What Primates Think
by Jill Locantore and Brendan Horton
When Koko the gorilla signals in American sign language, Koko again bad, after biting a trainer, is she using language to communicate? When the chimpanzee Yeroen acts as though a wound is much more painful than it really is, is he being intentionally deceptive? When Indah the orangutan correctly indicates which pile of grapes is lesser in number, is she doing math?
Scientists are hotly debating these questions, but one thing is clear: differences between humans and other primates arent as black and white as once was thought. Intelligence appears more a matter of degree, developing gradually throughout the primate lineage rather than sprouting magically when humans first arrived on the scene. Many of the features of our brain that support higher cognitive functions, such as language and mathematicsor at least their precursorsmay well be present in ape and monkey brains, and in the brains of long-extinct relatives like the Australopithecines.
Brain size by itself doesnt explain why we humans and our simian cousins are intelligent. The brain of a horse (Equus caballus), for example, is more than six times bigger than that of the relatively diminutive rhesus monkey (Macaca mulatta). However, if the two animals were the same size, the monkeys brain would be 20 times larger than its equine counterpart. Mr. Ed notwithstanding, scientists generally dont consider horses to be exceptionally bright barnyard animals.
Scientists, like Damon Clark of Princeton University, think that the most important differences concern the relative size of different brain parts. Consider the cerebellum. This pair of bun-like mounds at the base of the brain plays an important role in coordinated muscle movement. It accounts for a remarkably consistent proportion of the mammalian brainabout 13 percent by volume, according to Clarks calculationsregardless of total brain size.
In sharp contrast, the size of the neocortex varies widely across species. Fish, reptiles, birds, and amphibians, which emerged millions of years before mammals, lack a neocortex entirely. Unique to mammals, the neocortex is the layered, outer shell of the brain. (Cortex is Latin for barkas in tree barkand neo refers to the relative newness of this part of the brain, evolutionarily speaking.) Primate evolution has been characterized by a dramatic expansion of the neocortex. A whopping 80 percent of a human brain is taken up by the neocortex, compared to less than 20 percent of the brains of shrews (family Soricidae), for example.
Not only is the primate neocortex relatively larger than that of other mammals, it is more wrinkled, with many ridges and valleys. These increase the surface area of neocortex, allowing more nerve cells to be crammed inside the skull.
A Healthy Appetite
Primates boast brains 2.3 times larger on average than non-primates of the same body weight. But what fueled this development? Within the primate order, species that eat fruit tend to have larger brains than species that dine mainly on leaves. Although leaf-eating howler monkeys (Alouatta spp.) are closely related to fruit-eating spider monkeys (Ateles geoffroyi), the latter possess much larger brains. While leaves are ubiquitous and easily found, fruit resources are less abundant, and available only at certain times and in certain places. Successful harvesting of fruit requires animals to remember where fruit-bearing trees are located and anticipate when they will be in season. A larger brain may have helped fruit eaters deal with this kind of environmental variation.
Two challenges had to be overcome on the way to a large, metabolically active brain: greater energy use (at rest, our brains use 20 to 25 percent of our energy, whereas the brains of other primates use about eight to nine percent); and a greater amount of some basic brain building blocks, fatty acids known as arachidonic acid, or AA, and docosahexanoic acid, or DHA. According to University of Colorado physiologist Loren Cordain, our ancestors overcame the issue of increased energy use by the brain by getting rid of a roughly equal amount of intestinal tissue. We have less intestinal tissue than great apes, which require more in order to extract nutrients from a poorer diet. Our ancestors likely fed on richer foods, both in terms of calories (animal fat and bone marrow) and the essential fatty acids, which they got pre-packaged rather than relying on metabolic production.
Even with increased availability of AA and DHA in the diet, it was necessary for evolution to give larger brains more time to develop. Slower developmentlonger gestation, slower juvenile growth, and later onset of sexual maturitygave our hominid ancestors more time to accumulate AA and DHA from a diet still limited in these nutrients, says Cordain. Thus humans have taken advantage of both strategies. We are the slowest maturing primate and we are the most carnivorous primate, he says. In a study he co-authored last year, Cordain argues that our hominid ancestors would have needed a varied diet in order for the brain to evolve to a size larger than those of great apes. Based on his field work in Africa, such as diet would include freshwater fish and shellfish; the liver, brain, and muscle of large game; as well as wild nuts, roots, tubers, and vegetation.
