Science 23 June 2006 Science Magazine
Social Animals Prove Their Smarts
A new generation of experiments reveals that group-living animals have a
surprising degree of intelligence.
Figure 1 Groupthink. Social living may have fostered the evolution of
CREDIT: K & K AMMAN/GETTY IMAGES
- A Fox observed a Crow in a tree with a piece of cheese in her mouth.
Hungry for the cheese, he thought up a ruse to get it. He said, "What a
noble bird I see above me! Her beauty is without equal. If only her voice is
as sweet as her looks are fair …"
The Crow was greatly flattered, and to show the Fox that
she could sing, she opened her mouth and gave a loud caw. Down came the
cheese. The Fox, snatching it up, said, "You have a voice, madam, I see:
What you want is wits."
--Retelling of a fable by Aesop
In Aesop's time, it was common to endow animals with qualities of the human
mind. In addition to the flattering fox, Aesop told of a deceitful eagle that
lured a turtle to its death and a compassionate lion that exchanged favors with
a shepherd. But although folktales often feature scheming or generous animals,
scientists have spent most of the past few centuries thinking of other species
as "dumb," or at least driven by innate behaviors. Even when biologists,
anthropologists, and psychologists finally began to appreciate the complexity
of animal cognition in the 1950s, they tended to focus on the mental advantages
that still separated humans from the rest of the animal kingdom.
Even 10 years ago, most researchers considered the
intellectual chasm between humans and animals too broad for even primates to
begin to bridge. A few claimed that animals have advanced cognitive skills, but
early studies were chiefly anecdotal and convinced few hard-core experimental
biologists, says Michael Tomasello, a developmental psychologist at the Max
Planck Institute for Evolutionary Anthropology in Leipzig, Germany. "From a
scientific point of view, most of the evidence [for higher cognition] was not
very good," he says.
In the past decade, however, the field of animal cognition
has taken off, galvanized in part by a once-obscure idea that the development
of social skills drove the evolution of general intelligence (see sidebar). The
thinking is that the need to remember and track peers sharpened social animals'
ability to do other useful cognitive tasks, such as remembering where and when
particular fruit trees were ripe for the picking, or learning tool use from a
particularly creative peer. From this perspective, abilities such as
remembering the identity of dominant individuals are crucial steppingstones to
the most advanced cognitive abilities, such as learning how to interact with
those dominants for personal gain--something scientists assumed only humans
Of course, humans are masters of social intelligence. We
judge friend from foe and head honcho from underling by the raising of an
eyebrow. We scheme, deceive, and sometimes help others with no gain to
ourselves. But it turns out that other animals can do these things too, at
least to some degree. Researchers using rigorous tests of such abilities in
animals are finding numerous examples. Crows deceive each other, as do apes;
hyenas keep track of social hierarchies. There are enough parallels that now
"everyone is interested in discovering the similarities between animals and
humans," says Bennett Galef, an emeritus animal behaviorist at McMaster
University in Hamilton, Canada.
Together, the new studies, particularly those of apes and
birds, are providing provocative evidence that perhaps humans aren't as special
as we might like to think, says Brian Hare, a biological anthropologist also at
the Max Planck Institute for Evolutionary Anthropology. What was once
considered a sharp line separating humans from all other animals is becoming a
blurry gray area, with various animals possessing certain parts of the set of
skills that we consider advanced cognition.
In large part, that's because we're not the only species that
has evolved to cope with the demands of living in groups, says Nicola Clayton,
who studies animal cognition at the University of Cambridge, U.K. People in
villages, chimps in troops, ravens in flocks, and hyenas in packs all need to
be able to size each other up and modify their behaviors as needed.
Not all researchers are impressed with animals' newly
demonstrated social ingenuity, and there is disagreement about its
implications. All the same, says Marc Hauser of Harvard University, for
cognitive scientists the research questions have changed, from what sets humans
apart, to what animals reveal about the building blocks of higher cognition.
