Screeches, yowls and the hum of cicadas echo alongside yelps, squawks, splashes and drips. It’s first light in the Peruvian Amazon and my ears are enlivened with the joyous din of nocturnal symphonies jostling with the waking dawn chorus.
Then, I hear it: a long growl, like an approaching storm. I peer into the towering canopy above to try and decipher any rain clouds in the dark mist.
But biologist Jackeline Mendoza tells me the eerie sound is in fact the territorial warning call of the red howler monkey. I can barely believe that such a loud, resonant sound could be created by an animal.
- What are the smallest monkeys and primates in the world? Meet the tiny creatures of the monkey world
It reverberates as though emanating from at least half the sky, rolling and croaking in formidable undulations. In its rumbling roar, it swallows the already explosive chorus of cicadas and the cacophony of hisses, ribbits and chirps from countless other creatures hiding in the rainforest.
Smiling with encouragement, Jackeline nods at me to point my parabolic microphone in the direction of the noise. It’s a clever, rather futuristic piece of kit, comprising an acrylic, dish-shaped reflector that focuses soundwaves on to a small microphone, acting like an audio ‘zoom lens’ to capture faraway sounds while filtering out background noise.
As I listen through the headphones, the disc amplifies the howls and draws in soundwaves that cascade across the recording screen. It’s an utterly captivating way to experience the rainforest.
Jungle jam
I am making these recordings in support of an initiative called 8Primates, based at the Tambopata Research Centre, deep in the National Reserve of the same name. Founded in 2016 as part of a collection of citizen science projects known as Wired Amazon, it’s spearheaded by Mark Bowler, a spatial ecologist from the University of Suffolk.
The project isn’t only about how monkeys communicate. Rather, the idea is to use bioacoustic recordings, blended with behavioural studies, to track and monitor the distribution of the eight primates that call this region home.
“It’s about surveying populations as efficiently as possible, covering as much ground as we can,” says Mark.
- Do monkeys get drunk?
- "It looks like Gollum from The Lord of the Rings." The weirdest, most peculiar (and most wonderful) primates on the planet
Subsistence hunting by communities is an ongoing pressure in Tambopata, and habitats are becoming fragmented as a result of illegal gold-mining and logging. There’s plenty that these monkeys need protecting from, and “studying their distribution can tell us a lot about the state of conservation in the forest,” says Mark.
“Safeguarding the Amazon is essential to mitigate climate change and biodiversity loss, so we are trying to create something that can be widely replicated. We’re starting in the buffer zones of protected areas with good levels of ecotourism, where we can make the largest gains for our efforts.”
The idea of Wired Amazon was dreamed up by a collective of researchers, including Mark, and tour company Rainforest Expeditions, and the name refers to the project’s goal of using cutting-edge technology to ‘plug’ the rainforest into the wider scientific world.
Rainforest Expeditions manages three lodges in the reserve, and guests and locals can participate in the science, their visits partly funding the project. In tandem with the roving recordings, the project also utilises 150 static microphones, positioned strategically throughout the forest.
These microphones record continuously, capturing thousands of hours of sound as wildlife passes through. AI algorithms, trained using the roving data, then analyse the static data. All this enables the project to correlate species distribution with human activities, offering valuable insight into the impact of disturbance.
“Tambopata Reserve is more than 270,000ha in size but there are only 50 rangers working here,” says Jackeline, also the field coordinator of 8Primates.
“Citizen science is a simple, inexpensive method of studying primates across the whole reserve – in the first year alone, we involved 360 people. Their input makes a huge difference to what we’re doing.”
Tuning into calls
I tune in to the calls of the red howlers, concentrating hard on what I’m hearing. I’m aiming to isolate six vocalisations associated with their most common behaviours, such as contact, food and alarm calls, plus the calls of juveniles learning how to communicate.
Red howlers can be heard from up to a kilometre away, says Jackeline, and at dawn their calls are frequently picked up on the mics.
“This is the best time to record them as it’s when they are most active,” she says. “They’re out looking for breakfast.”
With their smart, russet coats catching the early sun as they bolt through the canopy, the howlers are arguably the stars of the show, but 8Primates is also monitoring seven other primates.
These are the shock- and large-headed capuchins, the Peruvian squirrel and spider monkeys, Azara’s night monkey, Weddell’s saddleback tamarin monkey and Toppin’s titi monkey. Most belong to habituated wild groups that live along the Tambopata River and are accustomed to the presence of humans.
Howlers are able to vocalise at such magnitude thanks to their specialised hyoid bone, which works like a charm to amplify sound. We humans also have these U-shaped bones in our throats, but ours are tiny in comparison – howler hyoids are box-like and half the size of your palm. It’s this design that produces that storm-like sound, acting as a resonating chamber to amplify low-pitched noises.
