Sloth versus Howler Monkey
The rainforests of Central and South America are home to some of the most fascinating creatures on Earth, including sloths and howler monkeys. At first glance, these two animals might seem worlds apart, although some people often assume sloths are a kind of monkey! One is known for its slow, solitary lifestyle, and the other for its loud, social nature. Yet, sloths and howler monkeys have more in common than it may seem.

Both animals have adapted to life high in the canopy, surviving on a leaf-based diet and facing the unique challenges of their environment. By understanding their similarities and differences, we can better appreciate the role each plays in their shared ecosystem and the importance of protecting their habitat.
What’s on the Menu? Leaves!
Unsurprisingly, given that howler monkeys and sloths both share the rainforests of Central and South America, they also share a common dietary trait: they are both folivores, meaning their diet consists primarily of leaves. This diet, while abundant in their rainforest habitat, is nutrient-poor and requires specialized adaptations for digestion and energy use. Sloths are a low-energy species, with a famously slow metabolism that allows them to conserve energy while their stomachs slowly digest plant material over several days.

Similarly, howler monkeys have a slower metabolism compared to other primates, which helps them process their leafy meals efficiently, though they supplement their diet with fruit and flowers. For both howler monkeys and sloths, these dietary strategies influence the energy levels and activity patterns of both species, showcasing their fascinating adaptation to a foliage-based lifestyle.
The Art of Doing (Almost) Nothing
When it comes to activity and behavior, sloths and howler monkeys are known for conserving energy, however their daily routines differ. Sloths are famously lethargic; only moving when necessary, but they sleep around 8 to 10 hours a day. Their energy-saving lifestyle is essential for survival, as their folivorous diet provides limited calories.

Howler monkeys are slightly more active. However, they are also known to be among the “laziest” primates since they often spend their time lounging around in treetops. They are considered diurnal and known to move around and socialize, but for only 45 minutes each day. They balance long periods of rest with brief bursts of activity for foraging, social interactions, or vocalizing.
Loner vs Social
Sloths and howler monkeys differ greatly in their social structures. Sloths are not social animals; they are primarily solitary, with limited interaction except during mating or when mothers care for their young. Sloths rarely interact with other species, even when their home ranges overlap. However, sloths do tolerate other animals nearby. During our fieldwork, we’ve often seen them sharing the same tree with other sloths, monkeys, porcupines, iguanas, squirrels, and more.
Howler monkeys, on the other hand, are highly social and live in groups of six to fifteen animals, usually consisting of one alpha male, females, juveniles, and offspring. When a young male reaches adulthood, he typically leaves the troop to join a new one or start his own. Living in groups provides howler monkeys with protection from predators, opportunities for grooming, and the ability to defend their territory collectively.
Life in the Treetops
Both sloths and howler monkeys are perfectly equipped for an arboreal lifestyle. Living high in the rainforest canopy demands specialized adaptations, like prehensile tails or strong claws.
Howler monkeys have strong limbs and a powerful prehensile tail, which means their tail can grip and act as a fifth limb. Their tail allows them to grip branches securely and move with ease through the trees. The underside of their tail even features a soft, sensitive pad, like the fleshy pads on the limbs of two-fingered sloths, which helps them navigate their environment with precision.

In contrast, sloths rely on their curved claws and strong limbs to navigate through the branches, moving slowly and deliberately to conserve energy and detection. While three-fingered sloths have a short vestigial tail that helps with balance when climbing, it is not as versatile (or long!) as a howler monkey.
Communication in the Rainforest
Sloths and howler monkeys have very different styles of communication. While sloths are mostly silent, howler monkeys are famous for their loud, unique howls. They are considered the loudest of all monkeys, and among the loudest animals. Male howler monkeys are known to address each other in the morning, with what is considered a guttural sound like a lion’s roar. Their howls can carry as far as 4km (3 miles), even in the densest of jungles. They tend to call out at dusk as well as a warning if a trespasser gets too close.
Their strong vocal calls are produced by their overly large hyoid bone in their throat. Females also have loud howls and groans that usually signal distress or call their stray infant.
Sloths are quiet stealthy animals; they mainly rely on their camouflage to protect them from predators. Female three-fingered sloths are known to make a high-pitched scream when receptive and looking for a mate as well as all three-fingered sloths make a high-pitched squeal if in distress.
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When threatened, they use their claws and teeth as defense and they can also make a deep growl to help scare off predators. Two-fingered sloths make a similar growl when angry or distressed, but otherwise make no vocalisations.
Howls and Fur, How to Identify a Male
In all three-fingered sloth species, with the exception of maned sloths (B. crinitus and B. torquatus), adult males can be identified by a bright yellow or orange patch of shorter fur on their backs. This patch, called a speculum, is likely linked to sexual selection.
Male howler monkeys are also easy to identify, thanks to their distinctive white testicles. Interestingly, there is a correlation between the size of their testicles and the volume of their howl: the louder and deeper the howl, the smaller the testicles.
In the end, it seems to be an evolutionary trade-off to maximize reproduction: large gonads to produce more sperm, or a large vocal-tract hyoid bone to produce a deep call that intimidates competitors and attracts females. Either way, it seems to get the job done!
Conservation Challenges
Both sloths and howler monkeys face many of the same conservation challenges, often where they live near humans. Both species are highly visible and frequently encounter human infrastructure, leading to issues such as electrocution on power lines, vehicle collisions, and habitat fragmentation. Efforts to mitigate these dangers include canopy bridges, initially designed for sloths. These Sloth Crossing canopy bridges have also proven beneficial for howler monkeys, who frequently use them to navigate fragmented forests.

Protecting these animals requires addressing their shared challenges, from habitat loss to conflicts with human infrastructure and urbanization, highlighting the importance of conservation initiatives that benefit multiple species living in the same ecosystem.
It is important to appreciate their unique behavior and understand their ecological importance so we can better support initiatives to protect not only the species, but the delicate rainforest ecosystem they call home. Conserving their habitat ensures future generations can admire, learn, and understand the slow, quiet nature of sloths and the loud booming calls of howler monkeys in the wild.
-Darlene Buibas
Writer & Collaborator
References
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