The Gap

by Thomas Suddendorf

  FIGURE 2.7.

  Kigale, a female Western Lowland gorilla, at the National Zoo, Smithsonian Institute, Washington, DC (photo Emma Collier-Baker).

  In captivity, gorillas have long been known to be adept at using tools, much like orangutans and chimpanzees. In the wild, such behavior had not been reported until quite recently. In 2005, a gorilla was observed using a stick to dig up tubers and to check water depths while wading through a swamp. One reason for the paucity of tool use in the wild may be that the gorillas’ immense strength provides them with alternate avenues to solve problems. My occasional collaborator Andrew Whiten told me that he once gave a puzzle box to a gorilla, a box that had previously been used to investigate chimpanzees’ imitative capacity at manipulating cogs and levers, and quickly learned that the gorilla had an easier way of getting at the treat inside: he simply smashed the entire thing open.

  Mountain gorillas are critically endangered, with an estimated 680 individuals surviving in the wild.8 The Eastern lowland gorilla population comprises only a few thousand animals. The IUCN lists Eastern gorillas as endangered and Western gorillas as critically endangered. The Western subspecies known as Cross River gorillas are down to only about 250 individuals. The total number of Western lowland gorillas used to be estimated at more than 90,000 individuals, but the population appears to be rapidly declining. In 2006, an Ebola epidemic killed some 5,000 gorillas. Yet for once there has been welcome news about numbers in the wild. In 2008 a previously unknown large population, possibly up to 100,000 individuals, was found in the Congo—a fantastic discovery. Alas, there is little hope that any other uncharted habitats will be found in the future.

  CHIMPANZEES ARE SUBDIVIDED INTO TWO species: the common chimpanzee (Pan troglodytes) and the pygmy chimpanzee or bonobo (Pan paniscus). Because chimpanzees are our closest living relatives and bonobos are in some aspects quite distinct from common chimpanzees, I shall introduce them separately.

  Common chimpanzees live in large areas across the sub-Saharan forests of central Africa and are often subdivided into four subspecies (Western, Eastern, Central, and Nigerian chimpanzee). Young chimpanzees regularly feature in movies (from Tarzan’s Cheetah to Ronald Reagan’s Bonzo), and this has created a public impression that chimpanzees are small and cute. Adult males can in fact be large and ferocious. They are a lot stronger than most humans, and it is dangerous to get in their way when they are excited. They can also be temperamental. We sometimes work with two male chimpanzees at Rockhampton Zoo in Australia, and they are exceptionally friendly, sitting on the other side of the mesh attending our psychological tests. However, a loud lawn mower, a bus, or some other trigger can set them off into a frantic rage. They scream, beat the mesh, jump and charge hysterically through the enclosure, and spit. In other words, they go ape-shit. You certainly do not want to be carrying one of them on your shoulders like Tarzan.

  In the wild, common chimpanzees are by far the best studied of all the apes. There are now data from several study sites that have observed specific groups for dozens of years. Chimpanzees live in large groups with typically forty to sixty members (some with over a hundred), but they usually travel and feed in smaller subgroups. Membership of these subgroups changes, and the whole group comes together only from time to time. This flexible system is known as fission-fusion—a lot of coming and going. It provides opportunity to spend time with selected individuals but poses special social cognitive problems of keeping track of changes in relationships and in the social hierarchy.

  Females migrate to join another group when they become sexually mature, whereas males tend to stay in their group of birth. Achieving high rank is important to males, as it secures greater sexual access to females. Female chimpanzees advertise their sexual readiness through distinctive pink genital swelling and during that time may mate between five and an astounding fifty times a day. Mind you, intercourse usually only lasts seven seconds. Copulating with many males, even when already pregnant, may be a strategy to reduce the likelihood of males in the group killing the baby. Unlike gorilla silverbacks, which monopolize access to females, chimpanzee males cannot be certain about their paternity.

  FIGURE 2.8.

  The common chimpanzee Ockie at Rockhampton Zoo.

