Spider Monkeys Are the Trapeze Artists of the Treetops

Imagine living almost your whole life 100 feet (30 meters) up in the tree canopy, rarely ever venturing down to the first floor. Spider monkeys, named for the spidery impression they give as they swing, jump and crawl through the trees by their long, spindly arms, legs and ultralong tails, may only see the ground a few times in their life.

All seven species of spider monkey are large — they're among the largest of the New World monkeys, existing only in Central and South America from Mexico to Brazil. An average adult weighs around 13 pounds (6 kilograms), but the largest species, the critically endangered black-headed spider monkeys (Ateles fusciceps), can weigh in at around 20 pounds (9 kilograms). It may not seem like much, but it's a lot if you consider the acrobatics required of them up in the treetops — they're so strong and agile that they can easily sail across a 40-foot (12-meter) gap in the tree canopy and land safely on a branch on the other side.

Spider monkeys are very vocal, making a variety of howls, barks, growls, screeches and whinnies (which sound like a horse's whinny) at each other through the trees. Vocalizations are important because they spend a lot of their time in large groups of around 50, but which break out into subgroups that change size and composition of individuals several times a day. The females choose mates and make decisions for the group, even down to how large the groups will be.

Spider Monkeys Live Mostly on Fruit

Together a group of spider monkeys forages for fruit, but also bird eggs, leaves and insects, but mostly they eat whatever fruit they can get their thumbless hands on. That's right — spider monkeys have extremely reduced thumbs, so small that they're not even noticeable. And it makes sense — when your entire existence revolves around agile locomotion through the tree canopy, an awkwardly-jutting thumb is a liability. What if it snags on something? So, no thumbs needed, but as it is, spider monkeys are incredible trapeze artists thanks to their four long fingers shaped like hooks and powerful prehensile tails (with a bald patch at the end similar to the palm of a hand), which can hold up the weight of their entire bodies.

Another reason spider monkeys don't need thumbs: Unlike some primates, they don't use tools, though, truth be told, they probably don't use tools because they don't have thumbs. Although we sometimes link intelligence with tool use, whether an animal uses tools or not depends partly on cognitive ability, and partly on need.

"I do not like to talk about intelligence because it is tricky," says Filippo Aureli, research professor at Universidad Veracruzana, in the Mexican state of Veracruz, in an email. "For example, spider monkeys are not good tools users, as they have a very reduced thumb as an adaptation for their acrobatic arboreal locomotion, but they are excellent in inhibiting behavior and flexibly changing their response — abilities known to involve the prefrontal cortex."

So, spider monkeys have a lot of social intelligence — since they live in such large groups that often change in composition and size, they have to be able to get along in a constantly shifting social environment, which can be difficult even for humans. According to Aureli, in a series of experimental tests with various monkeys and apes, spider monkeys performed better than the other monkeys and gorillas when it came to coping with this kind of social elasticity, and fared similarly to orangutans, chimpanzees and bonobos, all of whom live with similar group dynamics.

Since spider monkeys are large and live in big groups, they require huge tracts of undisturbed forest habitat to range in.

Spider Monkeys Are An Endangered Species

"They are now considered among the world's 25 most endangered primate species," says Aureli. "The main threat is habitat destruction. In some areas they are also threatened by the illegal pet trade."

Spider monkeys are also a favorite food for indigenous people, both because they're so large and because they are a preferred meat. And as usual, climate change plays a part in their dwindling numbers because tropical and subtropical areas of the planet will feel its effects sooner and more powerfully than others.

These threats are compounded by the fact that spider monkeys have a slow reproductive rate — females become sexually mature at around 4 years of age, give birth for the first time at around age 7 and can produce one baby about every three years. This means that once a population takes a hit, it takes a long time for it to recover.