Creatures of the Amazon's Forests and Rivers: From Jaguars to Uncontacted Tribes

The area of the Amazon rainforest varies depending on how it is defined, but the video explains that it is generally put at around 5.5 to 7 million square kilometers. Since Japan covers less than 400,000 square kilometers, that works out to an area roughly 15 times the size of Japan covered in jungle. The forest is estimated to contain 390 billion trees. Their tall branches spread sideways to form a canopy, and almost no sunlight reaches the ground. This unique environment shapes the lives of many organisms.

Said to reign at the top of the Amazon's food chain is the jaguar, the largest cat in the Americas. It weighs around 60 to 110 kilograms, making it somewhat smaller than lions and tigers. But the jaguar has a feature that sets it apart from them: its bite force, or the strength of its bite. Its bite isn't as strong as a lion's or a tiger's in absolute terms, but because the jaguar is far lighter than either, it is thought to have the strongest bite relative to body weight.

With its short jaw, large canines, and heavy skull, the jaguar boasts a bite force well in excess of 500 kilograms-force. When hunting, it targets not the throat but the skull, piercing it with its thick canines. Even caimans and turtles can fall victim.

As its attacks on caimans and turtles suggest, the jaguar is so at home in water that it could be called semi-aquatic, a lifestyle hard to picture from the usual image of a "cat." In 2024, a jaguar was found on a small island and identified as the same individual that had been recorded on the mainland a few years earlier. This meant the jaguar had swum to the island, which would have required swimming at least 1.27 kilometers without stopping, even by conservative estimates.

Living in the canopy is an animal that lives up to its name: the sloth. It spends nearly its entire life in the trees, and its defining characteristic is an extremely low metabolism. According to estimates from a study cited in the video, the three-toed sloth, a 4-kilogram animal commonly seen in the Amazon, has a metabolic rate of about 110 kilocalories a day, the lowest of any mammal. Even living normally, it burns less energy than is contained in a single rice ball.

One of the defining traits of mammals is that they are warm-blooded, maintaining a constant body temperature regardless of their surroundings. Humans stay around 37°C, and a deviation of about 5 degrees from that becomes life-threatening. Sloths, however, are known to experience body temperature swings of up to nearly 10 degrees over the course of a day. Their temperature is lowest from late at night to early morning and highest at midday. Unlike other mammals, sloths cannot warm themselves by shivering their muscles, so they raise their body temperature by basking in the sun. The video describes this as "a lifestyle that seems to half-abandon being warm-blooded."

This feature of their body temperature also connects to the well-known story that sloths can starve to death on a full stomach. Almost all of a sloth's food consists of abundant leaves, but sloths cannot digest leaves directly and instead obtain nutrients with the help of bacteria in their digestive tracts. When their body temperature drops, the bacteria's activity slows dramatically, and the sloth can starve to death because it cannot digest the food even though its stomach is full.

On top of that, sloths go to the trouble of climbing all the way down to the ground roughly once a week to defecate. This is the most dangerous moment for a sloth, making it easy prey for birds of prey and jaguars. And because it has to climb back up afterward, it is estimated to spend about 8% of its daily energy expenditure on this "ritual." The video states that why sloths take such risks to defecate on the ground is still not understood.

Appearing amid the all-green jungle with blue, red, and gold wings is the macaw. Beyond its vivid colors, its beak is known to be the most powerful of any bird. In one study, its bite reached a maximum of 540 newtons, or about 55 kilograms-force, enough to crack macadamia nuts, said to be the hardest nuts in the world.

Macaws have a habit that puzzled biologists for many years. Every year, hundreds of them gather at a waterside site in Peru and eat the soil there. At first, it was thought the soil had some kind of detoxifying effect. But closer investigation showed that the behavior occurred mostly during the breeding season, and that the soil they particularly favored had an extremely high sodium content. The Amazon rainforest is known for having little salt in its soil, especially inland, so it became more likely that macaws lacking sodium chloride were replenishing their sodium from the waterside soil.

Macaws are also important to plants. According to a theory introduced in the video, the animals that once carried large seeds through the rainforest and helped plants reproduce were large mammals, but because most of them have since gone extinct, those plants could no longer carry their seeds far, which hampered their reproduction. In Bolivia, however, macaws have been observed carrying large fruits more than a kilometer, eating the flesh, and then discarding the seeds. In fact, young palm trees are found in large numbers near trees where macaws perch. The video says that without macaws, many trees might not have been able to spread, and the Amazon could have looked completely different.

The toucan, a relative of woodpeckers, is best known for its giant beak, which in large individuals makes up a third of its body length. Even so, the beak is very light relative to the bird's body weight. That's because the inside of the beak is hollow, and the beak itself has a mesh-like structure with a high proportion of empty space.

