an Old World monkey

The range of many monkey species is determined by rivers. Since most monkey species don't swim, they generally don't cross large rivers except under extraordinary circumstances. Some large rivers have completely different monkey species on each side of their banks. Sometimes the species are similar enough that it seems conceivable that as a breeding couple of one species floated on logs across the river a hundreds of thousands of years ago and over time established a new species.

The predators that monkeys have to worry about most are eagles and humans. Snakes and cats are largely terrestrial animals who are no match for agile monkeys in the trees.

Old and New World Monkeys

Scientists divide monkeys into Old World monkeys — those from Asia, Africa and Europe — and New World monkeys — those from North and South America. There are no monkeys in Australia, New Zealand or islands in the major seas unless they have been introduced there.

New World Monkey

The main difference between Old World and New World monkeys is their tails. Old World monkeys never developed their tails into a adaptable, prehensile, fifth limb as New World monkeys did. They use their tails for balance when they scamper along branches and to help control their jumps and landings. Scientists distinguish between Old World and New World monkeys based on their noses. The noses of New World monkeys are flat and broad with widely-spaced nostrils that are pointed towards the side of the face whereas the noses of Old World monkeys are thinner and straighter and the nostrils are narrower and point forward. The scientific name of New World monkeys from South America is platyrrhines — flat-noses — while the scientific name of Old World monkeys is catarrhines — hooked noses.

Some scientists speculate that because many species of African monkeys spend a lot of time on the ground developing an agile tail was not a priority. In the Old World there are many species of monkeys who spend a lot of time on the ground while in the New World there are none.

Old World monkeys also have calloused patches on their rump, longer hind limbs than forelimbs, two pre-molar teeth and flattened nails on their digits. All are diurnal and most are at least partly terrestrial. David Attenborough wrote: “Old World monkeys have ear drums, which like our own, lie deep in the head, at the bottom of a tube. New World monkeys have eardrums so close to the ear surface of the head that they can easily be seen within the ear.”

Old World Monkeys

There are 21 genera and about 100 species of Old World monkeys. They live in Africa, southern Spain, parts of the Arabian peninsula, southern Asia and Japan. There are two main types of Old World monkeys, based largely on dietary adaptions: 1) Cercopithecines and 2) Colobines

Cercopithecines are mostly fruit-eating monkeys. They have rounded molars, cheek pouches used to store food, a short tail, well-developed thumbs and arms and legs of similar size which are useful in walking with all fours. They include macaques, baboons, some mangabeys, and guenons.

Colobines are mostly leaf-eating monkeys. They have high-cusped, shearing molars, no cheek pouches, multi-chambered stomachs for fermenting fibrous leaves, long legs for leaping, short or absent thumbs and very long tails. They include langurs and colobus monkeys.

Monkey Eating Habits

Primates generally use their incisors, canines, lips and fingers when they eat. How they use them depends on what they eat. Almost all primates eat fruit, seed and leaves. Many eat small prey, mostly insects, other arthropods, small animals and birds’ eggs. Even so there is some specialization.

In the wild monkeys are very finicky eaters. They sniff, feel and nibble what they eat and often spit out what they don’t want, leaving behind a trail of half eaten fruit and leaves on the forest floor. Sometimes this because they eat fruits that have worms and insects in them while leaving undisturbed fruit alone. For many monkey species worms and insects are their primary source of protein.

Fruit-Eating Monkeys

Ice-cream-eating monkey

Monkeys are also divided into fruit-eating monkeys and leaf-eating monkeys. Fruit and nut eating primates generally have a larger brain than leaf-eating apes and monkeys because they need more sophisticated reasoning, scientists suggest, to find food which is only found in some areas at certain times of the year while leaves are generally available everywhere throughout the year. The brain of the leaf-spider monkey, for instance, is only half the size of the fruit-eating howler monkey, even though occupy roughly the same terrain.

Fruit-eating monkey have a higher rate of metabolism than leaf-eating monkeys. This allows them to move more quickly and dexterously through the large area of forest to seek out of fruiting trees. To keep going they need high energy-rich food such as fruit. Fleshy fruits are easy to digest. Food passes through their bodies quicker and they defecate frequently.

Fruit-eating monkeys can get to widely scattered sources of food more quickly and efficiently than leaf-eating monkeys. They endure a kind of feast and famine existence in which they go long periods without food were no fruits are available and travel a long distance to find it. But once a food source is found they gorge themselves until it is depleted.

Different fruit eating monkeys, such as spider monkeys, capuchin and titis, often compete for the same fruits trees. The largest monkeys or the one in the largest group usually win out. ruit trees only fruit at certain times and sometimes these times are unpredictable Fruits are generally low in protein. Small primates often supplement their diets with small animal or insect prey or nectar-filled flowers. Large ones eat leaves.

Fruit eating monkeys generally have long arms and tails which allows them to swing through the trees like acrobats. They generally have oversize large intestines, which help them digest fruit. Their incisor teeth tend to act like scoops and are broader and larger that those of leaf-eating monkeys.

