IT IS hard to take a monkey seriously when it’s dressed in a red vest and matching fez. But even when the little capuchin monkey isn’t posing as an organ grinder’s sidekick it doesn’t get the respect it deserves. For decades scientists have thought of the capuchin as a misfit – along with all the other monkeys of Central and South America. Because New World monkeys are not as closely related to humans as African and Asian primates anthropologists have neglected them. But now all that is set to change: New World monkeys have suddenly become central to research into primate evolution.
Crucial to this reversal of fortune was a bold reappraisal of the evolutionary relationships between humans and other primates. Last year, Anthony Di Fiore and Drew Rendall of the University of California, Davis, published a paper in the Proceedings of the National Academy of Science claiming that New World monkeys such as the capuchin are not just an unimportant offshoot of the primate family tree. They are part of the stock from which all apes and humans evolved. The field of primate research was ripe for such a finding. More and more researchers were going out to study New World monkeys in the wild, and the jungles of Brazil, Ecuador, Venezuela and Costa Rica were fast becoming the “in” spots to work.
As a result primatologists have had to overhaul their notions of the “typical” primate. According to the established view primates typically live in groups where the females form close ties and the males leave at sexual maturity. But now anthropologists have to incorporate into their tales of hominid evolution the sexual flexibility of New World monkeys, their lack of hierarchies, and their broader range of social structures.
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Model behaviour
In the 1960s, when anthropologists took to the field to watch other primates, their goal was clear. Nonhuman primates – members of our taxonomic order and therefore our relatives – could serve as models for early human evolution. Watching these animals on their home ground and understanding how they formed social groups and interacted with each other would surely provide clues to the behaviour of our earliest ancestors. Chimpanzees looked like the most obvious candidates for this approach – humans and chimpanzees shared a common ancestor about 8 million years ago and we still have about 98 per cent of the same genes. But the two species have spent those millions of years evolving and adapting to fit very different lifestyles so researchers needed to find a less specialised primate as the model for early humans.
Baboons seemed to fit the bill. Although more distant relatives of humans, baboons share our African origins and spend most of their time out on the savanna, presumably dealing with the same problems our ancestors faced. Researchers discovered that baboons, and many of the 40 other African and Asian monkey species in their order, the cercopithecines, share a common pattern of behaviour. In their typical social system females remain with their mothers while males leave at sexual maturity and enter new groups. As adults, these unrelated males do not form strong social bonds and they must also actively compete for females in heat. Females, on the other hand, form close ties because they stay put. Both male and female cercopithecines interact through a network of social status, where one monkey ranks over another, and males outrank females. This pecking order is reinforced on a daily basis; they push each other from favoured places, turn their rears in a sign of submission to more dominant baboons and groom each other to bolster important connections
Old World monkeys and baboons in particular, soon became the anthropologists’ “model primate”, the best-guess picture of an ancient blueprint from which we humans evolved. Now some researchers are suggesting that for the past 30 years this “baboon bias” has warped the story of human origins. As Rendall points out, “Everyone latched on to the cercopithecine model because they were so well studied and they all seemed to be alike, and so everyone called that the normative higher primate.” So the “female-bonded” system has become the standard monkey pattern against which all other systems should be compared. The apes, where females are most often solitary, were considered specialised – branching out at some point from the more basal baboon pattern. And prosimians and New World monkeys, with their various lifestyles, were simply ignored by scientists who felt they offered little insight into human evolution.
But now it looks as if the baboon model is on its way out. Karen Strier, an anthropologist at the University of Wisconsin, recently published a major review paper in which she challenges the cercopithecine standard and calls for a broader view. “Anthropological interest in nonhuman primate models for human behavioural evolution has tended to focus on a relatively small number of species,” claims Strier. It is time, she suggests, to move away from the cercopithecines, acknowledge the diversity of primate behaviour and figure out what this diversity means for human evolution.
Oddly enough, as Strier and others acknowledge, the major players in this revolution are the once maligned South American monkeys. About 25 million years ago, when monkeys were as common as mice, the world looked quite different from today. South America was rather distant from North America, but closely aligned with Africa. Although no one is completely sure how monkeys came to be in South America (there is no fossil evidence of ancestral prosimians from which they could have evolved) the most reasonable explanation is that several species of African monkeys inadvertently floated the short distance from Africa to South America on rafts of swampland. As the two continents drifted farther apart, these monkey species became isolated, but they took advantage of their new territory and radiated across South America and eventually into Central America when the two Americas joined. The animals flourished – 16 genera now occupy diverse ecological niches.
Evolutionary outcasts
Despite their traditional states as evolutionary outcasts, animal behaviourists have been studying several species of New World monkeys for decades. Anthropologists found no place for these studies in the larger picture of primate and human evolution. Nonetheless, dedicated behaviourists learned that New World monkeys are best described by their variation and flexibility. Most notable is the wide array of mating systems they display. Some live in monogamous groups where one male and one female mate, and the father cares for infants, even twins. In at least one species, the saddleback tamarin, each female regularly lives with, and mates with, two males which then carry her twin offspring. Other species live in a communal fashion where females mate with all the available males who patiently wait their turn – rarely fighting over the females as Old World monkey males do.