Throughout history, researchers have swayed back and forth on the question of
animal intelligence. In the 1600s, nonhuman animals were considered little more
than breathing machines. But after Darwin implied that differences between
humans and other species were a matter of degree, dozens of examples of "smart"
animals came to light, only to be subsequently debunked. During the 19th and
early 20th centuries, the prevailing idea became that most animals, primates
included, didn't reason but instead had sets of rules that dictated their
behavior. And animal-cognition researchers avoided inferring states of mind
from an animal's behavior. They took their cues from 19th century psychologist
C. Lloyd Morgan, who argued that complex behaviors don't necessarily require
complex thought, and that researchers should look for simple, mechanistic
explanations for even the most complex animal behaviors.
But starting in the late 1970s, some researchers went against
the grain. In 1976, psychologist Nicholas Humphrey of the London School of
Economics stirred the pot by suggesting that getting along with others required
more brainpower than other aspects of daily life, and that social animals might
have humanlike smarts. "It was very much at odds with what everyone was
thinking at the time," Humphrey recalls.
Figure 2 Tool savvy. An eager student learns how to retrieve a treat
from outside its cage (left), while another ape takes a "grape-retrieval"
tool to save for later use (right).
CREDITS: (LEFT TO RIGHT) ANDREW WHITEN; JOSEP CALL/MAX PLANCK
Two years later, psychologists David Premack and Guy Woodruff of the University
of Pennsylvania proposed that chimpanzees might be able to think about what
they are doing and to understand what others are thinking, an ability they
called a "theory of mind." Even as children, humans can read each other's minds
at least to some degree. Maybe chimps could as well, if we could only find a
way to communicate with the apes, Premack and Woodruff proposed. And in the
late 1980s, Andrew Whiten, an evolutionary psychologist at the University of
St. Andrews in Fife, U.K., and his colleagues suggested that the relatively big
brains of humans and other primates evolved not to see, smell, or fight better
but to recognize and deal with social dilemmas.
But "it took a while for people to start thinking about these
ideas," says Clayton. Today, psychologists recognize "theory of mind" as a
critical cognitive skill, underlying teaching, deception, and perhaps even
language (Science, 16 May 2003, p. 1079). It's also seen as a steppingstone to
consciousness, or thinking about one's own thoughts--often considered the
ultimate in higher cognition (Science, 25 June 1999, p. 2073).
Yet scientists disagree on exactly what theory of mind is,
and the literature is filled with conflicting reports about its existence in
animals. As recently as 6 years ago, Hauser argued that chimps didn't have even
the basics of a theory of mind. Today, "the field has been completely turned
upside down," says Hauser. "The provocative question is not do they have a
theory of mind; it is thinking about the components that are going into theory
Reading the primate brain
Corporate meetings, playground games, and bargain shopping all require complex
negotiation skills and a keen sense of who one's allies are. It makes sense
that apes, our closest kin, should be political as well, but it has taken
decades for scientists to come to grips with the idea that apes have street
smarts akin to ours. Beginning in the 1960s, field researchers such as Jane
Goodall were reporting sophisticated politics among chimps, for example, but
controlled experimental evidence was rare.
Figure 4 Hide, no seek. Ravens (inset) turn away from each other to
keep secret the location of their buried food.
CREDIT: THOMAS BUGNYAR
Apes rarely did well on self-awareness, memory, gaze-following, gesture,
spatial learning, and other tests at which even young children excel. For
example, children will follow another person's gaze, showing that they are
aware that the tester is in fact looking at something. But chimps confronted
with humans with or without blindfolds on their heads didn't discriminate among
who could see--and therefore deliver a reward--and who couldn't.
Then 6 years ago, Hare and his colleagues showed that under
the right circumstances, chimps could pass some of these tests with flying
colors. The secret was that chimps are exquisitely tuned in to their
competition, particularly when food is involved, and will do everything they
can to get a treat.
In one experiment published in 2001, Hare, Tomasello, and
their colleagues paired a dominant chimp with a subordinate and manipulated the
two apes' view and access to food. If both could see the food, the subordinate
deferred. But if the dominant chimp couldn't see the treat, the subordinate
quickly snapped it up.
