Take any of my classes in biological anthropology, and you figure it out right quick. I find all nonhuman primates fascinating, but not equally so. Great apes—the orangutans of Asia, and the chimpanzees, bonobos, and gorillas of Africa—are the spice of my anthropological life. To my great good fortune, I’ve spent time observing (and in some cases filming) all four in captive settings.
I pretty much buy into—and thus teach and write about-- the cognitive watershed idea: The great apes, speaking now in terms of patterns, are capable of significantly greater cognitive feats than any other nonhuman primates. Theory of mind, or perspective-taking on a social partner’s knowledge or emotional state; degree of abstract thinking about situations related to what we humans call “dominance” and “kinship”; and sophisticated problem-solving with human symbol systems (in captivity) and with tools (in the wild and captivity) are some of the abilities that set the great apes apart.
It’s not that I don’t root for the monkeys—after all, my dissertation research had me wandering around southern Kenya for over a year, in view of stunning Mt. Kilimanjaro, watching some pretty smart baboons.
Because I always welcome new data showing that monkeys are pushing back against the cognitive-watershed idea, I was happy to come across a new article available online at the journal Animal Behaviour titled “Wild bearded capuchin monkeys place nuts in anvils selectively.” It is written by written by Qing Liu, Dorothy Fragaszy, Barth Wright, Kristin Wright, Patricia Izar and Elisabetta Visalberghi.
Liu et al. review previous work showing that bearded capuchin monkeys crack open nuts and seeds using hammer stones carefully selected as to weight and tendency to crumble. The monkeys also select heavier stones to crack the harder-to-crack nuts and prefer easier-to-crack nuts when available. In other words, the monkeys make strategic choices when it comes to tools and nuts. But what about the anvils, the striking platforms used in the nut-cracking sequence? Is there any selectivity at work in how monkeys choose these?
The scientists answer this question via a neat experimental intervention. On private property in an area of dry woodland in Brazil, a group of bearded capuchins (Cebus libidinosus) is habituated to observers. There, the researchers put out palm nuts, a single stone, and three log anvils experimentally manipulated in dimensions of the pit (where the stone is laid on the anvil).
“Anvil pits varied,” Liu et al. note, “in horizontal dimension (small, medium and large) in experiment 1 and depth (shallow, medium and deep) in experiment 2. In both experiments, three different pits were simultaneously presented, each on one anvil.” Nine monkeys – six imales, three females—voluntarily participated in the study.
Results show that the monkeys preferred the large and shallow pits—exactly those pits most effective for cracking the palm nuts. In this context, effectiveness is defined as “the number of nuts cracked with 100 strikes in that pit.”
Very clearly, the monkeys act selectively regarding anvils. However, the data offer a neat twist: “The magnitude of their preference for a particular pit (50% of total strikes in the large pit in experiment 1 and 75% in the shallow pit in experiment 2) was not as large as their preference for heavier stones… or their preference for nuts that were easier to crack” as was revealed in previous studies.
Interestingly, the monkeys did not preferentially observe or follow the more proficient nut-crackers in their group, a finding known from other capuchin work, but they were more likely to use the pit chosen by the preceding individual to nut-crack. Further, monkeys explored the anvils continuously throughout the experimental period. Interaction with the anvil, though, is qualitatively different from interaction with a nut or a stone that can be handled directly, and this fact may explain the somewhat reduced selectivity where anvils are concerned.
In other words, feedback to the monkey about the anvil comes in large part through the action of striking. “The stone serves,” the authors write, “as an extension of the hand, until the point of contact between the nut and the stone, when rebound, vibration, sound or other movement of the stone and the nut inform the body.” The monkeys’ cognition, then, is embodied. Looking at the nut-stone-anvil nut-cracking complex, we can see that “the boundary of the animal-environment system (what constitutes animal and what constitutes environment) shifts dynamically” according to context.
The monkeys probably learn about anvil selectivity via both social observation and embodied cognitive exploration. Embodied cognition is a hot topic in studies of human perception and action these days; bringing it to bear on monkeys’ intelligent tool-using behavior is an excellent move.
Naturally enough, I think that the complex and cultural nut-cracking, termite-fishing, and spear-hunting patterns carried out by chimpanzees cry out for this kind of analysis too.
But for now—score one for the selective monkeys!