Primate foraging is also more complicated than is sometimes assumed. They feed not only on bananas but on a great variety of things including leaves, roots, sap, and meat in the form of insects and small mammals. To obtain such foods some primates employ tools. Capuchin monkeys, for example, open nuts with stones. Others have developed sophisticated processing techniques. Gorillas, for instance, carefully fold nettles to avoid being stung. Yet others cooperate, as chimpanzees do to hunt monkeys. In these ways primate species exploit a diverse range of niches.
Taxonomists subdivide primates into groups based on a variety of traits. Not all primates are monkeys. Much of this has to do with noses—there are two suborders, one that comprises prosimians such as lemurs and lorises, which have wet noses (called Strepsirrhini), and the other primates that have dry noses (Haplorrhini). The latter includes the two groups known as monkeys: the new-world monkeys, whose nostrils point away from each other and the old-world monkeys, whose nostrils are in parallel. New-world monkeys, as implied by their name, are only found in the Americas and include tamarins, marmosets, howler monkeys, spider monkeys, squirrel monkeys, and capuchins. Some of these species have evolved a prehensile tail allowing them to grasp and hang from branches while feeding using their hands. They sometimes use it like an arm: when I presented a choice task to a spider monkey, she made her selection with her tail about as often as she did with her hand. Old-world monkeys live in Africa and Asia and do not have a prehensile tail. They tend to walk on top of branches on all fours, rather than hang below them, and their tail only helps with balance. They can sleep sitting upright and therefore often have thick red calluses on their behinds (so-called ischial callosities). Well-known old-world monkeys include macaques, baboons, mangabeys, colobus monkeys, and langurs. Apes and humans belong to a group of dry-nosed, old-world primates that have lost their tails altogether.2 Let’s have a look at our closest remaining animal relatives.
APES ARE GENERALLY LARGER THAN other primates. They have relatively long arms, a broad chest, and no snout. Apes usually depend on trees for a living, but because of their weight they typically hang below branches rather than balancing on top of them. A rotational ability in the shoulders serves this mode of locomotion and is essential not only for our capacity to hang off branches or high bars but for throwing spears or balls with precision. Ancient apes were also relatively large and, up in the trees, probably quite safe from predation. Such security allows species to live longer and slower lives. Indeed, living apes grow up slowly, with long periods of gestation and of parental care. They reach sexual maturity late and have an overall long life span of up to about fifty years. This extended life history is a fundamental ape adaptation that provided the opportunity to grow bigger brains.
Apes were once diverse and widespread, but both their numbers and distribution have radically been reduced. The most likely culprit is the climatic changes that destroyed the rainforest habitat to which they were adapted. Humans, of course, became ground-dwellers and are responsible for much of the deforestation in recent times. We are the most widespread and abundant primate—over seven thousand million of our one species compared to a few hundred thousand apes of all other species combined.
When apes were first brought to Europe in the seventeenth and eighteenth centuries, people were immediately struck by their resemblance to humans, even if they were perceived as grotesque distortions. They were often considered half human and half beast. In German, monkeys are referred to as Affen, and apes are known as Menschenaffen (“humanapes”). In Indonesian and Malay, orang utan means “man of the forest,” and an early European anatomist adopted a Latin version with the same meaning, Homo sylvestris, to describe an initial specimen from the African apes. Ever since Carl Linnaeus systematically classified organisms and placed humans among the primates, there have been debates about the appropriate groupings of apes and humans on the tree of life.
FIGURE 2.1.
Phylogenetic tree of humans and their closest animal relatives.
Although the relationships between primates are well established, the groupings and labels have been revised repeatedly in light of new data, most recently from genetics. In the latest and most widely used classification, and the one I will follow in this book (though only sparingly), humans and all apes are a group called hominoids. Humans and great apes, but not small apes (i.e., gibbons), are classified as a family group called hominids. Finally, the term “hominin” is used to refer only to humans and our extinct relations since the split from the last common ancestor with chimpanzees.