Swelling Heads
At a recent meeting of anthropologists, Dean Falk of Florida State University was among those discussing the use of techniques that allow more sophisticated investigation of ancient brains from excavated skulls that contained them. Its a method that seems to turn the sham science of phrenology outside in. Instead of reading bumps on the outside of ones head, Falk uses magnetic resonance imaging, or MRI, to elucidate the outer shape of the brains from the impressions left on the inside of the skulls. While humans make suitable subjects for direct imaging of the brain, other primates and our long-extinct human ancestors must be interpreted through such endocasting studies.
The progressive enlargement of the hominid brain, which likely started some two million years ago, has resulted in a threefold increase in endocranial volume. By cataloging the expansion of the brain from one species of human ancestor to the next, divining such details as how convoluted the neocortex was and how many blood vessels served it, scientists may be able to show changes in the shape and size of neocortex structures as they appeared in the human family tree. These can then be compared with the activity of a modern human brain as seen by a slightly different techniquefunctional magnetic resonance imagingin hopes of understanding when certain intellectual capabilities developed during human evolution.
Using this technique, Falk and her colleagues have identified changes within the brains of Australopithecines, human ancestors that lived one to three and a half million years ago. The differences showed up in one of the four species they studied, Australopithecus africanus, which had features in its frontal lobes that had evolved beyond the great apes and along the road to humans. According to Falk, The cerebral cortex of this species was unlike that of any living primate. Functional magnetic resonance imaging has shown this part of the frontal lobe to be important for focusing on and completing tasks, as well as for highly abstract thinking and planning.
Neocortex Specialization
One particular feature of the neocortex makes it ideal for supporting the kind of flexible behavior that scientists consider the hallmark of intelligence: plasticity. This characteristic refers to the ability of the brain to change in response to stimulation. Change occurs in the neocortex when connections are formed or strengthened along highly used neural pathways, or as unused connections are weakened or lost. Mammals are born with many more connections between neurons than they will ever use. As newborns playfully explore the world around them, connections that are repeatedly stimulated by input from sense organs and motor activity are strengthened, whereas unused connections are eventually lost. This kind of plasticity is what underscores learning. Primates have an especially long period of juvenile develop-ment during which the wiring of their big neocortex is established through learning.
As young mammal brains are shaped by interactions with the environment, different parts of the neocortex become specialized to perform different functions. The more important the function, the larger the area of neocortex devoted to it. Bats that rely on echolocation have an expansive portion of the neocortex devoted to hearing. The neocortex of star-nosed moles (Condylura cristata), by contrast, exhibits a large area devoted to input from the 22 finger-like appendages on the end of its sensitive nose. Significant areas of the human neocortex are devoted to the face and lips, reflecting the importance of communication through spoken language and facial expression.
Many parts of the neocortex develop spatial maps in response to input from sensory organs. For instance, when young mice scurry about investigating their environment, their whiskers are constantly bumping up against things and sending signals to the brain. This leads to the formation of barrel-shaped structures in the neocortex, one barrel for each whisker. Barrels corresponding to adjacent whiskers are adjacent in the brain, forming a map of the area around the mouses snout. If a whisker is removed early in the mouses development, a corresponding barrel will never be formed in the neocortex.
Primates have sensory maps in their neocortecies, too, corresponding to tactile and visual input. A large proportion of the primate brain is devoted to visual processing, accounting for as much as 50 percent of the neocortex in some monkey species, more than any other sense. However, if a human is born blind, parts of the visual cortex may develop to support auditory processing instead, a dramatic example of the plasticity of the neocortex. Scientists, such as John Allman at the California Institute of Technology, have discovered that there is not just one, but many different visual maps in the primate neocortex. Each map is specialized to analyze different kinds of information about the visual scene, such as color, shape, or movement.