The experiment, coupled with a related but simpler one
published a year earlier, was revolutionary. "There was a big change in
perspective," says Clayton, and a flurry of more ecologically appropriate
experiments--geared to what motivates chimps in the wild--followed. For
example, in a new study in Cognition, Hare and his colleagues designed another
competition over food. They had chimps go head-to-head against a human who
pulled food out of reach as a chimp went to grab it. If the chimps were given
the option, they sneaked up behind a barrier to get to the food instead of
approaching it directly. Thus, the chimps demonstrated not only that they knew
what the human could see but also that they knew how to manipulate the
situation to stay out of sight. Other studies have shown that chimps can
recognize when a human is imitating them.
They can also sense the motives of others. A study a few
months ago showed that chimps kept track of partners who best collaborated in
retrieving inaccessible food and chose that same partner again in the next
trial (Science, 3 March 2006, p. 1297). New experimental designs are helping to
demonstrate chimp smarts outside the social realm, too: Studies show that they
can reason about the movement of things they cannot directly see and plan for
the future by taking account of past experiences.
Figure 5 Meal planning. Western scrub-jays remember where and when
they buried wax worms in ice cube trays.
CREDIT: PHOTO COURTESY OF NICOLA CLAYTON
In parallel, other researchers are demonstrating that social primates are smart
enough to help their cause through teaching and learning. Chimps apparently
learn tool use from one another, and communities in different regions of Africa
develop what some researchers consider cultural differences in tool use. The
idea is still controversial, but field and, more recently, lab work are
strengthening the case.
Last year, Whiten and his colleagues demonstrated "social
learning" of traditions in two groups of captive chimpanzees. The researchers
trained one female from each group either to pull or to lift a stick tool to
retrieve a reward. After watching the female for just 20 minutes a day, each
group learned its respective technique within a week. Not only were the chimps
able to copy the lifting or the pulling, but lifters also almost never tried to
pull or vice versa, suggesting a strong tendency to conform to the local norms,
Taken together, this work shows striking parallels with human
abilities, says Hare. But do chimps have a theory of mind? They lack the most
advanced skill identified by Woodruff and Premack: the ability to realize that
another individual is thinking something wrong, or that it has a false belief,
points out cognitive scientist Daniel Povinelli of the University of Louisiana
in Lafayette. In his view, to have a theory of mind, a species must pass the
false-belief test. And so far, chimps fail it. "People who [keep] insisting
that 'It's got to be there, at least a little bit,' in dogs, cats, chimpanzees,
my cousin Ned's horse are really missing the point," Povinelli says.
But Hare argues that theory of mind is "a whole suite of
abilities." The new results indicating that chimps can judge what others are
thinking, manipulate others through deception, and so on "are shooting down the
all-or-none hypothesis about theory of mind," he says.
He adds that the false-belief test is so challenging that it
foils children up to about age 4. In one test, for example, a child and a
companion watch a tester put candy in a box. When the companion leaves, the
candy is moved into a bucket. Because the child doesn't yet have a sense of
false belief, she thinks the companion will know to look in the bucket, whereas
an adult realizes that the companion still thinks the candy is in the box.
Hare argues that experimenters simply haven't found a good
way to figure out where a chimp expects the companion to look for the treat,
and that therefore we don't know yet whether chimps pass or fail this test. "We
have not been able to come up with a convincing experiment with nonhumans," he
Picking bird brains
So far there's no evidence--and no good tests--of understanding false belief in
birds. But contra the opinion of the fox in Aesop's tale, Clayton and her
colleagues have found that crows and their relatives, including ravens and
scrub-jays, have social intelligence on par with primates, apparently deceiving
others in order to win more food. In Clayton's studies, she takes advantage of
the natural tendency of many birds to stash surplus food in anticipation of
lean times, and for other birds to steal those caches.
She and her husband Nathan Emery have recreated this behavior
in her lab at the University of Cambridge, providing captive birds with sand-
filled trays in which to bury wax worms. Sometimes the duo switches the food
after it's been hidden; in other cases, they allow another bird to witness the
burial. "They are putting birds in different situations and showing that the
birds do all sorts of flexible things," says Hare.
