A DARK, chiselled face looks at us from way up in the branches of a vast rainforest tree. Deep-set, inky eyes peer suspiciously through the foliage, throwing an occasional glance towards movement in a tree beyond. This is Ruby, a mature female bonobo. She lingers, almost as if to separate us from the rest of her party, who are moving on in search of more fruit. Then she leaps away. We wait, hoping to pick up more calls, but that’s it for today. The apes have returned to their world.
Such encounters are typical of the pleasure and frustration of studying bonobos in their natural habitat. Found only in the heart of the Democratic Republic of the Congo, these were the last great apes to be discovered and are the least studied. Their range, beneath a huge arc made by the Congo river, is fragmented across a region of lowland rainforest that approximates 350,000 square kilometres, about the size of Germany. It is a hauntingly beautiful landscape, encompassing both swamp and dry forest, all of it inaccessible, with travel mostly restricted to following forest trails by foot or trail bike, or navigating the rivers in unsteady flat-bottomed pirogues.
“It is a hauntingly beautiful landscape encompassing both swamp and dry forest, all of it inaccessible”
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But physical remoteness is not the only reason why bonobos are so elusive. A succession of wars in the Democratic Republic of the Congo between 1996 and 2003 has severely disrupted research. In 1998, scientists were forced to leave the area where I am staying with primatologist Amy Cobden from Emory University in Atlanta. After the war, research here was patchy until 2009 when the African Wildlife Foundation set up the , (see Map). In the interim some apes were hunted for bushmeat so it is not surprising that Ruby and her group are wary. Cobden knows it will be many months, possibly years, before the apes once again become sufficiently used to humans to behave naturally. With so much still to learn about these animals, she is hopeful that her patience will be rewarded.
What particularly interests Cobden is the degree to which the bonobos’ environment is responsible for their particular way of socialising, a question that promises to throw light on the puzzling differences between their behaviour and that of their closest cousins, chimpanzees. Work with wild chimps in more than 50 locations, and with captive animals, reveals that chimp society is characterised by hierarchical coalitions of aggressively dominant males who divide and rule females, kill infants, engage in wars, bully lone individuals and generally play nasty politics. Bonobos, on the other hand are famed for their effusive sex lives and egalitarian, peaceable society. Bonobo males do not dominate, females are relatively gregarious, groups are generally more stable, and neither infanticide nor group warfare have ever been observed. So here are two closely related species that shared a common ancestor less than 2 million years ago and inhabited very similar environments. Why they have taken such seemingly different evolutionary tracks is a mystery that continues to intrigue us, not least because of the light it might shed on another ape species that shared an ancestor with them both around 5 million years ago – humans.
Apes in Eden
Cobden’s research addresses the theory that bonobos lack the levels of aggression found in chimps because they evolved in their own Eden, with abundant food and little competition for it. Although in purely botanical terms chimp and bonobo forests are very similar, there is one key difference – bonobos are the only great apes living south of the Congo river whereas chimps evolved on the north bank, where they share their habitat with gorillas. In theory, that could explain their characteristic temperaments, since chimps must compete with gorillas for food. When food is scarce, the need to guard it creates a squeeze that drives the formation of despotic alliances and unpleasant politics, or at least the temperamental propensity for them. Limited food also influences group size, because bigger groups will strip food sources more quickly and therefore have to waste more time and energy travelling between sites.
Competition does seem to influence chimp behaviour. “Gorillas are serious competitors for ground foods, which means that in chimp forests, there are generally fewer edible herbs. Chimps just can’t rely on finding high-quality ground foods when fruit is scarce, so they often have to split up to forage,” says primatologist from Harvard University. What is more, among chimps living in western parts of Africa, in forests where there is plenty of ground food and no gorillas, groups are bigger and more stable and individuals generally behave in a more peaceable, bonobo-like manner.
The lack of detailed information about bonobo behaviour in the wild makes it hard to know for certain whether their calm temperament has arisen from nutritional abundance, and if so how this happened. However, Cobden and others are convinced that bonobos’ environment is not as idyllic as it might seem. She has seen them in direct competition for favoured fruits with other species, including monkeys and hornbills. What is more, bonobos and chimps seem to have a similar flexible approach to life. Both species live in highly fluid “fission-fusion” communities, in which small sub-groups split off to forage or nest in different parts of their range, with the community occasionally reuniting, particularly in places where food is abundant. When fruits are scarce, bonobos, like chimps, feed on herbs and reduce the size of their feeding parties. Cobden wants to know whether bonobos also get stressed by food scarcity and, if so, what happens when they do.
Bonobo watching
Her approach entails watching the apes, recording what they eat, the size and composition of their feeding parties and details of their behaviour. She estimates that between 25 and 40 bonobos live in two separate communities in her field site. Some are still extremely shy, but she regularly encounters, and can easily identify, eight adult males and eight adult females, along with a handful of juveniles. As well as studying the animals, she monitors a sample of 1800 forest trees twice a month to evaluate the availability of fruits. Finally, she collects samples of faeces and urine for hormonal analysis to gauge the animals’ nutritional status and stress levels.