THE SMALL APES OR GIBBONS are, indeed, the smallest apes.3 The first one I ever encountered used my arm like a tree branch as it swung past me to land on the path ahead. I stood flabbergasted somewhere in a Sumatran rainforest. Small apes have extremely long arms and are famous for their acrobatics. They are true brachiators, swinging from branch to branch (or arm to branch as the case may be). No other mammal comes close to their ability to rapidly move through the canopy. But when faced with one on the ground, you will see a much less elegant form of locomotion. The gibbon was walking toward me on two legs with its arms swinging awkwardly in the air, in a style reminiscent of John Cleese’s funny walks. I was very much endeared by the cute little ape waddling up to me. I had been told that there were some semihabituated white-handed gibbons around, and so I was happy to make its acquaintance. Later on it even jumped on my lap. I stroked it and promptly got bitten.
FIGURE 2.2.
Siamang at Adelaide Zoo (photo Andrew Hill).
The line that led to the small apes split off from the ancestors of great apes and humans some eighteen million years ago. Today small apes live exclusively in Southeast Asia. They comprise four distinct genera: siamangs (Symphalangus), hoolocks (Bunopithecus), crested gibbons (Nomascus), and lar gibbons (Hylobates). These genera differ in various ways including the form of their skulls and even in the number of their chromosomes (ranging from thirty-eight in hoolocks to fifty-two in crested gibbons). There are several species of crested gibbons and lar gibbons, with little overlap between their habitats—only siamangs (the largest small apes) and white-handed-gibbons (a lar gibbon species) share territory in the forests of Sumatra and peninsular Malaysia.
Unlike great apes, but like many humans, gibbons live in small, monogamous families composed of a mated pair and several dependent offspring. Pairs stay together for years, and males invest time and effort in the rearing of the young. Siamang males even become the primary carrier of their offspring when the infant has reached the age of one year. When gibbons become sexually mature, they leave their “nuclear” family to start their own.
Each family typically has a territory of several hectares, which it vehemently defends. Gibbons mark their territory through their characteristically clamorous songs, which tend to last several minutes and are typically produced at specific times of the day. Different species of small apes can be identified through their songs, which range from booming and piercing cacophonies to haunting wails. Pairs often sing different parts in duet. The vocalizations of the small apes are far more diverse in range than those of the great apes. It has been argued that their ability to voluntarily produce such a diversity of sounds may be a better model of what our hominin ancestors turned into speech than the vocalizations of great apes. However, the functions of these small ape vocalizations seem to be restricted to territory marking, mate attraction, and, in case of the duets, pair bonding.
Surprisingly little is known about gibbons’ cognitive abilities. What work has been done is largely limited to studies on lar gibbons. Even these studies have been few relative to the myriad of psychological tests conducted on great apes. I found this a curious omission, given their close relationship to us, but soon learned the reason. It turns out that psychological testing of gibbons is rather difficult. Unlike great apes, captive gibbons do not tend to sit still opposite a human experimenter and then readily follow an experimental procedure. Perhaps because they are more vulnerable to p
redation, they are generally more fearful and flighty. In the studies my colleagues and I conducted, they frequently interrupted testing by indulging their penchant for energetic, high-speed acrobatics.
Small apes used to be widespread in Asia. Early Chinese texts report their presence as far north as the Yellow River. Today, however, they are confined to ever smaller areas of primary forest in Southeast Asia. Habitat destruction, hunting, and illegal trade have made them critically endangered. In fact, several gibbon species are alarmingly close to extinction in the wild.4 The silvery gibbon (Hylobates moloch) is down to about four thousand individuals and the western black crested gibbon (Nomascus concolor) to fewer than two thousand. The worst situation, however, is that of the Hainan black crested gibbon (Nomascus hainanus). This species, at last count, consisted of twenty-two individuals.