Every imaginable mating system, diet type, and social system can be found in the lifestyles and daily behaviour of these animals. Leaf eating howler monkeys, for example, spend their day in large groups, hanging out in trees, digesting. Small groups of more agile spider monkeys flit from tree to tree, scouring the highest reaches of the canopy for fruit. Huge communities of squirrel monkeys bound noisily through the Amazonian forest on the lookout for insects to supplement their fruit diet. No one spends much time grooming or working out a pecking order. And usually females, as well as males, migrate from their birth place to another troop or territory. Only one species, the capuchin, displays a female-bonded pattern of social interaction. The rest are a magnificent prism of social, behavioural and mating systems.
Still, all this knowledge might simply have reinforced the New World monkeys’ quirky image, if Strier had not gone to Brazil in the early 1980s. Her aim was to explain the odd behaviour of the woolly spider monkey or muriqui. But following the monkeys through the forest she was struck by a heretical idea. “Maybe I had been looking at muriquis all wrong,” it occurred to Strier. “Maybe they weren’t the extreme exceptional case. Maybe they were actually part of a continuum of diverse primates. And maybe the extreme case were the Old World cercopithecines because they, not the New World monkeys, were so constant.”
Central to Strier’s thinking was the fact that in at least half the primate genera the male transfer “rule” did not operate. In many species of apes, monkeys, and prosimians both females and males disperse, or even just the females. And when females are not bonded, other “typical” patterns of social life disintegrate: no more matrilines, kinship takes a back seat and hierarchy loses its relevance. When males are left behind, it’s in their interests to cooperate. In 1990, Strier published what she now calls “tentative feelers” prompting anthropologists to step back and look at the model primate. Four years later, a convincing argument has evolved suggesting that our primate roots lie in the flexible social and mating systems of the New World monkeys rather than the cercopithecines.
Odd one out
Support for Strier’s broader, less exclusive, view of primate life came this year when Di Fiore and Rendall at the University of California, Davis, catalogued 25 behavioural characteristics of 33 primate genera and let a computer assess their similarities and differences. They discovered that the New World monkeys are a large, highly variable group, while the Old World monkeys come out as a cluster set apart from the other primates.
This analysis highlights the strong similarities among the primates if you exclude the Old World monkeys. For example, it is the female chimpanzees who leave home and the males who live the socially bonded life. This kind of social system is seen in South American spider monkeys as well. And some lemurs, some South American monkeys, and the small gibbon apes all lead monogamous family-oriented lives. In other words, there are analogous systems that cross taxonomic categories and indicate a common heritage, as long as the female-bonded rule is eliminated.
The new blueprint for a typical primate might include features such as female dispersal, the ability to negotiate social situations without pulling rank, and a variety of mating systems from monogamy to polygyny. And this pattern holds good for apes as well as New World monkeys. “If you take out the cercopithecoids and look at the apes, the prosimians and New World monkeys, the apes then look like part of the gang,” points out Peter Rodman, professor of anthropology at the University of California, Davis, and a recent convert to New World monkeys. From this standpoint New World monkeys, with their wide array of systems and behaviour, become more important for understanding our ancient human past.
But where does this leave the newly ostracised baboons? Have they and other cercopithecines seen the last of researchers tailing them across the savanna? Hardly. “We will continue to be fascinated by the Old World monkeys simply because they are so [genetically] close to us,” says Irven De Vore, anthropology professor at Harvard University and one of the first researchers to describe baboon behaviour. “Also,” De Vore adds, “we’ve been studying baboons for over 35 years, and some of the most fascinating information has only come out in the last five.
Lynne Isbell of Rutgers University, New Brunswick is at work on her third species of African monkey, and she takes issue with any idea that there was a typical cercopithecine pattern in the first place. Even among the Old World monkeys, she points out, there are major differences in their patterns of behaviour. So she would advocate more, not less study of cercopithecines. “We also have to start looking at ecological differences between the Old and New Worlds, and figure out why female dispersal, the key factor, happens so readily in the New World, but not in the Old.”
This new perspective has expanded the stage on which human evolution occurred. No longer can anthropologists turn to one simple monkey model and script our past. And while New World monkeys might not yet have upstaged their Old World kin as the stars in this play, they have finally achieved what any understudy might aspire to – equal billing.
How to spot a New World monkey
JUST about anyone can tell the difference between a baboon (monkey) and a chimpanzee (ape). The ape is much bigger, has no tail, and most important, sports a large brain. But telling a New World monkey from an Old World monkey takes a bit more scrutiny.
The face is the best place to start. New World monkeys have wide noses, with a broad septum running down the middle, and their nostrils tend to point to the sides. Old World monkeys, in contrast, have narrow noses and their nostrils angle down.
New World monkeys have also retained a third premolar or bicuspid tooth (which will be difficult to see unless your subject is sedated) while Old World monkeys, like humans, have only two premolars.
Although it is not obvious from the outside, their ears are different too. In humans and other Old World forms, the inner ear extends into a long bony tube called the auditory meatus, but South American primates don’t have this tube.
All monkey bodies are designed for tree travel. They generally have arms and legs of equal length. The hands have long fingers for grasping and picking, and nails with soft pads on the underside accentuate their powerful sense of touch. But some of the smaller American monkeys have retained some claws to help them cling, like squirrels, to tree trunks.
A few New World monkeys have specialised tails. These so called prehensile tails are made of strong muscle and used like a fifth limb while hanging from, or climbing about, the canopy. And since all American monkeys are arboreal – they spend most of their time in trees – those with prehensile tails have a special advantage.
But if none of these characteristics are definitive enough, find out where your monkey lives. There’s no disputing this. Old World monkeys come from Asia and Africa but New World varieties hang out in the jungles of Central and South America.