  As Jane Goodall famously documented, the social lives of chimpanzees are complex and intriguing. For instance, gaining the support of the alpha female may elevate a male’s position. Coalitions may allow lower-ranking males to topple the reigning alpha male. In turn, other alliances may retaliate against usurping individuals and their associates. As the primatologist Frans de Waal aptly observed, there is good cause to refer to “chimpanzee politics”—which is what he called his seminal book on the subject. The notion that humans are the only political species, as Aristotle proposed, can only be upheld with appropriate restriction of the term’s definition.

  War is often said to be an extension of politics. The territory of each chimpanzee community has distinct boundaries that male groups patrol. Goodall observed how chimpanzees on these patrols killed chimpanzees from a neighboring group. At that time it was widely believed that only humans had that deplorable trait to cooperate in the murder its own kind. The ferocity of the killing shocked a lot of people. The outnumbered victims were held down while assailants beat and mauled them, dragged them back and forth along the ground, and attacked them long after they had stopped defending themselves. Many examples of chimpanzee raids and violence against neighbors have been recorded since. Given that humans and chimpanzees share this cruel potential, it may be an ancient trait indeed, which, as already noted, may have had an important role in the very creation of the gap.

  Chimpanzees also share the human penchant for hunting. Some groups supplement their diet with small animals and even baboons. Hunting primates, such as swift colobus monkeys, seems to involve sophisticated cooperation. For example, one chimpanzee may drive the prey toward others that appear to hide in ambush. What cognitive abilities are involved, however, is subject to considerable debate. (Various pack animals, such as lions and wolves, also engage in cooperative hunting.) In 2007 chimpanzees in the savannah of southeastern Senegal were reported to sharpen sticks and spear bushbabies—small nocturnal primates—hiding in tree holes.

  Although significant, meat is by no means a major food source for most chimpanzees. About half of what they typically consume is fruit, and other vegetable matter such as leaves and bark is also commonly eaten. Some groups have been observed digging for tubers. There have even been reports that chimpanzees seek out medicinal plants when sick. Many chimpanzees obtain protein by eating ants and termites and have developed ingenious ways of fishing them out of their holes. Small sticks are stripped of their leaves, inserted, and retracted once insects have gathered on the tool.9 Protein requirements are sometimes further supplemented by consumption of nuts. In the Tai forest, for instance, chimpanzees spend considerable time cracking open nuts with stone hammers and anvils, and appear to have done so for a long time in this region. At one site an archeological study indicates that chimpanzees used such stone tools 4,300 years ago—the chimpanzee stone age predates human farming in the area. Chimpanzee foraging is much more complex and diverse than once thought. They have developed ingenious ways of obtaining varied food sources, and some of these methods are passed on, apparently over thousands of years, through social learning.

  Groups differ in what tools they use and how they use them. Leaves are used for cleaning, branches are shaken and stones are hurled in aggressive displays, rocks and logs are used as hammers and anvils, and sticks are employed to obtain objects otherwise out of reach. Some tools are made to suit the task (e.g., stripping and sharpening a stick and adjusting its length), and the same kind of objects may be employed for a range of different purposes (e.g., leaves may be used as toilet paper, sponges, and umbrellas). In short, as we will see throughout the book, our closest animal relatives have rather clever minds.

  Common chimpanzees used to be, well, common in
Equatorial Africa, living in over twenty countries. Current estimates put the total chimpanzee population size to above 170,000 but less than 300,000. This comprises over 20,000 Western, 90,000 Eastern, and 70,000 Central chimpanzees. There are fewer than 6,500 Nigerian chimpanzees remaining. Though these numbers are higher than for most of the other apes, it may be instructive to compare them to the populations of a few towns and to imagine those settlements were all that is left of humanity on the planet. Numbers are declining primarily as a result of habitat destruction and degradation, as well as hunting for the bush meat trade and the pet trade. The common chimpanzee is thus classified as endangered by the IUCN.10

  BONOBOS, OR PYGMY CHIMPANZEES AS they used to be known, were only described in 1929. They are more petite than their better-known cousins; have a black, relatively flat face, pink lips, and a higher forehead; and look well groomed, with their hair often parted neatly down the middle. Together with a relatively straight posture when standing upright, which they do about a quarter of the time when on the ground and especially when carrying things, they look eerily like one might imagine an early human ancestor.