Using this approach, Clayton and her Cambridge colleague
Anthony Dickinson have shown that western scrub-jays remember what they have
buried, and when and where they buried it, a phenomenon called mental time
travel. They retrieve perishable food before it rots, for example, while
waiting longer to retrieve nonperishable items. Many animals can remember where
food has been placed, but rarely have researchers demonstrated that an animal
can keep track of when an event occurred and use past events to figure out what
to do in the future. This ability was demonstrated in bonobos and orangutans
only recently, in an experiment published online in Science last month (16
June, p. 1662). The study showed that these primates could select the proper
tool for a task even though they wouldn't need it until the next day. And in
this week's issue of Current Biology, other researchers demonstrated that
mangabeys, a primate found in Uganda, will take note of unripe fruit and come
back to pick it after a few sunny days.
For birds, anticipating the future enables them to realize
when they must take evasive action to protect stashed food. Working with Joanna
Dally, then a graduate student, and Emery, Clayton showed in another experiment
that western scrub-jays that see a potential thief will hide food far away from
the other bird and sometimes move their supplies several times. In other cases,
they wait to stash food until the onlooker is distracted. The jays take none of
these precautions if no other birds are in sight. "There's flexibility at
multiple levels," says Clayton.
Furthermore, birds who have been thieves themselves are more
likely to take these evasive actions than birds who have not been so nefarious.
The jays' behavior implies that they are aware of the onlooker's intentions and
are using their past experience to predict the future actions of the potential
thief, says Clayton.
Figure 6 Keeping track. Hyenas remember the players--and their
relatives--when bickering breaks out.
CREDIT: LAURA SMALE
In addition, like apes, the jays track the social status of their competitors
and change their behavior accordingly. In the lab, scrub-jays try hard to hide
food from dominants but not from breeding partners, whose pilfering is
tolerated, Clayton's group reported. All this hints that jays do have elements
of a theory of mind, says Clayton.
Lab work on ravens supports this idea. In most cases, a raven
poised to grab another raven's stashed food doesn't hesitate to act when
bystanders might beat them to it, Thomas Bugnyar and Bernd Heinrich of the
University of Vermont in Burlington reported in 2005. But if the stash belongs
to a dominant member of the flock, the thief will briefly search elsewhere, as
if to allay suspicion. Such actions seem intentional and suggest that the
thieves understand what other birds are seeing, says Bugnyar. "There's no
question that birds are more intelligent than anyone thought they would be,"
But researchers still don't agree on how to interpret these
results. Cognitive ethologist Marc Beckoff of the University of Colorado,
Boulder, sees little difference in social prowess between humans and other
species, and he suggests that animals should be treated more like humans.
Other researchers still draw a line separating the minds of
humans and animals, even other social species. The new experiments highlight
how "various species have remarkable cognitive skills for the problems they
must solve," but they stop short of showing a theory of mind or other advanced
cognitive skills, says Povinelli. Humans, by virtue of having language, have a
fundamentally novel cognitive system, he points out. Tomasello agrees, noting
that humans excel at many skills: They are better teachers, for example.
Furthermore, what looks like humanlike cognition may not be.
Dogs, for example, seem to know what their owners are thinking. But "they are
not reading people's minds but our behavior," cautions Clive Wynne, a
psychologist at the University of Florida, Gainesville. For example, those
ravens avoiding the wrath of dominant birds could be picking up on subtle
behavioral cues that humans can't read, he says.
To resolve whether external cues or internal decision-making
underlie seemingly intelligent behavior, researchers need to expand their
studies to include more species, Wynne says. "We're only studying a tiny, tiny
fraction of animals," he says. "We really don't know what's out there."
Those studies are beginning, and by looking across the animal
kingdom, researchers are gleaning the conditions that predispose a species
toward social intelligence. For example, Kay Holekamp, an ethologist at
Michigan State University in East Lansing, has observed hyenas for 18 years and
concludes that these scavengers can recognize not just their own status
relative to the pack leader but also the status relationships of other pack
members. Other researchers are trying to measure social intelligence, albeit
often in indirect ways, in ungulates, elephants, and dolphins. And in this
week's issue of Current Biology, researchers demonstrated that fringe-lipped
bats learn to listen for unfamiliar prey from fellow bats.
All these studies suffer from the same limitation, however.
Researchers still can't read the minds of their subjects, warns behavioral
ecologist Anne Engh of the University of Pennsylvania: "Until we can come up
with creative methods of testing, we won't know whether complex behaviors are
the result of animals actually knowing what they are doing or whether they are
able to do complex things using cognitive short cuts."
