Why Monkeys Matter: 10 Primate Facts That Will Blow Your Mind


An aye-aye in Madagascar. (© E.E. Louis Jr.)

This is the second post in Human Nature’s “Why Monkeys Matter” series. In case you missed it, check out yesterday’s post by leading primatologist — and CI president — Russ Mittermeier.

There are innumerable reasons why we should protect primates.

Some of them serve as indicator species for the health of their habitats; when they disappear, it’s not a good omen for tropical forest ecosystems. Others disperse seeds, helping to maintain forest cover. And in many places, they serve as an important draw for tourists, who bring revenue into local communities in exchange for a chance to see these iconic animals face to face.

But that’s not why I’ve been a self-defined “primate geek” since I began learning more about them as an anthropology major in college. To me, they’re just cool, and a world without them would be a less interesting place.

Here’s some proof: 10 facts guaranteed to amaze, impress, and/or weird you out.

1. Aye-ayes hunt by tapping on trees.

This odd prosimian — which differs greatly from its closest relatives — has a long, bony middle finger on each hand. The aye-aye uses these digits to tap on the branches and trunks of trees, listening for insect larvae inside, in a method known as “percussive foraging.” It then bites through the bark and use its bony fingers to scoop out the prey. These animals essentially fill the same ecological niche as woodpeckers in other parts of the world.

2. Capuchins use rocks of half their body weight to crack open palm fruits.

Chimps may be the primates most famous for using tools, but they’re not the only ones. In Brazil, bearded capuchins position palm fruit on a large, flat rock and use a smaller rock — which can weigh one-third to one-half their body weight — to smash the outer shell of the fruit. Between hits, they inspect the fruit and reposition it if necessary. Young capuchins learn this skill from their mothers.

Philippine tarsier

Wild Philippine tarsier; see fact #5. (© CI/photo by Russell A. Mittermeier)

3. Male howler monkeys howl in order to size up potential competitors.

Male primates must constantly compete for mates, but engaging in constant battles can be dangerous (or sometimes lethal). Many species use their voices to defend their turf, including howler monkeys, whose vocalizations can be heard 5 kilometers (3 miles) away. Depending on the vocal response they receive from other individuals, they decide if they want to pursue an actual physical fight.

4. Dominant female tamarins and marmosets will suppress ovulation in the other females, which keeps them from getting pregnant.

This isn’t a conscious action; rather, it happens naturally through the release of hormones by the dominant female. Tamarins and marmosets are also the only primate family that regularly produces twins. Other members of the family group — including the males — will often help carry and care for the infants, a behavior known as “cooperative breeding.”


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5. A tarsier’s eyes are each the size of its brain.

Out of all the world’s primates, these nocturnal animals from Southeast Asia may be the most alien in appearance. If humans had eyes that were the same size in proportion to our faces, our eyes would be the size of grapefruits. Tarsiers’ eyes are fixed in their sockets, meaning they have to turn their heads to see around them; the tarsier can rotate its head almost 180 degrees in either direction, like an owl.

Verreaux's sifaka "dancing"

Verreaux’s sifaka “dancing”; see fact #8. (© CI/photo by Russell A. Mittermeier)

6. Bonobos use sex to alleviate tension.

Bonobos — also known as pygmy chimpanzees — are known for their “non-conceptive sexual behavior,” which occurs across all age and sex categories within their social groups. In a striking contrast with their larger cousins, the common chimpanzees, male bonobos rarely fight over access to females. Instead, all group members use sex as a way to diffuse tension in the group, which allows them to cooperate without aggression.

7. Some primates pollinate plants.

Some species of uacari and tamarin subsist almost exclusively on nectar when there’s no fruit around. As they extract nectar from these plants, the monkeys pick up pollen on their fur, which helps the plants colonize new habitats. Some plant species are actually adapted to being pollinated by mammals rather than birds or insects, and have evolved to be more robust.

8. When traveling on the ground, sifakas “dance” on two legs.

These large lemurs from Madagascar spend most of their time in the trees, where they leap between branches and tree trunks. On the occasions where they must descend to the ground, they move in a “bipedal gallop” in which their legs sashay and their arms are held up in the air as a counter-balance. It may look comical, but this movement is actually the most efficient way for sifakas to travel on the ground, minimizing their chances of being caught by a predator.

9. Some New World monkeys have patches of bare skin on their tails, allowing them to function as “fifth limbs.”

Contrary to what popular culture tells us, only a few genera of monkeys have prehensile tails. Spider monkeys, howlers, woolly monkeys and muriquis have patches of bare skin on their tails that allows them to hang from them, grasp things and generally use them as “fifth limbs.” Capuchins also have prehensile tails, but they are fully “furred” and can’t support the monkeys’ weight.

white bald-headed uacari

White bald-headed uacari. (© CI/photo by Russell A. Mittermeier)

10. Bald uacaris may have evolved red faces to advertise their health and suitability as mates.

These monkeys are not known (by humans, anyway) for their good looks; however, their bright red faces may serve as an indication of health. Monkeys with malaria and other diseases develop pale faces. Given their secluded habitat — the monkeys live mostly in the flooded forests of the Amazon Basin — uacaris are very hard to observe in the wild.

Molly Bergen is the managing editor of Human Nature. Read the next post in our “Why Monkeys Matter” series.


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  2. William lee says

    I worked for 15 years at a FDA research center which used Rhesus monkies . One study was on fetal alcohol syndrome on the primate fetus which were removed via c-section and connected by mini pump and replaced. Just before birth the fetus was exposed to disconnect the pump and replaced allowing to deliver normally and monitored the infants. Another was using marijuana to decrease their aggressive nature which it did at doses low enough to allow them to function without effecting them or their ADLs.during that time we studied many of the other primates yet this article taught me things I did not know about some of the primates. Surprised me but in a great way.

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