There are various theories about why toucans have such large beaks, including attracting partners and peeling fruit. The theory currently considered most likely is that it serves to regulate body temperature: by increasing blood flow to the beak, the bird can release internal heat into its surroundings. According to one account, a toucan's beak has a temperature-regulating capacity comparable to an elephant's ears.

Bright colors can also be danger colors that warn others to stay away. On the floor of the Amazon live poison dart frogs, which carry some of the most potent toxins on Earth. They are famous because indigenous people coated the tips of their blowgun darts with the poison for hunting, but of the more than 170 species of poison dart frog, only three species, all found in Colombia, were used this way.

The most toxic among them is the shimmering golden poison frog. Its skin secretes a toxin called batrachotoxin, said to be enough to kill 10 people. When batrachotoxin enters the bloodstream, it locks the sodium channels in nerves and muscles in the open position, and in the worst case, the victim becomes unable to breathe and dies.

However, the video also explains that there's no need for excessive fear. Indigenous people handled the toxin by applying it to their darts, and some modern researchers handle these frogs with bare hands. Batrachotoxin is not toxic unless it gets inside the body. If you have a small cut on your hand, you'll know right away because of a burning pain, but even then it apparently isn't life-threatening.

It is known that poison dart frogs produce their toxin from the food they eat. As a result, frogs kept by humans can't make toxin from the food they're given and become completely nontoxic. That's why poison dart frogs, which should be deadly, are sometimes sold as pets.

Leaping nimbly from branch to branch in the upper canopy is the spider monkey, with its long arms and tail. Spider monkeys have almost no thumbs. Because thumbs would get in the way when grabbing branches mid-leap, they are thought to have gradually shrunk over the course of evolution, and the hands serve as little more than hooks. The tail is longer than the arms and functions like a fifth limb; the monkey can hang from a branch by its tail alone, supporting its entire body weight. At the tip of the tail is a hairless patch bearing a pattern that, like a fingerprint, differs from individual to individual.

Spider monkeys are also highly social animals. They live in groups of several dozen, but when foraging, they frequently split into several smaller groups and then reunite later. When they reunite, spider monkeys often hug one another. This behavior is thought to have evolved as a way to show a lack of hostility in order to avoid pointless conflict within the group.

Spider monkeys also contribute greatly to seed dispersal. One study that observed a single group estimated that it dispersed the seeds of 133 species in total over one year. They dispersed 195,000 seeds larger than 1 millimeter, 35,000 of which were larger than 3 millimeters, carrying them an average distance of more than 400 meters. It has been pointed out that declines in their numbers directly affect the rainforest.

If the jaguar rules the ground, the video says, the master of the underground may be the tiny leafcutter ant. A single colony sometimes houses millions of ants and can easily exceed the size of a tennis court. Worker ants march like an army back to the nest, carrying pieces of leaves heavier than their own bodies.

But leafcutter ants don't eat leaves. The leaves are used not as food but as compost. The ants cultivate a fungus inside the nest, and the leaves they bring in feed this fungus. The grown fungus produces nutritious cells for the ants, which the ants then eat. The video calls this "farming" and describes it as the oldest agriculture on Earth, practiced for roughly 50 million years. It also introduces a theory that this began because of climate change caused by the asteroid impact 66 million years ago that drove the dinosaurs to extinction.

Leafcutter ants and their cultivated fungus have such a symbiotic relationship that neither can live without the other. Leafcutter ants cannot synthesize arginine, a type of amino acid, in their bodies, and depend entirely on the fungus for it. The fungus, meanwhile, has reportedly lost its abilities related to stress responses and detoxification. Because the ants diligently thin it out and dispose of waste, the fungus no longer needs those abilities.

Their role in the rainforest as a whole is also significant. A single colony can excavate about 40 tons of soil, and by carrying waste outside the nest, the ants move nutrients through the soil. They function as "ecosystem engineers" that alter soil structure and the forest's nutrient cycling, and the video says it would be no exaggeration to say that much of the Amazon's soil has been shaped by leafcutter ants.

The Amazon's most famous tarantula is the Goliath birdeater, the heaviest spider in the world. It can weigh up to about 170 grams and have a leg span of as much as 30 centimeters. Despite its name, it is thought to rarely eat birds; its diet consists mostly of insects, with the occasional small frog or mouse. It immobilizes its prey with large venomous fangs, carries it back to its burrow, injects digestive enzymes, and sucks out the liquefied insides with its straw-like mouth.

It doesn't pose much harm to humans. Its venom isn't very potent to people, and the pain is said to be about as bad as a bee sting. What you should really watch out for are the urticating hairs that most Amazonian tarantulas have, which they scatter around them when they sense danger. If these touch the skin, they can cause an itchy rash.