The larger the hyoid, the louder the monkey. Given that red howlers howl to protect territory and prevent other males from taking over their groups, it’s a crucial adaptation.
Yet, interestingly, other species of howler, such as the black-and-gold and mantled howler, have adopted alternative strategies for passing on their genes. Some form coalitions with one or more fellow males, teaming up to defend territory. But this means sharing and competing for females.
“They compete by producing more sperm, for which they need bigger testicles – but you can’t have everything,” says Mark.
“So, they compromise by surrendering the size of their hyoid bone and, therefore, the volume of their howls. In short, the howlers with the loudest voices have the smallest testicles.”
- What's the biggest monkey in the world? Clue – it comes equipped with fearsome canine teeth that are amongst the longest of any primate
- Can animals ask humans for help?
A forest full of noise
As we continue our journey through the forest, now in full daylight, a troupe of spider monkeys – one of the largest of the eight primates – engulfs the canopy, moving so quickly through the trees that I barely manage to point my dish at them in time.
As I listen, I attempt to count them. It’s a large group, about 30 or 40-strong, but I’m told the monkeys will splinter into sub-groups of up to 10 later in the day.
“Their most common call is a coo-coo-caw-coo when they are ‘chatting’, but it’s a lower pitch than the howlers, so is more difficult to record,” says Jackeline, delivering a perfect imitation of her subject.
The species’ Quechuan name is maquisapa, which translates as ‘big or long hands’. With limbs and tails clinging to and snapping the vines, it’s easy to see how these lively primates earned such a moniker.
“The length of their arms and legs often look so similar that when they jump, they are like tarantulas moving above you,” says Jackeline.
We move on, heads bowed against several brief but torrential downpours. An 8Primates guide, Joss Silva Gomez, now takes the lead, expertly navigating a narrow, muddy trail. Before joining the team in 2010, Joss was a bushmeat hunter and gold-miner.
He is long accustomed to the lay of the forest and deeply attuned to its sights and sounds, picking out the plaintive, flute-like call of a broad-billed motmot and spotting a tiny poison dart frog complete with tadpoles on its back.
Suddenly, the vegetation morphs into a thick bamboo forest. Joss is just demonstrating how and where the shoots can be chopped to create flask-like receptacles from which the monkeys can drink, when a racket that resembles the braying of a donkey echoes through the air.
It’s a Toppin’s titi monkey making itself known, and that brazen hee-haw call is actually a duet, Jackeline tells me. The female sings the first part, the male the second. The couple sings together and mates for life, and some researchers say the stronger the synchrony of the call, the stronger the bond between the pair, with each partner taking turns to vocalise ever-louder hees and haws.
Though all eight primates are habituated, some are less expressive than others. The most challenging one to record is the shock-headed capuchin, a rare species at Tambopata, whose soft, nasal squeak is not easy to discern – particularly when the jungle’s insects are at their loudest.
In 2024, the 8Primates team only observed this species four times, and only once managed to record its calls.
- Best singers in the animal kingdom - forget birdsong, whales, frogs and monkeys are nature's top vocalists
- It can scale vertical cliffs, shuffles along on its bottom grazing like a cow, and has flowing blond hair and a blazing red chest
Protecting the ‘mother of the jungle’
Leaving the bamboo behind, we soon come across a Brazil nut tree towering over us at more than 30m tall. This tree’s produce, sought after around the world, is a vital resource, generating US$3–8 million annually and supporting more than 30,000 jobs in the Madre de Dios region.
The tree is known by the local Ese Eja people as ‘mother of the jungle’ and logging in the vicinity of Brazil nut trees is illegal.
Brazil nuts are only fertilised when growing in rainforests, meaning the nuts must be harvested in the Amazon rather than on commercial farms. The rights to harvest are owned as concessions by the Ese Eja, some rights going back centuries.
“This is one of the most sustainable uses of the forest, along with responsible tourism,” says Mark. “Comparing the diversity with the surrounding forest and monitoring through time is therefore a really useful thing to do.”
Detecting the presence of chainsaws through audio research is a particularly important way to protect the Brazil nut tree. Noise data gathered by 8Primates informs the Ese Eja about illegal logging, helping them to monitor the habitat.
Heading back towards the research centre, I’m pinching myself. It’s not every day you get the chance to tune into the Amazon and eavesdrop on its iconic primates. It’s exciting that people from so many walks of life are able to come together to safeguard this precious habitat.
“Logging and mining are easy ways to earn money, but we want to offer alternatives and spread our knowledge through communities and tourists,” says Jackeline.
“Conservation doesn’t mean ‘don’t touch the forest’, it means learning and improving. That’s my dream.”
