Food properties influence grasping strategies in strepsirrhines

Abstract

Although hand grasping is ubiquitous in primate species, its origins remain uncertain. This is in part because uncertainty about hand skills and grasping strategies persists in strepsirrhines, a monophyletic group of primates located near the base of the primate tree. In this study, we report and discuss our observations of the different grasping strategies adopted by 85 captive individuals belonging to 22 species of strepsirrhines during the grasping of food items of different sizes and consistencies. Our results indicate that although strepsirrhines do not present variability in their hand-grip types (sole whole-hand power grip), they are able to adjust their grasping strategy depending on the properties of the food. Notably, they use the mouth when more precision is needed (i.e. to grasp small items). Moreover, grasping strategies adopted for big items differ depending on food consistency, revealing a new and potentially essential factor to consider in future research on grasping abilities. We believe that by looking across this important set of species in unconstrained standardized conditions, this study provides valuable insight for further comparative research on the potential selective pressures involved in the evolution of hand grasping.

Food mobility and the evolution of grasping behaviour: a case study in strepsirrhine primates

“…For the mobile test procedure, for each species, one food type was chosen from their usual diet in order to maximise the motivation of the species to participate in the experiment (Table 2). A prior study showed that the size, Young's modulus and fracture toughness of the food significantly impact the grasping strategies adopted by strepsirrhines (Peckre et al, 2019). In particular, small items were associated with an increase in mouth grasping compared with strategies involving the hand(s).…”

“…Choosing a relative measure of the size of the item allowed us to control for the high variability of body size across the species tested. As Young's modulus and fracture toughness did not impact small item grasping strategy (Peckre et al, 2019), we examined 21 different small items across a broad range of Young's modulus and fracture toughness values (Table S1). Published data for Young's modulus (median 1.4 MPa, range 0.2-50.4 MPa) and fracture toughness (median 308.6 J m −2 , range 20.0-1030.6 J m −2 ) in a sub-sample of the small items (N=12) indicate a significant positive correlation (r=0.71, P=0.01) between the two material properties (raw data and citations provided in Table S1).…”

“…Strepsirrhine species share a phylogenetic history and, therefore, cannot be considered as independent data points (Felsenstein, 1985). Previous studies showed that there is a significant phylogenetic signal in grasping behaviour associated with large items but not with small ones (Fabre et al, 2018;Peckre et al, 2016Peckre et al, , 2019. We tested the presence of a phylogenetic signal in the coefficients of the effect of food mobility on grasping behaviour using the multivariate K-statistic using the R 'physignal' function included in the 'geomorph' package (Adams, 2014; https://CRAN.…”

“…Contrary to catarrhines, they seem to show a unique (i.e. one) whole-hand power grip with no digit individualisation (Bishop, 1962(Bishop, , 1964Christel and Fragaszy, 2000;Peckre et al, 2019;Regan et al, 2001;Reghem et al, 2013;Schöneich, 1993). Nevertheless, all strepsirrhine species appear to be able to grasp with the hands, and significant variation seems to exist in the propensity of these species to use their hand to grasp static objects (Peckre et al, 2016).…”

“…First, while the mobile property of living prey is associated with increased hand use compared with oral prey capture, these tests compared food items that differ not only in their mobility but also in their size, Young's modulus and fracture toughness. These are properties that are known to influence grasping strategies (Nekaris, 2005;Peckre et al, 2019;Petter, 1962;Pouydebat et al, 2014;Scheumann et al, 2011;Toussaint et al, 2013Toussaint et al, , 2015. Toussaint et al (2015) were the first to test the specific use of the hands to grasp a mobile version of an otherwise static food item by swaying a piece of banana on a string.…”