The daily routine involves rising soon after 3 am and marching through the forest at breakneck speed to reach bonobo nest sites before the animals wake. The apes tend to sleep in different trees each night, weaving nests from flexible branch ends and grouping together high above the forest floor. Cobden and her team locate the nests easily, having watched the bonobos build them the evening before. Creeping close in the dark and sitting in silent expectation of movement above is undoubtedly the most thrilling part of the job – especially on occasions when the apes pierce the dawn with ear-splitting squeals.
Once awake the bonobos are fickle. Sometimes they will tolerate their human observers, peering down from on high. Other times they quickly descend from the treetops and lose themselves in the undergrowth. Then, while the bonobos have some private time, the researchers tackle the less rewarding business of collecting faeces and urine – each animal generally having obliged by leaving a pungent deposit beneath its nest. Faeces are collected using twigs as chopsticks and a bag made of large rubbery leaves. Urine, found as dark-coloured droplets, is pipetted into small plastic test tubes. Once Cobden and her team have what they need, they carefully label all the samples and carry them back to camp.
If the apes permit it, the researchers will follow them all day. More usually, the humans spend their afternoons and evenings weighing faeces and drying small samples in a fire-driven clay oven, ready for hormone analysis back in the lab at Emory University. The rest is then washed so that undigested solid remains can be identified. What they have found indicates that the bonobos consume about 30 different species of fruit, and half a dozen herbs, plus the young leaves of a few trees. Red hair, probably from a small antelope called a duiker, is a rare addition, along with the mousey brown fur of the odd tree hyrax. One sample even contained a pair of bird’s feet. Until quite recently, bonobos were thought to be strict vegetarians, not partaking in the “chimp-like” behaviour of hunting. Now meat eating is seen as normal for them too.
After spending 18 months tracking the bonobos, Cobden has returned to her lab and is trying to make sense of the information she collected. She has already found signs that food is not always plentiful. Preliminary tests of urine samples regularly reveal the presence of ketones, products of fat metabolism that suggest the animals are not always getting enough calories from their diet. She will also be able to find out how much fruit an individual has been eating by measuring levels of C-peptide, a metabolite of insulin. These levels reflect the insulin high that follows from rising glucose levels after a meal rich in carbohydrates. The next part of the puzzle will be to see whether food shortages lead to an increase in stress hormones such as cortisol, and whether this coincides with the splintering of groups. The hormone analyses will take about a year to complete.
There are already some hints about what Cobden may find. In the 1980s, Frances White from the University of Oregon, Eugene, observed that bonobos living near Cobden’s group reduced their feeding party size as fruit supplies diminished. However, they handled the size reduction in a very different way from chimps, where males usually remain together at the centre of their range and the less sociable females tend to go it alone. In contrast, White found that among bonobos, when food was short it was males, not females, who were pushed to the periphery ().
This is particularly intriguing in the light of data amassed from two decades of research at the longest running bonobo study in the Luo Scientific Reserve. Analysing these findings, pinpoints several ways in which the behaviour of females appears to contribute to the equanimity of bonobo society (Evolutionary Anthropology, in press). Female bonobos spend more time together than do most female chimps and they have greater influence on what their group does. By sticking together they temper the worst excesses of violence that persist in “chimp-like”, male-dominated hierarchies where males fight over, rape and batter lone females.
Furuichi has observed behaviour that goes some way to explaining this. In general, bonobo females spend longer than chimps in oestrous – signalling fertility with a large sexual swelling. Noting that rates of conception differ little between the two species, he concludes that bonobos give dishonest oestrous signals for longer than chimps do. With more sexually attractive females together at the same time, it becomes harder for males to monopolise a single one. The effect is to reduce sexual competition between males, a factor considered very important in driving chimps to the extremes of aggression for which they are famous.
So, could the answer to boosting social equability be as simple as “girl power”? Not quite. Wrangham points out that an obvious consequence of females being sexually attractive is that this pulls in males, which in turn increases group size, and so ratchets up competition for food. When times are good this is not a problem, but when they are lean, females must defend themselves against male competition for food. And that brings us full circle, back to ecology. Wrangham believes that as well as prolonged oestrous, there must have been something in the evolutionary environment that initially supported the aggregation of bonobo females, allowing them to benefit from female solidarity and still get enough to eat. His best guess so far is that the key was ground food and no competition from gorillas for it. But more research is needed – especially on wild bonobos.
Cobden can only hope that all her mosquito-ridden, early morning meetings with the apes will provide a clue. Ideally, her hard-won observations will reveal subtleties of bonobo ecology that have gone undiscovered until now, yet are potent enough to explain the difference between the despotic male dominance of chimp society and the peaceful female rule of bonobo groups. At the very least, her fieldwork challenges stereotypical ideas about bonobos and chimps. “You’ve got angry chimps and mellow bonobos – that’s the cliché,” says Cobden. “But it assumes that bonobos are peaceful as a default, yet the issue is really one of degree and kind.” Likewise for chimps. Perhaps this realisation will help us better understand our own complex nature.