Shaping The Cortex
Selection for better visual skills may have kick-started the evolutionary expansion of the primate neocortex. Keen vision and superb eyehand coordination likely helped early primates find ripe fruit, use tools to crack nuts, or get at insects. As primates became more social, good vision also may have been useful for recognizing and gauging the emotional expressions of friends and foes. In fact, studies of brain activity in rhesus macaques indicate that certain areas of the neocortex are particularly responsive to images of faces and facial expressions.
The visual neocortex, located toward the back of the brain, seems to have attained its maximum size in the great apes. Primatologist Robin Dunbar of the University of Liverpool suggests that a point of diminishing returns perhaps was reached, in which primate vision became so advanced that adding even more brain power did not do much to improve visual processing further. At the same time that the evolutionary expansion of the visual cortex was leveling off, however, non-sensory areas in the frontal parts of the neocortex continued to increase in size.
Expanding Through Evolution
So what are these non-sensory, frontal areas of the neocortex doing in the primate brain? Scientists are just beginning to figure out the answer to this question. Intriguing evidence from studies of brain activity in humans and other primates suggests that the frontal neocortex is involved in mental manipulation of information. For example, French researcher Emmanuel Mellet and colleagues showed that the frontal neocortex becomes active when people are instructed to build objects mentally by stacking imaginary cubes in their minds eye.
Frontal areas of the neocortex may also be involved in the use of visual information to plan and execute fine motor movements. When a monkey performs an action like reaching out and grasping a small object with its fingers, neurons in a certain part of the frontal neocortex fire wildly. Interestingly, Giacomo Rizzolatti from the University of Parma found that these same neurons fire when the monkey simply observes another monkey, or even a human, performing the same action. These mirror cells, as Rizzolatti dubbed them, may play a role in imitative behavior and learning through observation.
As the phrase monkey see, monkey do suggests, primates seem quite good at aping each other. In fact, striking examples of what appears to be observational learning come from studies of tool use in great apes. Primatologist Jane Goodall reported that infant and juvenile chimpanzees at Gombe National Park in Tanzania watch intently as their mothers break off small branches and poke them into holes, fishing for termites. The young chimps then make awkward attempts to copy their mothers.
Golden lion tamarins (Leontopithecus rosalia rosalis), studied by the Smithsonian National Zoos associate director of biological programs Benjamin Beck, also use twigs as tools for foraging, back-scratching, and grooming. Two of the tamarins observed by Beck made the behavioral innovation of using their radio-collar antennae as a grooming tool. It just so happens that these two tamarins were mates, suggesting that one member of the pair made the innovation first, and the mate quickly learned through observation. This kind of social learning probably underlies cultural differences that exist between different populations of the same species. One can see also how communication among group members would be important to early human groups as they gathered more and more technologygroups that communicate better get wider use of new technology and its benefits.
Mirror cells may have also played an important role in the evolution of language. The same area where mirror cells are found in the monkeys neocortex is called Brocas area in humans and is involved in speech production. People with brain damage in Brocas area have a very difficult time speaking. Brain-imaging studies have shown that this area is also activated when people make or observe hand gestures. Rizzolatti and his colleagues propose that observing and repeating gestures may have been a precursor to gesture-based communication, which eventually evolved into vocal language in humans.
Also interesting, but most speculative, is the notion that mirror cells are important precursors of the ability to take the mental perspective of others. Individuals who possess this ability are said to have a theory of mind, meaning that they can speculate about what others are thinking, feeling, or intending. Without a theory of mind, humans would be unable to empathize with others. Nor would they be able to deceive or intentionally manipulate others. People who have suffered damage to the frontal neocortex are often less empathetic and sympathetic, and miss social cues leading to inappropriate judgments.
For instance, psychologist Don Stuss at the University of Toronto has demonstrated that patients with frontal brain damage have difficulty getting jokes or realizing when they are being duped. Various non-human primate species also engage in seemingly empathetic, deceptive, or manipulative behavior. For example, primatologist Frans de Waal of Emory University describes an incident involving two dominant male chimpanzees, Yeroen and Nikki. Yeroen sustained a minor injury to his hand during a fight and was seen limping afterwardsbut only when Nikki was in the vicinity. When Nikki was out of sight, Yeroen walked normally.
