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Whishaw, I. Q., Murillo Chacon, E., Guadamuz Meléndez, Y. F., Tellez Arias, W., & Melin, A. D. (2026). The exclusive use of a reach versus a grasp when foraging by the mantled howler monkey (Alouatta palliata). Animal Behavior and Cognition, 13(3), 256-281.  https://doi.org/10.26451/abc.13.03.04.2026

Howler monkeys (Alouatta) are characterized as energy minimizers, featuring a reduced daily foraging range, an above-branch quadrupedal positional repertoire and a reduced forelimb motion. The present study quantifies how these behavioral features influence the reach and grasp of the mantled howler monkey (Alouatta palliata) for harvesting a diet of leaves, fruit, flowers and vines. Wild howler monkeys, habituated to human observers, were filmed in Sector Santa Rosa (SSR), Área de Conservación Guanacaste in northwestern Costa Rica. Video recordings of the monkeys harvesting 20 food species were subject to frame-by-frame analysis. The monkeys suspended themselves by their tail for 58% of 2208 reaches. On 99% of reaches they hooked a branch with a power grasp and pulled it toward their mouth. Reaching for a branch featured a pronated hand and upper arm abduction/adduction with minimal hand or arm rotation. Fruit and flowers were plucked from the branch using the incisors and leaves were plucked using the molars. Plucking a food item from a branch was aided by lower arm extension that relaxed the hand’s pull on a branch, allowing the recoil of the branch to jerk the food from the branch. Independent grasps by hand (249 observations) were only used to adjust food in the mouth or take food from the mouth to reorient it for selective biting. Howler monkey tail prehension and branch withdraw feeding thus represents an almost exclusive use of a reach strategy with little/no use of an independent grasp by the hands or mouth. In requiring little hand or finger skill, branch pulling may contribute to the howler monkey’s energy minimizing lifestyle. This reduced motoric commitment to eating may enable a concordant reduction in brain size as a further source of energy minimization. The findings support the idea that variation in the use of the reach and the grasp contribute to the feeding diversity of primates.

Branch pulling, Dual visuomotor theory, reaching, hand use, prehension, teeth grasps, positional behavior, folivory, energy minimizing