THE GREAT APES COMPRISE THREE genera: orangutans, gorillas, and chimpanzees. As the name suggests, they are larger than the small apes, with some male gorillas reaching over 200 kilograms.5 Great apes differ widely in the way they make a living, where they spend their time, and their social structure. However, unlike the small apes, they build nests in trees or on the ground to sleep in. They also make vocalizations akin to laughter when they engage in chasing games, wrestling, and tickling. As in humans, the armpits and the belly are particularly ticklish. The apparent joy of these activities does not seem to disappear with age—nor does their immense curiosity.
In the 1960s the curiosity of Louis Leakey, already famous for his discoveries in paleoanthropology, spawned long-term field studies of our closest living relatives. He sent out three female researchers to study each of the great ape genera in the wild. They are sometimes affectionately called “Leakey’s Angels.” Jane Goodall went to Tanzania to study chimpanzees, Dian Fossey to Congo and Rwanda to research gorillas, and Birute Galdikas to Borneo to investigate orangutans. Their approaches were initially considered unorthodox, but their persistence in recording behavior over long periods had a lasting influence. While Fossey’s efforts ended in tragedy in 1985 (as dramatized in the film Gorillas in the Mist), Goodall’s and Galdikas’s projects are ongoing. These long-term studies, together with a few other committed undertakings such as Toshisada Nishida’s work in Tanzania and Christophe and Hedwig Boesch’s work in the Ivory Coast, have yielded an extraordinary amount of new knowledge about our closest relatives.
ORANGUTANS ARE RED APES THAT live in the remaining jungles on the islands of Sumatra and Borneo. They are classified as two species (Pongo abelii and Pongo pygmaeus), but they can interbreed in captivity. Orangutans hardly ever descend from the trees. They prefer the lofty heights and have a corresponding air of aloofness about them when they gaze down at us ground dwellers. At least that is what it felt like to me, as my neck started to ache from prolonged upward staring. Male orangutans can weigh over 80 kilograms, making their climbing relatively slow and considered. I sympathized with their climbing efforts, as the branches move or break in roughly the way they would if I were trying to do the same thing. They have one distinct advantage over us, however: they have four hands, each with an opposable thumb at their disposal.
FIGURE 2.3.
A male orangutan in Ketambe, Sumatra (photo Emma Collier-Baker).
Orangutans spend a good deal of the day in search of fruit. They supplement their diet with juicy stems and insects. Occasional meat eating has also been observed. For example, Sumatran orangutans sometimes kill slow lorises—a slow-moving, wet-nosed primate. Perhaps because fruiting trees can only feed so many orangutans at a time, they do not live in social groups like other great apes. Instead, adult males usually live a largely solitary life.6 Females travel with their offspring, as males do not partake in the rearing. Adult males grow to twice the size of females, and some develop large cheek flanges and throat sacs that they use to emit long calls. They range over a particular territory, and receptive females join them occasionally for a period of up to three weeks. They tend to repeatedly copulate during this time, including face-to-face intercourse. It can look very intimate.
FIGURE 2.4.
Utama, a female Sumatran orangutan at Perth Zoo (photo Andrew Hill).
For some time researchers thought there was another smaller species of orangutan. Today, it is clear that this is a second form of mature male, sometimes called the Peter Pan morph. These mature males are transient and stay in the subadult (unflanged) stage for many years. In spite of the usually gentle nature of orangutans, these males may physically force females to copulate with them. As they look like adolescents, they may escape the ire of resident adult males. Upon some cues, possibly when an old flanged male disappears, a Peter Pan morph can grow into a flanged full-grown male.
The tendency to attempt forceful intercourse in some circumstances appears not to be restricted to Peter Pan morphs, or even to males. One researcher who had regular contact with a female orangutan once told me that he had to turn away a direct advance, upon which the snubbed orangutan would no longer cooperate in his research efforts.
