to crack palm nuts and other encased fruits in relation to the age and sex of the individuals. Our first goal, was to test two alternative hypotheses which have been proposed to explain tool use in primate populations: the necessity and the opportunity hypotheses. According to the first hypothesis (Fox et al., 2004), since tool use allows the access to food otherwise not available, it should be more frequent when food resources are scarce (Boesch & Boesch-Achermann, 2000; Fox et al., 2004; Moura e Lee, 2004). In contrast, the second hypothesis (Beck, 1980; Fox et al., 2004) predicts that favorable local conditions enhance the emergence and the maintenance of innovative behaviours, including tool use. Our second goal was to determine the frequencies of tool use in the different sex-age classes in order to verify whether, as we predict on the basis of the high sexual dimorphism and the energetic effort required to crack open nuts, male capuchins perform this behaviour more frequently than females. The study area (13 km2 ) is located at Fazenda Boa Vista in the north-eastern Brazilian state of Piauí (9º 00’ S; 45º 00’ W), 21 km north-west of the town of Gilbués. The physical geography of the field site is a sandy plain placed approximately 420 m above sea level. The climate is seasonally dry (annual rainfall 1156 mm; total rainfall during dry season: April to September 230 mm; data from 1971–1990, source: Embrapa). This study was conducted on two well-habituated wild groups of C. libidinosus that were followed from June 2006 to July 2007. Activity budget were recorded by group scan sampling and the occurrence of tool use was scored by focal animal sampling (Martin & Bateson, 1993). The result shows that tool use is a habitual behaviour for the capuchins (sensu Whiten et al., 1999) since it is enacted repeatedly and regularly by several individuals of a group. Moreover, tool use can be defined as traditional (as defined by Fragaszy & Perry, 2003) since (i) the use of stones as percussor has been observed only in a limited number of populations of C. libidinosus (Ottoni & Izar, 2008), even if hammers and encased fruits are present in many other study sites and (ii) this behaviour appears to be transmitted via social influences (Resende et al., 2008; Silva, 2008). Food resources in Boa Vista do not appear more scarce than in other sites where capuchins live in which food abundance was evaluated with similar methodologies (i.e., Atlantic Forest, Izar, 2004; see also Nakai, 2007); consequently, access to resources fundamental for survival does not seem to have caused the use of tools in our groups. The lack of a marked seasonality in food availability did not allow to further test the necessity hypothesis (Fox et al., 2004; Moura e Lee, 2004). Our results are in agreement with the predictions of the opportunity hypothesis (Beck, 1980; Fox et al., 2004) since capuchins use of tools reflects the availability of nuts. As we expected, female capuchins use tools less frequently. This is in sharp contrast with what has been described for nut cracking in chimpanzees (Boesch & Boesch, 1984, Lonsdorf et al., 2004), and what is expected on the basis of the high energy requirements due to lactation and gestation (Clutton-Brock, 1977). Since using tools for capuchins is a strenuous activity, it is possible that it requires energetic efforts too elevated for females, whose body weight and force are lower than males (Liu et al., 2009), to be beneficial.