Understanding of causality involved in nut cracking by semi-wild
2nd Augst,2006 - 15th Augst,2006
Wild and semi-wild tufted capuchin monkeys (Cebus apella)
are known to crack open palm nuts with the hammer stone on the hard
surface. We examined experimentally whether they understand the physical
causality involved in this apparently intelligent tool-using behavior, at
the Tiete Ecological Park, Sao Paulo, Brazil. We did this in collaboration
with Dr. Eduardo Ottoni and colleagues at the Institute of Psychology,
University of Sao Paulo. For this particular investigation period, in fact
we prepared two anvils. One was made of hard cement blocks and the other
was made of soft sponge and rubber. We placed the two anvils on the site
where the monkeys often crack open nuts. We also placed a familiar hammer
stone and palm nuts between the two anvils.
During the investigation period, 6 individual monkeys were
observed to crack at this experimental cracking site. In all of the dozens
of attempts, the monkeys consistently used the hard anvil. The monkeys
were observed to do this on the soil next to the anvils in just two
attempts, but were never observed to do so on the soft anvil. The data
show that the capuchins at this park clearly understand that the hardness
of the anvil surface is an essential feature for the successful nut
cracking. We plan to extend this study toward the analysis of features of
hammer stones the monkeys recognize as essential. In particular we plan to
test the monkeys for their choice of had and soft hammers and hammers of
various shapes. Some of these tests are now ongoing by my collaborators.
A female capuchin monkey named Filo cracking a nut.
The experimental setup.
The anvil on the left is a soft one and that on the right is a hard
A familiar hammer stone and palm nuts are placed between the two anvils.