One interpretation is that Yeroen was intentionally trying to deceive Nikki. However, it is difficult to determine what a non-human primate really knows about the mental states of others. It is possible that Yeroen understood that Nikki would leave him alone if he limped and had no real concept of what Nikki believed. The presence of mirror cells suggests that monkeys and apes are at least able to make a connection between their own behavior and similar behavior by others, a tentative first step toward developing a theory of mind.
Many of the skills that seem to be supported by the neocortexrecognizing facial expression, learning through observation, communicating in a sophisticated manner, and reading mindsgive individuals an obvious advantage in social environments. Darwin first suggested more than a century ago that the evolution of intelligence is linked with living in social groups.
Group Think
Although social complexity is a difficult concept to measure, scientists have used social group size as a crude indicator. In separate studies, John Allman and Robin Dunbar each found that primate species that live in larger social groups tend to have bigger neocortices. This relationship is particularly strong when just the frontal, non-visual parts of the neocortex are taken into consideration. Gibbons and siamangs (Hylobates spp.), for example, live in small family groups of two to six individuals and have the smallest neocortices of all the apes. In contrast, chimpanzees often form troops of 70 or more and have one of the largest neocortices in the primate order. Chimpanzees also have a particularly complex social system in which the formation of coalitions to compete for food, mates, and status plays a major role.
The correlation between social group size and neocortex volume can even be extrapolated to humans. It has been estimated that the largest group of people in which everyone knows each other well is about 150 people; fittingly, this is the approximate size of a modern army unit. Based on the robustness of this correlation, and the lack of a correlation between neocortex size and other factors, such as foraging habits, Dunbar argues that the primate neocortex evolved to meet the demands of a complex social life.
Irene Pepperberg, an ethologist and evolutionary psychologist at the University of Arizona, has drawn similar conclusions about the evolutionary forces behind the exceptional intelligence of the African gray parrot (Psittacus erithacus). (Pepperberg is famous for her work with Alex, a gray parrot whose abilities include naming more than a hundred objects and discriminating them by color, size, and shape; counting to six; and achieving language abilities that rival that of chimpanzees.) In the wild, gray parrots congregate in groups ranging from a few dozen to thousands, transitioning from one extreme to the other in the course of a single day. Preliminary results indicate that complex social structures, mediated by elaborate vocalizations, form the basis of these groups.
Carnivores exhibit a strikingly similar pattern of neocortical expansion in species with larger social groups. Spotted hyenas (Crocuta crocuta) are conspicuous among carnivores by virtue of their unusually large social groupspacks numbering up to 100and their large neocortex. They exhibit intensely social behavior, including ritualized greetings and pre-hunting ceremonies, and are among the few non-primate species to form coalitions. However, carnivores didnt take this path quite as far as primates. Dunbar speculates that this is perhaps because the carnivore social universe is dominated by smells (evident from how dogs greet each other in the park).
Evolving social intelligence may have allowed even greater complexity in primate societies, in turn creating an even greater demand for this type of intelligence. Thus, primate evolution may have entered a feedback loop that sent this mammalian order spiraling up the intellectual flagpole. Neuroscientist Stanley Rappaport from the National Institutes of Health suggests that this frantic bout of evolution was facilitated by the plasticity of primate brains. The ability of the neocortex to change in response to stimulation, both from sensory input and from the act of thinking itself, gives primates the mental flexibility to cope with a complex, dynamic social environment.
Ultimately, it seems that the primate brain evolved to not just perceive information, but to learn and think about information, too.
MORE! Easy as 1,2,3 (Orangs Counting) and Gene Machines
A former ZooGoer intern, Jill Locantore is now a communications specialist at the National Academy of Sciences.
ZooGoer 31(4) 2002. Copyright 2002 Friends of the National Zoo. All rights reserved.
