Demography, Diet and Range Size in a Population of Black-handed Spider Monkey's

Abstract

#1 Studies of wild Ateles geoffroyi yucatensis have occurred, or are ongoing in Mexico, Guatemala and Costa Rica, but no sites have been established in Belize. Across study sites, group composition, average sub-group size, diet and ranging patterns vary as a function of ecological and demographic variables particular to respective locations. Here we describe the age and sex compositions, average sub-group sizes, range sizes, activity budgets and diets of two recently habituated groups of spider monkeys at Runaway Creek Nature Preserve in Belize. Habituation and data collection began in June 2007 using a combination of all occurrences recording and group scans. Two separate and adjacent monkey groups were identified – Groups 1 and 2 – numbering 33 and 31 individuals respectively. Range sizes for both groups were determined using a minimum convex polygon of GPS location points. Group 1 ranged over an area of 114.43 ha (N=678) and Group 2 over 193.67 ha (N=181). Average sub-group size for all individuals of all age/sex classes in Group 1 group was 5.16 individuals, with a mode of 2 (N=415). In Group 2, average sub-group size was 4.48 individuals, with a mode of 3 (N=106). Between June 2007 and July 2009 the spider monkeys from both groups were observed feeding from 70 different plant species. These and future data emanating from this new study site represent an important addition to a small but growing number of studies that will further our understanding of within-species variability in response to differing socio-ecological variables across the range of Ateles. #2 Infant handling by adults other than the mother occurs to varying degrees across primate species and social organizations. Among males infant handling may reflect kinship-based affiliation, bond formation or a reproductive strategy that facilitates access to the mother. Spider monkeys (Ateles spp.) exhibit male philopatry and therefore males may preferentially handle male infants as they could be potential future allies. To investigate this hypothesis, all occurrence data were collected from January 2007 to December 2009 on a community of 35 wild spider monkeys at Runaway Creek Nature Reserve, Belize. During 555 hours of observation, 59 infant handling bouts and 12 infant handling attempts were recorded. All of the 11 infants were handled by individuals other than the mother during the study period. Handling of infants by adult, sub-adult and large juvenile males was the most common [N=48, or 81% of all bouts]. Infant handling by adult, sub-adult and large juvenile females was less common [N=11, or 19% of all bouts], but a higher proportion of handling bouts by females resulted in prolonged infant carries [0.36 for females; 0.10 for males]. Individual adult and sub-adult males varied with respect to their probability of handling an infant [Pearson X2=14.25, df=4, p<0.05]. The sex of the infant did not affect how often it was handled by males [Pearson X2=3.84, df=1, p=0.36]. As all infants born in a male philopatric group are presumed to share some degree of paternal kinship, we suggest that male infant handling in A. geoffroyi reflects kinship-based affiliation or tolerance. #3 Sexual segregation, the social and/or spatial separation of males and females, has been characterized in many animals; however, no systematic analysis has yet been undertaken to measure sex segregation in primates. Using data from a 7-month study on a community of spider monkeys in Belize, we used the Sexual Segregation Aggregation Statistic (SSAS) to determine if the sexes segregate or aggregate. We then determined if the patterns are driven by social, habitat, or reproductive differences between males and females. SSAS values range from 0 (complete aggregation) to 1 (complete segregation). The overall SSAS value suggests that male and female spider monkeys are slightly more segregated (SSAS=0.56) than aggregated. However segregation varies monthly; there is more segregation during the dry season months (January-April: 0.66-0.85) and more aggregation during the wet season months (May-July: 0.27-0.45). Several hypotheses have been proposed to explain the causes of segregation, such as the forage selection and predation risk hypotheses for habitat segregation and the social preferences and activity budget hypotheses for social segregation. Despite low levels of dimorphism in spider monkeys, high fission-fusion dynamics may lead to different activity budgets for males and females, which in turn favors segregation to meet different reproductive agendas. Males and females differ significantly in time spent traveling, feeding and socializing. Spider monkeys also prefer to associate with same-sex partners. Since males and females do not differ in habitat use, segregation in spider monkeys is best explained by sex differences in activity budgets and social preferences.