Apes, not Monkeys, Ace IQ Tests
DURHAM, N.C. -- The great apes are the smartest of all
nonhuman primates, with orangutans and chimpanzees consistently
besting monkeys and lemurs on a variety of intelligence tests,
Duke University Medical Center researchers have found.
"It's clear that some species can and do develop enhanced
abilities for solving particular problems," said Robert O.
Deaner, Ph.D., who led the study as part of his doctoral
dissertation. "But our results imply that natural selection may
favor a general type of intelligence in some circumstances. We
suspect that this was crucial in human evolution."
The study was published online August 1, 2006, in the
Psychology. Funding was provided by the medical center's
Department of Biological Anthropology and Anatomy.
Experts in psychology broadly define intelligence as general
problem-solving skills -- "domain-general cognition" in the
parlance of the field. This intelligence allows an animal to
tackle new and unpredictable situations. Domain-general
cognitive ability is separate from domain-specific abilities
for solving certain environmental challenges, such as a bird
remembering where it cached food.
Intelligence testing of animals has repeatedly revealed that
some species perform better than others. This suggests that
some animals have better domain-general skills, Deaner said.
However, scientists have been hard-pressed to convincingly
prove these differences could be attributed to intelligence, he
"The trouble is that one species may outperform another in a
problem-solving test not because it's smarter, but because one
species is poorly suited to that particular testing situation,"
he said. For example, one species may be more comfortable
grabbing a joystick.
Deaner and his colleagues reasoned that they could refute
this premise -- that performance differences resulted from
particular testing situations -- by demonstrating that some
primate species surpassed others across a wide range of
problem-solving tests. Primates are an excellent comparison
group because their similar perceptual and motor skills means
that the same tests are generally appropriate for all of them,
Deaner said. But developing a suitable data set to test this
idea was not easy.
"At first we thought gathering the data would require a
lifetime," said Deaner, now an assistant professor of
psychology at Grand Valley State University in Allendale, Mich.
"Ideally, one would test several individuals from each of 20 or
30 species with dozens of cognitive tests, but this certainly
didn't seem practical. Then we realized the data had already
been gathered by an army of comparative psychologists."
The team first pored through hundreds of published studies,
then assigned each testing situation or experiment to one of
nine overall paradigms. For example, one paradigm was patterned
strings. During the test, a primate is shown an array of
crossing strings, only one of which is tethered to a treat. The
subject is allowed to pull only one of the strings and must
decide before pulling which string is actually attached to the
food. The paradigm taps the ability to form spatial
representations, considered crucial for intelligent
The results were clear: there were a few cases where one
species performed better than another one in one task and
reversed places in a different task, but, overall, some species
truly outperformed others. "Our research strengthens the
long-standing notion that some animal species truly are more
intelligent than others," Deaner said. The smartest species
were clearly the great apes -- orangutans, chimpanzees, and
gorillas – which performed much better than monkeys and
"The fact that great apes performed better than other
primates in these laboratory tasks is reassuring," said Carel
van Schaik, Ph.D., a study co-author and director of the
Anthropological Institute and Museum at the University of
Zurich. "After all, in absolute terms, their brains are the
largest and they show the most sophisticated behavior under
natural conditions -- deception and culturally-transmitted
behavior, including tool-use."
Though some species clearly outperformed others, there was
no evidence that any species performed especially well within a
particular paradigm. This result contradicts the theory that
species differences in intelligence only exist for narrow,
specialized skills, Deaner said. Instead, the results argue
that some species possess a broad, domain-general type of
intelligence that allows them to succeed in a variety of
Team statistician Valen Johnson, Ph.D., a professor of
biostatistics at the University of Texas M. D. Anderson Cancer
Center, created a new statistical technique to examine the data
for consistency across the various tests. "It was tougher than
it looks, because most species were only tested in a few
situations," Johnson said. "Conventional methods wouldn't do