The video then heads to Angel Falls, a waterfall in southern Venezuela. It isn't in the Amazon River basin, but it is considered part of the Amazon rainforest. Its total drop is 979 meters, the largest in the world, and even excluding the section where the water strikes rock partway down, its uninterrupted drop is 807 meters. Both are world records, and both are taller than Tokyo Skytree.

In the surrounding area are more than 100 table mountains, rising like islands from a sea of jungle. They are thought to have formed when erosion from rain and other forces wore away the surrounding land, leaving only the hard rock behind. Angel Falls itself pours off one of them. Because each is isolated, unique ecosystems have developed on them over tens of millions of years; they have many endemic species, and there may also be undiscovered organisms. In 1912, Arthur Conan Doyle published his novel The Lost World, set on top of such a mountain where dinosaurs had survived.

Because the drop is so great, most of the falling water turns into fine mist that drifts through the air. During the dry season from December to April, almost all of the water evaporates. There is no plunge pool; the spot where one would normally be is said to look as if it were being sprayed by a giant mister.

Indigenous people have long known about the falls and apparently called them Kerepakupai Merú. The video explains that the current name was given in 1937, following the discovery of the falls in 1933 by American pilot James Angel. When he landed on top of the table mountain, his plane got stuck in the mud and couldn't take off, and he and his three companions had no choice but to return on foot. The journey took 11 days, and according to the story that has been passed down, this adventure made the falls' name known around the world.

In parts of the Amazon are flooded forests, which are inundated at the same time every year and remain underwater for months. The video says these floods should be called not disasters but environmental changes that come with the rainy season, explaining that the entire ecosystem is built on the assumption of these floods.

At their peak, the waters reach all the way up to the canopy, the forest's uppermost layer, and fish rise up near the canopy too. During this period, fish eat fruits and seeds that fall from the trees and take on the role of dispersing the seeds. In fact, it has been shown that the extent of flooded forests and the richness of their tree species correlate with the abundance of fruit-eating fish. The forest feeds the fish, and the fish expand the forest.

Normally, trees submerged to this extent would quickly die because their roots couldn't breathe. But the trees of the flooded forest have hollow aerenchyma tissue running continuously from the roots to the upper parts of the tree, a mechanism that delivers oxygen to the roots. They have also evolved to be able to photosynthesize even when their leaves are underwater and sunlight is drastically reduced.

The Amazon River, the core of the Amazon rainforest, doesn't just have the largest discharge in the world; its discharge is greater than that of the world's second- through seventh-largest rivers combined. About 20% of the fresh water flowing into the oceans is said to come from the Amazon.

The Amazon River once flowed in the opposite direction. When the Andes began to form, it was cut off from the Pacific Ocean, and the Amazon became a giant lake. Eventually it reached the Atlantic Ocean to the east, and the present-day Amazon River took shape. Even today, the flow sometimes reverses at high tide. Called the pororoca, this phenomenon can form a surge up to 4 meters high that travels about 800 kilometers upstream from the river's mouth.

There is also a theory that another enormous river flows beneath the Amazon. A slow-moving body of groundwater 4 kilometers below the surface is said to run roughly parallel to the Amazon River, and although it is disputed, it is sometimes called the Hamza River.

The Amazon River is home to a fish whose actual behavior doesn't match its popular image: the piranha. Contrary to its ferocious image, the video explains, it is actually a timid fish. That image is said to have originated with an episode involving former U.S. President Theodore Roosevelt. To impress Roosevelt as he traveled through Brazil, local people set up a stretch of water holding a school of piranhas that hadn't been fed, then threw a cow into it. The cow was reduced to bones in no time, and the piranha's ferocious image spread to the public.

In reality, piranhas are omnivores that eat not only other fish and insects but also fruits and seeds, and they are known to prefer carrion over live prey. It isn't impossible for them to attack people or livestock, but in nature it almost never happens. Rather, piranhas, with their taste for carrion, play an important role as the river's cleaners. The tropical Amazon River is warm, and dead animals rot quickly and can spread infections to other fish. The video says it might even be possible to see the piranhas, by cleaning these up first, as protecting the ecosystem of the entire river.

The Amazon river dolphin, known for its pink skin, is called the boto in the region, where legend has it that it transforms into a handsome young man at night to seduce women, then returns to the river as a dolphin in the morning. The main reason for its pink color is that it has less subcutaneous fat than other dolphins, so its blood vessels run just beneath the skin, and the pink becomes especially noticeable when the water is warm. Unlike other dolphins, which need to maintain their body temperature in cold oceans, this dolphin, which lives in the tropics, apparently has no need for a thick layer of blubber.

It also has a body structure adapted to flooded forests. To move between tangled branches, the vertebrae in its neck are not fused, allowing it to bend its neck 90 degrees. It can swim sideways, backward, and even upside down, and can maneuver easily in tight spaces.

