INTRODUCTION
The ecological conditions under which prehension and their concomitant anatomical, physiological and neurobiological correlates have evolved in tetrapods have been under intense investigation since the beginning of the 20th century (Fragaszy & Crast, 2016; Pouydebat & Bardo, 2019; Sustaita et al., 2013). Within the literature on prehension in anthropoids, manual grasping (reach and retrieval) and manipulation have received the most attention due to their direct implication on intelligence, cognitive development (DeLoache, 2000; von Hofsten & Rosander, 2018) and encephalization (Gibson, 1986) and the indirect implications on the evolution of our own species (e.g. Iriki & Taoka, 2012; Johnson-Frey, 2004; Ramayya et al., 2010). Though a general consensus exists on the complex selective pressures leading to the evolution of grasping, like arboreal locomotion (jump, grasp-leaping) and feeding on precarious substrates (Fragaszy & Crast, 2016), few studies have highlighted the need for greater attention to ecological factors in addition to other factors (morphological, cognitive and physiological) that ‘first promoted hand use or advanced the diversity of prehensile capabilities’ (Pouydebat & Bardo, 2019; e.g. Peckre et al., 2016; Toussaint et al., 2013). Additionally, almost all studies on grasp, grip, reach, retrieval and acquisition in anthropoids have focused on describing phylogenetic similarities/dissimilarities within an ecological context to build an evolutionary model of limbic specialization and skilled dexterous movements. Surprisingly, selective forces on food acquisition operating across heterogeneous populations of a species have never been investigated.
Considering unabated habitat modification and rampant urbanization throughout the ranges of different primates (Estrada et al., 2017; Mittermeier et al., 2006), we were interested in studying the adaptive pressures of urbanization on food retrieval and food processing in primate groups distributed across the urban-forest gradient. The pressures of urban survival in synanthropic groups of primates have led to dietary expansion (Corrêa et al., 2018; Thatcher et al., 2020), foraging on packaged processed food (Kaplan et al., 2011; Katlam et al., 2018) and complex foraging skills (e.g., Brotcorne et al., 2017; Mangalam & Singh, 2013) among other facilitatory transformations like object neophilia (Forss et al., 2015), explorative tendencies and innovative ability (Dhananjaya et al., 2021; Huffman & Quiatt, 1986). Since manipulative abilities and dexterity presumably coevolved with foraging on embedded/encased food (Gibson, 1986; Melin et al., 2014; Parker & Gibson, 1977; Van Schaik et al., 1999), we reasoned that the demands of extractive foraging from packaged artificial food in an urban environment would accentuate the use of hands relative to non-urbanized groups under foraging contexts. Concomitantly, we selected two behavioral actions differing in their manipulative complexities and demands, namely food acquisition (FA) and food processing/extraction (FP) to evaluate the effects of urbanization and dietary dependence on food items that are commonly packaged. Food acquisition entailed the action of collecting food that is typically accomplished either with the mouth or with the hand in anthropoid species (Schwenk, 2000), whereas food processing/extraction is a relatively complex behavior composed of hierarchical sequence of mental, perceptual and motor elements varying in degree of oral-manual dexterity (Russon, 1998; Stokes & Byrne, 2001; Trébouet et al., 2018). Plucking a fruit from a plant is an example of food retrieval while deshelling and depeeling a peanut are examples of food extraction and food processing, respectively (Dhananjaya et al., 2021). Firstly, we offered a food retrieval and a food extraction/processing task to three social groups of bonnet macaques (Macaca radiata ) differing in urbanization within a standardized experimental condition. Secondly, we developed a framework for evaluating naturalistic instances of food retrieval and food processing from foraging and feeding videos of additional primate species followed by characterizing urbanization of their habitat and encounter with packaged food. We anticipated higher hand use among more urbanized groups and groups with higher probability of encounter with packaged food during food retrieval and food processing.