Leaf-Eating Monkeys

Leaf-eating monkeys generally much more slowly and deliberately through the trees. They tend to stick around one place, constantly munching on leaves they choose rather moving around a lot and picking and choosing.

Leaf-eating monkeys defecate less than fruit-eating monkeys, often when after waking up in the morning and tight after nightfall. Moreover, they have to be carful about what they eat because their slow metabolism absorbs toxins earlier than the quicker metabolism of fruit-eating monkeys.

Leaf-eating monkeys generally have a low metabolism which allows them to survive on large quantities of low-energy but ubiquitous food like leaves. They have developed jaws and chewing muscles and sharp crested molars that allow them break through the tough cell walls of leaves to get at the nutrients inside and make them easier to digest.

Leaf eating monkeys have chambered stomachs like ruminants such as cows. They eat large quantities of leaves and rely on bacteria in their stomachs to break down the cellulose in the leaves. These monkeys often spend several hours eating and several hours relaxing while food in their stomachs digest.

Leaf-eating monkeys have longer intestines for their body size than fruit-eating monkeys to allow for more time to absorb nutrients. Their digestive system is dominated by their small intestine, which absorbs the nutrients. Some have large stomachs or colons, which act as fermentation chambers to breakdown the cellulose and lignin in the leaves to make them digestible. They also have efficient kidneys and livers to filter out toxins and other chemicals the plant produce as a defense.

Leaf eating monkeys often do better in second growth forest than virgin ones because the monkeys sometimes eat so much fruit and vegetation from certain kinds of trees that overdose on toxic chemicals. In young forests the trees are not large, and do not bare as much of fruit and leaves, consequently they are less likely to provide chemicals in quantities that would cause the monkey to OD.

Monkey Group Behavior

Most monkeys live in groups led by a dominant male and comprised of lesser males, and females and their young. In most groups there are twice as many females as males, with non-group males either living alone or living in a group with other males. Most of the males within the group are juveniles that will eventually leave.

A male monkey usually gains control of a group one of two ways: either by defeating a dominant male in a group and taking over his group and by luring some females away from a group to form his own group.

Within a group the females often get along fairly well. They usually have some sort of ranking system. The males may live peacefully or fight depending in the group, its dynamic, the species and availability of food and females.

These rules don’t apply to all monkeys and apes. In some primate species males hold on to dominance thanks to the collective support of females. In others the females are dominant. In others still dominance and hierarchy are irrelevant concepts.

Male Dominance in Rhesus Monkeys and the Rostral Prefrontal Cortex

rhesus monkey

Commenting on the findings in a paper by Jerome Sallet and his colleagues at Oxford University published in the journal Science,Robert M. Sapolsky wrote in the Los Angeles Times: “Male rhesus monkeys are among the least Dalai Lama-esque of primates. They form strict, linear dominance hierarchies, and the highest-ranking guy holds his position through aggression and intimidation, something primatologists call a "despotic" dominance system. Rhesus males are often brutish and combative. [Source: Robert M. Sapolsky, Los Angeles Times, January 15, 2012. Sapolsky is a professor of neuroscience at Stanford University and the author of "A Primate's Memoir," among other books.]

Sallet and his team studied the behaviors of the rhesus males in different social groups, figuring out the hierarchies. Then, with those behavioral data in hand, the researchers scanned the brains of the monkeys using magnetic resonance imaging to determine the size of different parts of the subjects' brains. Looking at the brain this way is informative — because size matters. A region that has been working hard tends to increase in size, while those that are mothballed get smaller. Spend a summer learning how to juggle, say, and the part of the cortex devoted to hand-eye coordination expands. Go through a horrific experience that results in post-traumatic stress disorder and you'll probably end up with an enlarged amygdala, a part of the brain that is centrally involved in processing fear and anxiety. Conversely, lose your hearing and your "auditory cortex" will atrophy.

When the researchers scrutinized the monkeys' brains, all sorts of interesting findings popped up. One involved the issue of dominance. After controlling for such factors as age, weight and size of the social group, out popped a clear correlation: The higher a male's rank, the larger the average size of a particular brain region. Was this a region that controls the muscles required to pummel a rival? Were these the neurons that cause testosterone to be secreted? No and no.

The size increase was in a region far from the world of muscles and androgenic hormones, a region called the rostral prefrontal cortex. This area of the brain has to do with social smarts, with drawing inferences based on information about what other individuals can see, what they know, what they think (something psychologists call "theory of mind"). In humans, people with more gray matter in that region are better at understanding multiple layers of human interaction. They'd be good, for example, at the virtually impossible task of keeping track of who, in "A Midsummer Night's Dream," is in love with whom, and who thinks he might know who is in love with whom. In other words, this is a brain region central to being able to understand someone else's perspective.