Galef is particularly skeptical of researchers who have
concluded that chimps respond to peer pressure, that wolves and capuchin
monkeys have a sense of fairness, or that jackdaws are the avian equivalent of
the Good Samaritan. "It's gotten a little out of hand," he complains. And not
one species has yet passed the falsebelief test, he points out.
But does that matter? "It's not clear to me that you need [a
complete] theory of mind to be very skilled socially," says Hare. And for much
of the animal kingdom, those skills are good enough. Just ask Aesop.
Science 23 June 2006 Science Magazine
Man's Best Friend(s) Reveal the Possible Roots of Social Intelligence
When a chimp sneaks a banana behind another chimp's back, it's showing social
intelligence. So is the crow that buries worms behind a bush to prevent
bystanders from spotting the location of its stash. Recent controlled
experiments show that some social animals have evolved the flexibility and
intelligence to deceive and benefit from others and even predict what their
peers may do (see main text).
But why did these and related abilities evolve? In the 1970s,
Nicholas Humphrey, now of the London School of Economics, proposed that natural
selection favored the ability to distinguish anger from acceptance and to
respond to changes in the moods of one's companions. Individuals with these
kinds of social skills had advantages in gleaning food and mates--and avoiding
violence, he suggested. But such evolutionary scenarios are hard to test. Now
Brian Hare and Michael Tomasello of the Max Planck Institute for Evolutionary
Anthropology in Leipzig, Germany, and their colleagues are gleaning some clues
from studies of domestic dogs and their wild cousins, wolves and foxes.
Even as puppies, canines are adept at taking cues from their
owners--more adept than chimps, who are rarely able to follow a human's eyes or
hands to hidden food. That indicates a genetic component to this skill. For
decades, anthropologists have hypothesized that this behavior began when dogs
and humans were able to tolerate each others' company without aggression.
Togetherness fostered dogs' social skills, helping ensure their access to food
and other resources without having to resort to violence. Dogs better at
reading human minds were favored by selection, leading to a cycle of
interaction and cooperation.
That hypothesis is backed up by Hare's studies of foxes bred
for the past 45 years to be comfortable with humans. These foxes understand
human gestures--for example, when a human points to food--but untamed foxes
don't, even after extensive efforts to train them, Hare and his colleagues
reported in 2005. Studies done in 2002 and 2003 reveal "the exact same
difference between dogs and wolves," says Hare. Selecting foxes for
"togetherness" with humans also facilitated the evolution of the ability to
understand their two-legged caretakers.
A similar cycle of tolerance leading to increased
communication may have spurred the evolution of social intelligence within a
species, says Hare. As social tolerance increased, group members could get
close enough to an innovative, tool-using peer to imitate the behavior.
Selection could also favor even more congenial relationships, say for
cooperative food gathering or childcare, to the benefit of all involved.
Fellowship. Foxes bred to be tame are keenly tuned in to human
CREDIT: IRENE PLYUSNIA, PHOTO COURTESY OF BRIAN HARE
And the limits of social tolerance may partly explain differences in
intelligence among species, says Tomasello. For example, chimps have
competitive strategies down cold and can be quite sneaky. But they don't
cooperate very effectively, at least not intentionally; they would have come to
a bad end in Aesop's fable about the lion and the shepherd who traded favors.
In contrast, although humans too are competitive, we also
possess the capacity for more empathetic social skills. "We lie, but we can
also cooperate and coordinate planning," says Tomasello. "It's not that humans
have greater individual brainpower, it's that they have the ability to pool
their cognitive resources and benefit from what others have learned."
This evolutionary scenario sounds reasonable, but it will be
difficult to prove. Hare plans to compare higher cognition between bonobos and
chimps, which exhibit different levels of social tolerance, to see whether the
connection between sociability and cognition holds up. Bonobos are quite
tolerant; when they meet strangers, they have sex, whereas chimps often wage
war, he points out.
Even before these studies are done, other researchers are
taking notice--although they have yet to be convinced. "Evolutionary
modification of fearfulness and aggressive tendencies might be a critical
precursor to the evolution of social intelligence," says ethologist Kay
Holekamp of Michigan State University in East Lansing. "But I would certainly
be surprised if that were all there were to [it]."