Apart from holding large leaves as umbrellas when it rains, tool use among orangutans was long thought unusual. In recent years, however, primatologist Carel van Schaik and colleagues have documented a variety of cases. For example, some orangutan groups in Sumatra use stick tools to get to the seeds of Neesia fruit and to obtain insects from holes. This appears to be socially maintained behavior. In Borneo, Anne Russon and Birute Galdikas documented orangutans’ capacities as imitators. At the reintroduction center of Tanjung Puting, orangutans sometimes even copy peculiar human behavior. One orangutan, called Supinah, has been particularly curious about humans’ ability to light and control fire—not unlike King Louie in the Disney film adaptation of The Jungle Book. The ensuing experiments with kerosene and other materials understandably raised some concern but were ultimately unsuccessful.
Orangutans are able to solve a range of other problems. I once observed a subadult male reaching over from a somewhat isolated tree to the outer branches of its neighbor. He then stayed in a rather awkward horizontal position between the trees for what seemed to be far too long for comfort. He hung on until a juvenile, maybe three years old, made its way down from the top of the trunk and used him as a living bridge across the gap. In this way, the subadult offered himself as a tool to the juvenile.
FIGURE 2.5.
Orangutan bridge in Ketambe, Sumatra.
In spite of all the attention the enigmatic red ape receives from the public and governments, the remaining populations are rapidly declining. The latest population estimate for Sumatran orangutans is a mere 7,300 individuals. The figures for the orangutan of Borneo are slightly better, with estimates of between 45,000 and 69,000 individuals, but their numbers are also declining. Continuing habitat destruction (especially for palm oil plantations), bush fires, hunting, and the pet trade are reducing the population. According to the International Union for the Conservation of Nature (IUCN), Bornean orangutans are endangered, and Sumatran orangutans are critically endangered; that is, they are likely to be extinct in the near future.7
GORILLAS ARE THE LARGEST OF the great apes, so it is not surprising that King Kong was cast as a gorilla. In spite of the size of male silverbacks and their impressive chest-beating displays, gorillas are largely placid vegetarians. I once had the pleasure of visiting a habituated group of mountain gorillas in Uganda (something I highly recommend both as an experience and for the sake of conservation), as they were relaxing in the forest. The silverback was lying on his side studying his fingernails. He then casually grabbed his butt cheek, lifted it a little, and let one rip—a parallel to human behavior seldom discussed. The only chest beating I saw came from a one-year-old infant.
There are two generally recognized species of gorilla: the Western gorilla (Gorilla gorilla) and the Eastern gorilla (Gorilla beringei). In 2012 the first draft of the gorilla genome was published and suggested that these two species diverged some 1.75 million years ago, albeit with some subsequent gene flo
w. The majority of both species live in the lowland, but the Eastern species include the mountain gorillas. There are only a few hundred mountain gorillas left, and their habitat differs starkly from the typical rainforest most Eastern and Western lowland gorillas call home. Much of what is known about natural gorilla behavior comes from detailed studies of mountain gorillas first initiated by Dian Fossey.
FIGURE 2.6.
A mountain gorilla spying on us from behind a bush in Bwindi Impenetrable National Park, Uganda.
Mountain gorillas live in small family groups comprising an adult male, a few females, and their offspring. As they reach maturity, females leave to join another group. A full adult male is larger than the females in his harem and develops the distinctive silver-gray hair patch on his back. “Bachelor” adult males tend to live solitary lives until they can take over such a group.
In spite of their colossal weight, gorillas are generally good climbers. On the ground, gorillas usually move on all fours using their hand knuckles. Mountain gorillas mainly eat ground-level roots, shoots, and leaves. Lowland gorillas eat somewhat more fruit, which they gather in trees. Recent fecal analyses suggest they occasionally also eat mammalian meat. To maintain their size on a predominantly plant-based diet, gorillas, unsurprisingly, spend a lot of their time feeding. Some plants have serious defenses, such as thorns; others only have tiny cores that are edible. The psychologist Dick Byrne has documented gorillas’ painstaking procedures to get to these juicy bits without getting hurt. These techniques can be quite complex, involving multiple steps, and young gorillas acquire them by observing their elders.
The Gap Page 3
