How bearded capuchin monkeys (Sapajus libidinosus) prepare to use a stone to crack nuts

Abstract

Bearded capuchin monkeys crack nuts with naturally varying stone hammers, suggesting they may tune their grips and muscular forces to each stone. If so, they might use discrete actions on a stone before lifting and striking, and they would likely use these actions more frequently when the stone is larger and/or less familiar and/or when first initiating striking. We examined the behavior of (a) four monkeys (all proficient at cracking nuts) with two larger (1 kg) and two smaller (0.5 kg) stones, (b) 12 monkeys with one 1 kg stone, and (c) one monkey during its first 100 strikes with an initially unfamiliar 1 kg stone. Bearded capuchin monkeys used three discrete actions on the stone before striking, all more often with the larger stones than the smaller stones. We infer that the first discrete action (Spin) aided the monkey in determining where to grip the stone, the second (Flip) allowed it to position the stone on the anvil ergonomically before lifting it, and the third (Preparatory Lift) readied the monkey for the strenuous lifting action. The monkey that provided 100 strikes with one initially unfamiliar stone performed fewer Spins in later strikes but performed Flip and Preparatory Lift at consistent rates. The monkeys gripped the stone with both hands along the sides to lift it, but usually moved one or both hands to the top of the stone at the zenith of the lift for the downward strike. The findings highlight two new aspects of the capuchins’ nut‐cracking: (a) Anticipatory actions with the stone before striking, especially when the stone is larger or unfamiliar, and when initiating striking and (b) shifting grips on the stone during a strike. We invite researchers to investigate if other taxa use anticipatory actions and shift their grips during percussive activity.

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

“…The monkeys occasionally acted in ways that could enable them to perceive the situation's relevant affordances (i.e., how they could act with the hoes to achieve their goal), but they did not seem to do so intentionally. Persistent but low rates of haptic exploration characterize capuchin monkeys' actions in other settings as well-as when searching for sunflower seeds on an unseen, uneven surface (Lacreuse & Fragaszy, 1997) and preparing to use stones to crack nuts (Fragaszy et al, 2019).…”

“…Capuchin monkeys use manual exploratory actions when retrieving small objects hidden out of view (Lacreuse & Fragaszy, 1997); they use exploratory manual actions to identify if a nut is empty or full (Visalberghi & Neel, 2003) and to position nuts in anvils in preparation to crack them with a stone (Fragaszy et al, 2013). They use exploratory manual actions with stones prior to selecting one to transport (Visalberghi et al, 2009) and prior to lifting them to crack nuts placed on anvils (Fragaszy et al, 2019). Capuchin monkeys reposition canes, altering the spatial relation between the canes and a piece of food so that the canes can be used to retrieve the food (Cummins-Sebree & Fragaszy, 2005).…”