20 unexplained mysteries of the Amazon

The Amazon rainforest is the most biodiverse region on Earth. There are thousands of tree species, most of which grow to between 24 and 30 metres tall, developing huge networks of branches that make up the canopy layer. A few trees grow even taller and poke up above the canopy, forming the emergent layer – small flying and gliding animals like birds, bats, and butterflies are good at manoeuvring from tree to tree up where it’s windy. Below the main tree canopy, the understory layer is darker and stiller. Plants growing there often have extravagant, very fragrant flowers in order to attract pollinators without a lot of light. The forest floor layer is even darker, and few plants grow there.

The most active part of the rainforest is the canopy layer, which is the six or so metres of treetops that essentially form the roof of the ecosystem 24 metres above the ground. More animals live in the canopy than in any other layer – birds including macaws and toucans, monkeys, spiders, sloths, and hundreds of thousands of insects –  that eat the fruits and leaves of trees and sleep in the branches.

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Although there are places in the Amazon where the tree canopy is so thick that no light reaches the ground, there are other spots where humans (and there are a lot of them there) are farming, ranching, and engaging in other activities that change the landscape. Clear-cutting in recent years has revealed evidence of land use by earlier groups as well: 2000-year-old huge geometric earthworks form squares and circles that stretch as far as a city block. Some trenches are 3.5 wide and 4 metres deep. Researchers aren’t sure what the geoglyphs were used for, but a recent study in The Proceedings of the National Academy of Sciences found that ancient humans actively managed the forest using sustainable practices. “New estimates for the population of Amazonia in pre-colonial times range between 6 and 10 million people, which is many more than today,” says Jennifer Watling, an archaeologist who led the study. “These people had many ingenious ways of making the forest more productive without damaging it for future generations.”

If you saw the 2016 movie The Lost City of Z, you know about Percival Fawcett, the intrepid British explorer of Amazonia who vanished (with his son and another team member) in 1925 during one of his expeditions. His story made for great tabloid fodder in its day, and although he almost certainly perished in the Amazon through accident, illness, or at the hand of an indigenous tribe he’d insulted (writer and explorer Hugh Thomson wrote in the Washington Post that Fawcett was said to have stolen canoes and refused to share game on occasion), numerous expeditions were sent to look for him, and occasional reports of a white man in the rainforest would revive the story for decades.

Although Fawcett lacked manners when it came to his interactions with local tribes, he had a great sense of what contemporary newspaper readers back home in England wanted, and he seems to have spun some wild stories to help him garner funding for more adventures. One of his tales was about the Maricoxi, a sasquatch-like tribe of very hairy creatures that threatened his party with bows and arrows but could only grunt.

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About a million indigenous people live in the Amazon rainforest. There are around 400 tribes, most of which have had contact with outsiders for hundreds of years. They hunt, fish, and farm, and have access to Western medicine and education. But a dwindling number of tribal people have remained isolated. Although they’re often referred to as “uncontacted,” most isolated tribes actually know about outsiders and choose to keep their distance. That’s because most of the contact they’ve had has been extremely destructive: loggers, miners, and ranchers have killed and enslaved indigenous people in the region and isolated tribes still have little immunity to the diseases outsiders can introduce. In July 2018, Brazilian authorities managed to take a picture of a man known as the “indigenous man in the hole,” who is the sole survivor of a tribe whose other members were killed by farmers in 1995 – he has rejected outside visitors, though the government leaves him seeds and tools.

Many different tribes – even those that don’t communicate with one another – talk about a giant rainforest animal that they describe as either “roaring” or “fetid.” The accounts are so numerous that researchers have mounted expeditions to try to track down a 2.1-metres-tall beast with a stench so strong it can make hunters dizzy and disoriented. Although no bones or scat samples have turned up, some scientists think the descriptions of mapinguary might be based on passed-down stories of a time when humans in the Amazon interacted with the last giant ground sloths – possibly 10,000 years ago, when the creatures are thought to have gone extinct (or maybe more recently, if that date is found to be wrong). “We know that extinct species can survive as legends for hundreds of years,” David Oren, a former director of research at the Goeldi Institute in Belém, Brazil, told The New York Times in 2007. “But whether such an animal still exists or not is another question, one we can’t answer yet.”

A new species of plant or animal was discovered in the Amazon rainforest every three days, on average, between 1999 and 2009, according to a WWF report. They included a bald parrot, a tiny blind catfish, and a translucent frog, with skin so thin you can see its heartbeat. In fact, 1 out of every 10 known species lives in the Amazon.

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The biggest snake in the world is the green anaconda, which lives in the Amazon’s swamps and streams. Growing as long as 8.8 metres and weighing up to 249 kilos, anacondas beat out their main competition for the title, the reticulated python (native to South and Southeast Asia), which can grow slightly longer but tend to be much more slender. The anaconda spends most of its time in water; its eyes and nasal openings are located on top of its head to allow it to watch for prey while almost completely underwater. They catch wild pigs, birds, and even jaguars, squeeze them until they suffocate, and swallow their prey whole. Anacondas can go months without food after a big kill.

Weird wildlife in the Amazon isn’t always huge – over the past decade, scientists have been trying to figure out what is building tiny silk structures in Tambopata, Peru. Each has a ring of pillars connected by horizontal threads, forming a fence; in the middle is a cone. After collecting and observing numerous specimens, researchers finally started seeing spiders hatch out of the structure in the middle. Knowing the builders are spiders is enlightening, but scientists still aren’t sure what species they are, because no clear adult owners of the silkhenge structures were observed. No other spider has ever been observed laying only a single egg in an egg sac – in fact, most spiders keep a bundle of eggs on their own webs to protect until they hatch.