Wild bearded capuchin monkeys in Brazil select heavy stones, sometimes weighing more than a kilogram, carry them to anvils and use them to hammer open palm nuts — transporting both tools and food to the places where they can use them effectively.

Picture two animals doing what looks like the same job. One is a chimpanzee in the forests of Côte d’Ivoire, an animal that often weighs more than 40 kilograms, lifting a stone to smash open a nut. The other is a bearded capuchin in the Brazilian scrub, small enough for a child to pick up, hauling a stone that weighs almost as much as it does. Both crack nuts that resist a similar amount of force before they give way.

That mismatch, the tiny monkey pulling off a strongman’s version of the same feat, is what keeps drawing researchers back to compare them.

The two groups at the center of this comparison are the bearded capuchins at Fazenda Boa Vista in Piauí, Brazil, and the Western chimpanzees of Taï National Park. A team led by Elisabetta Visalberghi, Dorothy Fragaszy, Giulia Sirianni and Christophe Boesch has studied them side by side.

What the capuchins actually do

The Brazilian monkeys don’t grab the first rock they find. Hard stones are scarce at Fazenda Boa Vista, and the capuchins are picky about which ones they pick up and where they carry them.

A 2009 study found that they choose, carry and use the right stone for the nut in front of them. The authors wrote that “planning actions and intentional selection of tools is within the ken of monkeys and similar to the tool activities of hominins and apes.” That’s a firm claim from a single study, and “planning” is a loaded word in animal science, so it’s worth reading as the team’s interpretation rather than settled fact.

The stones are not small. At Fazenda Boa Vista, the hammers capuchins use to crack palm nuts weigh, on average, more than 1 kg. In one case the researchers describe, a dominant male weighing just 4.2 kg used a 3.5 kg stone to crack a piassava nut. Opening a nut still takes many blows. In field experiments, the most efficient capuchin needed about 6.6 strikes per nut, while a young male might take more than 75.

The scale problem

A capuchin’s hammer is a substantial share of its own body weight. The stones used at Boa Vista come to roughly 25 to 40% of an adult’s body weight. A chimpanzee’s hammer, by contrast, is only a small slice of its much bigger frame. Chimpanzees weigh many times more than capuchins, yet the toughest nut each cracks fails at a similar force.

The physics leads somewhere striking. To break a nut of a given toughness, the researchers note, a certain amount of energy has to go into the blow, no matter which animal delivers it.

Reading the choices as planning

What tips this from raw effort toward something more deliberate is how the monkeys choose between stones.

In field experiments, when two stones weighed the same, capuchins took the closer one. When stones differed in both weight and distance, some individuals preferred the heavier stone no matter how far they had to carry it, while others stuck with whatever was near. That same study found the monkeys’ body weight affects their decisions. Body size shapes what counts as a trip worth making. And when they can’t judge a stone’s weight by sight, capuchins gather that information by moving, lifting, or tapping it before committing.

The clearest sign that effort is matched to the task comes from experiments where the researchers varied how tough the nuts were. Given nuts of different toughness, the capuchins chose, carried and used the right stone for each one. A monkey doesn’t lug a kilogram of rock around for a nut a lighter stone would open. It saves the big hammer for the job that needs it. That’s an experimental result rather than a rule of capuchin life, but a telling one.

Where the two species diverge, and where the story stops

If there’s a single image worth keeping, it’s the one about scale. A chimpanzee swinging a hammer that weighs a fraction of its body is impressive. A capuchin the size of a house cat carrying a stone that runs to a large share of its own weight, choosing it by feel, and saving it for the nut that actually needs it, is arguably the greater feat once you measure it against the animal’s frame. What that says about the mind behind the choice is the part the researchers are still working out.