  Bonobos live in a limited region south of the river Congo. It may have been river barriers that led to their separation from common chimpanzees between one and two million years ago. They eat mainly fruit, complemented with some leaves and, on occasion, a small amount of animal protein. Only in 2008 were bonobos first described to collaboratively hunt monkeys and share the spoils. There has been no evidence yet of bonobos using tools in the wild, but that may only be a matter of time and patient observation. They certainly do use them effectively in captivity. Little is known about wild bonobos, with only two permanent study sites currently in operation.

  FIGURE 2.9.

  Young adult male bonobo Kevin (photo by and courtesy of Frans de Waal).

  Like common chimpanzees, bonobos live in fission-fusion societies and can typically be found in subgroups of up to twenty-five individuals. The entire group may be as large as two hundred individuals. What little study there has been of bonobos in the wild suggests distinct traits that separate them from common chimpanzees. They show far less aggression, are less male-dominated, and have a lot more sex. There is sex between all ages, genders, and ranks. Bonobos seem to enjoy sex and indulge in a variety of positions, including face-to-face intercourse, tongue kissing, and even oral sex. Bonobo sex, like human sex, is not just used for procreation. In certain situations sex seems to serve a tension-reducing function. After conflicts, for example, sex is often used as a means to establish reconciliation. Perhaps there is a lesson here.

  As enthusiastically documented by Frans de Waal, bonobos have a peaceful—some might call it “utopian”—society. De Waal argues that bonobos have compassion, empathy, and kindness. The relationship between the sexes is quite egalitarian. There is little violence compared to the frequent outbursts observed in common chimpanzees. Recall, though, that we know a lot less about bonobos than we do about common chimpanzees. More extensive research is highly desirable and may well reduce some of the apparent differences between these species of chimpanzee. It took years of observation before Jane Goodall found that common chimpanzees engage in collective killing.

  Estimates of bonobo population size range between 30,000 and 50,000 individuals. Bonobos are classified as endangered by the IUCN and face similar pressures from human activity as the other great apes. Since they only live in the Democratic Republic of Congo, the local political situation is crucially important in the continuing survival of this fascinating hominid.11

  AS OUR CLOSEST REMAINING RELATIVES, these species of ape provide context for our discussion about the nature and origin of the apparent gap that separates animal and human minds. Given that minds are generated by brains, I’ll close this chapter with a comparison of human brains and those of our animal relatives.

  Thomas Huxley found that mammalian brains are broadly equivalent in structure and differ primarily in size. Size matters. Even within our own species, there is some evidence that people with larger brains are more intelligent than those with smaller brains, at least as measured by IQ tests. Do humans, then, simply have the largest brains?

  The brains of small apes weigh about 80 grams and those of great apes between about 300 and 450 grams. Humans have by far the largest brains of all primates, typically weighing between about 1.25 and 1.45 kilograms, and containing some 170 billion cells, about half of which are neurons. Metabolically we heavily invest in brain activity. Our brain comprises about 2 percent of our body mass but consumes some 25 percent of our energy. (Thinking is exercise and costs you 20 to 25 watts to run. Yes, you are exercising right now.) However, brain size alone cannot explain the gap—alas, we do not have the biggest. Elephant brains can weigh over 4 kilograms, and whales have much larger brains still, weighing up to 9 kilograms.

  However, if you take overall body size into account, humans have much larger relative brain sizes than these giant creatures, whose brains comprise less than 1 percent of their bodies. Yet, in spite of its initial intuitive appeal, it is not entirely clear why relative brain size should matter. Perhaps larger bodies require larger brains in terms of innervations and neural management, but shouldn’t cognitive processing be independent of body size? After all, we don’t become more or less smart when we change relative brain size through losing or gaining weight, do we? Furthermore, some large animals, such as crocodiles, do fine with walnut-sized brains. Why should we adjust brain measures according to body size when large bodies can be run by small brains?

