How tufted capuchin monkeys (Sapajus spp.) and humans (Homo sapiens) handle a jointed tool.

The embodied theory of tooling predicts that when using a grasped object as a tool, individuals accommodate their actions to manage the altered degrees of freedom in the body-plus-object system. We tested predictions from this theory by studying how 3 tufted capuchin monkeys (Sapajus spp.) and 6 humans (Homo sapiens) used a hoe to retrieve a token. The hoe's handle was rigid, had 2 segments with 1 planar joint, or had 3 segments with 2 (orthogonal) planar joints. When jointed, rotating the handle could render it rigid. The monkeys used more actions to retrieve the token when the handle had 1 joint than when it had no joints or 2 joints. They did not use exploratory actions frequently nor in a directed manner in any condition. Although they sometimes rotated the handle of the hoe, they did not make it rigid. In a follow-up study, we explored whether humans would rotate the handle to use a 2-jointed hoe in a conventional manner, as predicted both by the embodied theory and theories of functional fixedness in humans. Two people rotated the handle to use the hoe conventionally, but 4 people did not; instead, they used the hoe as it was presented, as did the monkeys. These results confirm some predictions but also highlight shortcomings of the embodied theory with respect to specifying the consequences of adding multiple degrees of freedom. The study of species' perceptual sensitivity to jointed object's inertial properties could help to refine the embodied theory of tooling. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Measures of food intake in mantled howling monkeys.

Food intake (i.e., the amount of food consumed by an individual) is a crucial measure for studying feeding behavior, but its measurement requires high visibility of individuals and long recording sessions, which are often difficult to accomplish under field conditions. As a consequence, studies on the feeding behavior of primates typically do not estimate food intake directly, and focus rather on studying dietary patterns through indirect measures of food intake, such as time spent feeding, number of food bites and food intake rates. The aim of the present study was to determine the validity of these estimators of food intake in mantled howling monkeys (Alouatta palliata) by comparing the estimations with the direct measurement of food intake. We recorded 97 feeding episodes of two male and two female adults, during which we determined the number of ingested food units (i.e., number of leaves and number of fruits), the number of bites taken and time spent feeding. After weighing units of food similar to those consumed, we calculated food intake and mean intake rates per food type (ripe fruits, unripe fruits, mature leaves, and young leaves). The number of bites taken by mantled howling monkeys during feeding episodes was strongly related to food intake, and this relationship was not affected by the type of food ingested. In contrast, neither time spent feeding nor food ingestion rate were related to food intake. These results suggest that the number of bites could be used as a valid proxy to study food intake in this species, whereas the other two measures are likely to yield inaccurate estimates of food intake.