Urbanization accentuates hand use in the foraging activities of primates
Objectives: How a species uses its anatomical manipulators is determined by anatomy and physiology at a proximate level and by phylogeny and ecology at an ultimate level. Primates are often distinguished by their prehensile capabilities and manual dexterity. Theoretical explanations for intraspecific variation in the use of mouth and hands by nonhuman animals is non-existent. In the context of intense pressures of urbanization on natural habitats, we examined if hand and mouth use differed across the rural-urban gradient in food retrieval and food processing under experimental conditions and under naturalistic conditions in cercopithecids. Results: The use of hands in acquisition and processing food increased with urbanization and the converse was true for mouth use under experimental conditions. Even under naturalistic settings, all examined species of urban primates showed a bias in hand use during food acquisition. Discussion: Thus, it appears that the adaptive pressures of urbanization, like the manual constraints of extracting packaged food and perhaps the need for visual-haptic exploration of novel objects accentuates hand use in even simple tasks among synanthropic groups of nonhuman primates. Additional research is needed to determine specific factors of urbanization influencing the trend observed and to ascertain similar patterns in other primates occupying forest-urban niches. Materials and Methods: We observed acquisition and processing of peanuts under standardized conditions across groups of bonnet monkey (Macaca radiata) differing in their dietary dependence on packed food items. Next, we coded acquisition and processing behaviors in bonnet macaques, Japanese monkey and vervet monkey in a similar habitat gradient. The supplementary materials (Folder: Supplementary information) presents supplemental figures and supplemental table. The data file for each primate species is included in 'Mined Info-R data files'. These data files have been used for analyses in R. The excel file contains all the data used in the study and data description is provided in the text file titled,'Data usage details'.
Steps to reproduce
Experimental condition We selected three groups of bonnet macaques around Mysore, India based on their exposure to anthropogenic packaged food (Bull temple (BT) group-High, Foot hill (FH) group-Moderate, T. Betahalli (TB) group-Low). Based on home range, habitats of the 3 groups could be also classified along the urban dimension based on built up area in the following manner: BT-Urban, FH-Semi urban and TB-Rural/Natural. We offered an extractable food item, peanut on terrestrial substratum to isolated and passive macaques and recorded their use of hand/mouth to acquire peanut and to process peanut.The experiment was conducted in three phases involving a single group in each phase and behavioral responses were recorded using camcorders. Video analyses of naturalistic food acquisition and food extraction/processing of primate species We mined videos of 3 primate species from video databases/archives such that they spanned the urban-forest gradient alongside the possibility of acquiring packaged artificial food. Videos obtained covered foraging and feeding by primates in as many microhabitats of each species as possible and included their natural habitats, altered habitats and anthropogenic habitats. Based on availability of videos and relevant information, we included the following cercopithecid species in the analyses: bonnet macaque, Japanese macaque and vervet monkey. We identified the year of video recording/uploading and the geographical location of the primate group(s) based on the metadata of video. Among videos containing foraging and feeding incidences, we shortlisted instances in which food items met the conditions described in Table 1 (see preprint). The conditions for food inclusion were based on delimiting the overlap of mouth use and hand use for FA and the overlap of mouth use, hand use and hand-mouth use for FP. Most conditions elucidated in Table 1 (see preprint) were based on research literature on grasp, food acquisition, reach-for-food and reach-for-grasp in primates. The remaining conditions were determined based on the logical evaluation of an equal likelihood of hand/mouth use in FA and FP with regard to species-food pairs. The description and inclusion/exclusion criteria of each covariate are provided in Table 2. The covariates and their scopes described in Table 2 were also drawn from published materials. To characterize the degree of exposure to packaged anthropogenic food, we obtained information from either researchers familiar with the groups, videographers, published/unpublished research, residents of the region and/or from the narration in the videos.