  In any case, the outcomes of relative size comparisons have also not been supportive of humans’ sense of superiority. Some shrews and mice, it turns out, have brains that are up to five times larger than ours relative to body size. They can have an extraordinary 10 percent of their body be brain, compared to our 2 percent. This may please fans of Douglas Adams, whose fictional laboratory mice were smarter than us and conducted experiments on human scientists who thought they were experimenting on them, but there are no signs of extraordinary mouse intelligence I am aware of.

  Since we get beaten by large mammals in the first scheme and small mammals in the second, a third scheme has been devised that takes into account that as mammals get larger, brains get absolutely larger but relatively smaller. The psychologist Harry Jerison calculated so-called encephalization quotients, or EQs, which compare the actual size of a species’ brain to the size one would expect for an average animal of its size from the same taxon. Among mammals the average animal is calculated to be a cat. Table 2.1 lists some examples of mammalian EQs. In this influential scheme, humans emerge on top with a brain over seven times larger than that predicted for the average mammal of our size. Many other animals seem to be ranked in line with common assumptions. Some findings, however, are unexpected. Capuchin monkeys, for instance, have surprisingly large EQs that put them well ahead of chimpanzees. One may also worry about the influence of the reference group. If instead of comparing us to the average mammal, we narrow this down to a comparison to the average primate, or expand it to a comparison to the average vertebrate, the results change. Not surprisingly, then, debate continues over which measure is most informative.12

  TABLE 2.1. Some sample encephalization quotients

  Given the limitations of both absolute and relative measures, Andrew Whiten and I combined the two. We took Jerison’s EQs for primates and computed the absolute brain mass that is in excess of that predicted for an average mammal of the same body size (see Figure 2.10). Lo and behold, humans came out on top, with our closest relatives lined up in an order that appears to make intuitive sense. The great apes enjoy absolute neural resources well beyond that typically evident in a mammal of their size, and substantially more than monkeys. Humans have disproportionally more computing power still.

  FIGURE 2.10.

  Average excess brain in grams over and above that predicted by body size.

  This makes us feel good, perhaps, if a little uneasy. Are
we just massaging data to get the results we want? A lot of ink has been spilled on the pros and cons of various measures of brain size comparison, but it remains uncertain whether any of them have unearthed some hidden truth or whether we are simply using statistics to confirm our preconceptions.

  The obsession with size may be misleading. When the neurologist Korbinian Brodmann produced his seminal comparative brain maps at the beginning of the twentieth century, Huxley’s claim about structural similarity between mammalian species was confirmed. However, Brodmann appreciated that more sophisticated methods might eventually find differences in internal organization.

  New methods are indeed beginning to reveal some subtle differences. For example, new techniques for estimating the number of brain cells in mammals suggest that different scaling rules link brain size and number of neurons for rodents, insectivores, and primates. It turns out that ten grams of monkey brain contain a lot more neurons than ten grams of rat brain. However, these data also suggest that human brains are simply linearly scaled-up primate brains in terms of cell numbers.

  Brain characteristics other than cell numbers and overall size may be responsible for our peculiar minds. The human olfactory bulb (the brain area that processes smell) as well as the primary visual area at the back of the brain are relatively smaller in humans than they are in our close relatives. Such changes may reflect some brain reorganization that occurred in human evolution.13

  Modern neuroscience is beginning to identify finer differences. The first documented microscopic distinction between ape and human brains has been a unique cell organization in a layer of humans’ primary visual cortex—not an area typically associated with higher cognitive functions. There is also some suggestion that humans differ in the neural connections in the prefrontal cortex, an area very much associated with higher cognitive functions. While neurons in the back of the brain have relatively few connections, and human brains differ little from other primates in this respect, there are many connections in the prefrontal cortex. Their density is much higher in humans than in other primates that have been examined thus far. Much more fine-grained counting will be necessary to determine exact quantitative differences.