A study like this can't tell you whether a big rostral prefrontal cortex causes a male to become high-ranking, whether it's the other way around, or whether it is neither of those things. What it shows is that on average, the size of a part of the brain centrally involved in social intelligence is a good predictor of high rank, a cool and novel finding.

Male Monkeys Pay for Sex

In January 2, 2008 AFP reported: “Selling sex is said to be humankind's oldest profession but it may have deep evolutionary roots, according to a study into our primate cousins which found that male macaques pay for intercourse by using grooming as a currency. Michael Gumert of Nanyang Technological University in Singapore made the discovery in a 20-month investigation into 50 long-tailed macaques in Kalimantan Tengah, Indonesia, New Scientist reported.

On average, females had sex 1.5 times per hour. But this rate jumped to 3.5 times per hour immediately after the female had been groomed by a male -- and her partner of choice was likely to be the hunky monkey that did the grooming. Market forces also acted on the value of the transaction. If there were several females in the area, the cost of buying sex would drop dramatically -- a male could "buy" a female for just eight minutes of nit-picking. But if there were no females around, he would have to groom for up to 16 minutes before sex was offered.

Gumert told the New York Times that about 89 percent of the male-grooming-female episodes observed “were directed at towards sexually active females. When males groomed these females it appears they expected sex or at least genital inspection. By contrast when mothers groomed their young it appeared they did so to clean and sooth them.

Gumert told the New York Times, “What led me to think of grooming as a form of payment was seeing how it changed across different market conditions. When there were fewer females around, the male would groom longer, and hwn there were lots of females, the grooming times whet down. Males also grooomed females of high-rank considerably longer than ones of lower rank.

The work supports the theory that biological market forces can explain social behaviour, the British weekly says. "There is a very well-known mix of economic and mating markets in the human species itself," said Ronald Noe of France's University of Strasbourg. "There are many examples of rich old men getting young attractive ladies."


Homosexuality Among Monkeys

Paul Vasey, a researcher at the University of Lethbridge in Alberta, Canada, has spent many years studying homosexuality among Japanese macaques. The Economist reported: “Female macaques often form homosexual consortships. These are temporary but exclusive relationships that involve frequent sexual activity. Females in a consortship will mount each other tens or hundreds of times. In one group that Dr Vasey observed, females mounted each other as often as once every two minutes. Yet his observations suggest these consortships serve no adaptive function. He has spent many years testing hypotheses that might explain the behaviour, such as alliance forming, the relief of social tension and the communication of dominance. There is, he says, not a shred of evidence for any of them. Female mounting behaviour may have evolutionary roots, but he reckons the reason for it now is sexual gratification. That gratification is involved is known because when a female mounts another female she thrusts her pelvis against the mountee and masturbates her clitoris using her tail. [Source: The Economist , February 22, 2003]

“This activity, of course, excludes males. In one study, Dr Vasey found that when male monkeys courted a female involved in a homosexual consortship, 95 percent of such females rebuffed him and chose to remain with their girlfriend. This suggests, he says, that it is not simply males who are competing for sexual partners, as Darwin's theory predicts, but both males and females. And homosexual behaviour is documented in at least 15 other species, including Canada geese, gorillas, chimpanzees and humans. [Ibid]

”Such examples may not be enough to topple sexual selection, and it is likely that this part of Darwin's theory does indeed hold good for many species. But as Dr Roughgarden warns supporters of that theory, although any one of these problems with it might be overlooked, the “sheer number of difficulties is hard to deny. If these are not enough to falsify sexual-selection theory, then what would be?” Sex, it seems, has come a long way since Darwin.” [Ibid]

Monkeys and Television

The Yomiuri Shimbun reported: “Humans are not the only primates that enjoy watching TV. In an article published in a Swiss specialist journal a team from the Kyoto University Primate Research Institute found that monkeys enjoyed themselves based on monitoring the monkeys’ brain activity while it watched TV. Feelings of anger and fear were evident from the monkey's facial expression, but feelings of joy could only be induced. [Source: Yomiuri Shimbun, June 20, 2010]

The team, led by Prof. Nobuo Masataka, used optical topography to observe what parts of a rhesus monkey's brain were activated when it watched TV. Optical topography uses near-infrared light to record brain activity. The team found the frontal lobe of the monkey's brain was activated by watching TV. This is the same part that is activated in humans when they feel joy, such as when a baby sees its mother smile.

In the experiment, the monkey watched a video of an elephant and a giraffe performing in a circus, and another of a monkey grooming itself. Its brain showed more activity when it watched the circus. An increasing number of zoos have been showing their animals TV to break up the monotonous living environment. The research team has scientifically confirmed the effectiveness of the zoos' strategy, observers said.

Image Sources: Wikimedia Commons

Text Sources: National Geographic, Natural History magazine, Smithsonian magazine, Wikipedia, New York Times, Washington Post, Los Angeles Times, Times of London, The Guardian, Top Secret Animal Attack Files website, The New Yorker, Time, Newsweek, Reuters, AP, AFP, The Economist, BBC, and various books and other publications.

Last updated November 2012

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