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.