It is also known for using tools. In a 2008 study, dolphins were observed holding branches, grass, or lumps of clay in their mouths and slapping them against the surface of the water. This behavior was performed almost exclusively by males, peaked in the lead-up to the females' breeding season, and the males that displayed it most often fathered more offspring. In other words, it was a courtship behavior, or a way of showing off their appeal. It is said to be extremely rare for dolphins and whales to use tools in this way.

The Amazon River is also home to the anaconda, the heaviest snake in the world. The largest recorded individuals have reached 6.3 meters in length and 200 kilograms in weight. Anacondas are not venomous; they hunt by constricting prey with their long bodies, targeting capybaras, deer, caimans, large fish, and more. There have been several incidents in which human children were attacked, but adults are apparently rarely attacked. Females are overwhelmingly larger than males, and during the breeding season, as many as 12 or so males may cluster around a single female, forming a ball. Anacondas use a method that is uncommon among reptiles, hatching their eggs inside the body before giving birth, and the young are several dozen centimeters long at birth and can swim and hunt right away.

The caiman, a type of crocodilian, can reach 6.5 meters in length and 450 kilograms in weight in the largest individuals. As with other crocodilians, the sex of a caiman is determined by the temperature at which its egg incubates, raising concerns about the effects of climate change.

Caimans have a tragic history. In the mid-20th century, they were driven to the brink of extinction by overhunting for their skins, and in some regions, populations fell by 99% between 1950 and 1970. When caiman numbers plummeted, the capybaras and piranhas they had preyed on exploded in number, greatly increasing damage to crops and other fish. Even in small numbers, caimans were a keystone species with a major influence on the ecosystem. Helped in part by the large number of eggs they lay, their populations have now recovered, but the video says something similar could happen again at any time.

When people hear "otter," many may picture the small Asian small-clawed otter, but the one in the Amazon is the giant otter, the largest member of the weasel family. It can grow up to 2 meters long and mainly catches and eats fish. Giant otters live in family-based groups of four to eight, hunting together, defending their territory, and raising their young as a group. Contrary to what their appearance might suggest, they have a highly social way of life.

They are also notable for their rich range of vocalizations. A 2014 study found that adults have 22 types of calls and young otters have 11, and that each family uses distinct calls. Locally, they are reportedly called "lobo del río," the river wolf. But their numbers have plummeted, and fewer than 5,000 are believed to remain in the wild. They are on the endangered species list, and protection of their habitat is needed.

Finally, the video turns to humans. Around 30 million people are estimated to live across the Amazon, including tribes with as many as 300 different languages. But the full picture may never be known, because there are many uncontacted tribes that refuse contact with the outside world. In Brazil alone there are 124 uncontacted tribes, and although their numbers aren't known, they are thought to total around 10,000 people worldwide.

The video stresses that these people have neither been left behind by civilization nor possess only primitive intelligence. They are people who have chosen, of their own accord, to refuse contact with civilized society, and in many cases they once did have interactions with outsiders. The reality, the video says, is that the results were so bad that those interactions ceased.

Beyond direct violence, the impact of epidemics was enormous. It is known that between 1875 and 2008, 117 epidemics affected 59 tribes. Most of them were diseases such as measles, influenza, and malaria, to which many people have some degree of immunity. But for many Amazonian tribes, these were unknown diseases against which they had no immunity, and the fatality rate was extremely high, at around 25%; at least 11,000 people are estimated to have died. For this reason, organizations responsible for protecting and supporting uncontacted tribes today take the position of prohibiting attempts at contact. Some people who left formerly uncontacted tribes reportedly advise uncontacted people they meet not to go to civilized society.

As a figure who symbolizes all this, the video introduces a person known as the "Man of the Hole." He was the last survivor of a tribe about which little is known, and the whereabouts of its other members are unknown. From the 1990s until 2022, he lived alone in the Amazon rainforest, and he got his name because he often dug deep holes. Protection organizations made no attempt to contact him and watched over him from a distance.

In 2022, he was found dead in a hammock outside his hut. He was estimated to be 60 years old, and the cause of death was believed to be illness. With his death, his tribe's culture, language, and history were lost. Depending on how you look at it, it might seem that he lived out his natural life, but according to the video, although the details are unknown, it has been suggested that his tribe may have met the worst possible fate after being treated as an obstacle by illegal ranchers trying to expand their land for pasture.

That danger has not gone away. In recent years, gold mining has boomed in Brazil, and illegal miners are clearing the rainforest and causing mercury pollution in the Amazon River. As a result, the tribes living there are losing both their homes and their water sources. The government has taken countermeasures, but the video says the current situation is nothing more than an endless game of cat and mouse.

The video closes with the words that the Amazon rainforest cannot be fully described by its vast jungle, jaguars, and piranhas alone; etched into it, too, are traces of human life that has continued there for tens of thousands of years.