Sloths of Arenal Volcano: 7 Facts Only Experts Know (+ Where to See Them)

A sloth moves so slowly that algae — living, photosynthetic algae — grows on its fur.

That single fact contains an entire ecological story. Trichophilus welckeri, a green algae species found exclusively on sloth hair, gives the animal a greenish tint that functions as camouflage in the canopy. But the relationship goes further: the fur also hosts moths, beetles, and fungi in a miniature ecosystem riding through the forest on a mammal that sleeps up to 18 hours a day. Each sloth, in a very real sense, is a living habitat.

For most visitors arriving to Arenal, sloths are the animal they most want to see — and often the one they understand least. This field guide, written from years of guiding and wildlife observation in La Fortuna, covers the biology, ecology, and field identification of both sloth species found in our region, along with the practical knowledge needed to find, observe, and interpret them responsibly.

1. Two Species: Knowing Who You're Looking At

Costa Rica is home to two sloth species, both present in the Arenal lowlands:

Bradypus variegatus — Brown-throated Three-toed Sloth

The more commonly encountered of the two in our area. Bradypus is unmistakable in life: a rounded head with a flattened face, three long curved claws on each limb, and a distinctive facial pattern — a pale face bordered by dark patches around the eyes, giving it a permanently sleepy expression. Adult males develop a striking specular patch on the back, a yellow-orange area with a dark central stripe, used in studies for individual identification.

Key measurements: 42–80 cm body length; 3.5–4.5 kg. Tail present (short). Nine cervical vertebrae — uniquely among mammals, Bradypus has between 8 and 10 cervical vertebrae (most mammals have exactly 7), allowing it to rotate its head up to 270°.

Diet: Highly selective folivore. Feeds primarily on leaves of Cecropia (Cecropia spp.), Ficus, and Inga species. A single individual may use up to 40 different tree species but shows strong preference for a core of 5–8 species in its home range. Leaves are digested in a multi-chambered, slow-acting stomach — digestion can take up to a month for a single meal.

Choloepus hoffmanni — Hoffmann's Two-toed Sloth

Less frequently observed than Bradypus but reliably present in the forest patches around La Fortuna. Choloepus has two claws on the forelimbs (three on the hindlimbs), a more pig-like snout, and lacks the tail. Its face is broader, its fur coarser and often lighter — cream to pale brown — and it tends to occupy the mid-canopy and understory more frequently than Bradypus.

Key measurements: 54–74 cm body length; 4–8 kg — notably heavier than Bradypus. The larger body mass correlates with its more varied diet.

Diet: More generalist than Bradypus. In addition to leaves, Choloepus consumes fruits, flowers, bark, and occasionally invertebrates. This dietary flexibility may explain its success in secondary forests and forest edges around human settlements.

2. The Physiology of Extreme Slow

The sloth's slowness is not laziness — it is a sophisticated energy strategy refined over approximately 60 million years of evolution.

Metabolic Rate and Thermoregulation

Both sloth genera have a basal metabolic rate approximately 40–74% lower than predicted for a mammal of their body size (McNab, 1978; Gillooly et al., 2001). This extreme metabolic reduction is functionally linked to their low-quality, high-toxin diet: leaves are nutrient-poor and contain secondary compounds (tannins, alkaloids) that require costly detoxification. By operating at minimal metabolic cost, sloths extract sufficient energy from food that would be inadequate for most mammals.

Unlike true hibernators, sloths do not enter dormancy — they remain active year-round. However, they are functionally heterothermic: body temperature fluctuates with ambient temperature to a degree unusual among non-hibernating mammals, ranging from 24°C to 35°C in Bradypus (Pauli et al., 2014). On cool, cloudy mornings in Arenal — common during the rainy season — you may observe a sloth descending to a sun-exposed branch specifically to thermoregulate. This is not aimless movement; it is a calculated thermal strategy.

The Descent to the Ground

One of the most energetically costly and ecologically significant behaviors in sloth biology is the weekly descent to defecate. Bradypus descends to the forest floor approximately once every 7–10 days, digs a small hole at the base of its preferred tree, deposits feces and urine, and buries them before climbing back up. The entire process takes 20–30 minutes and consumes roughly 8% of the animal's daily energy budget — disproportionately costly for an animal with such limited energy reserves.

Why descend at all? Multiple hypotheses exist. The most supported: the descent facilitates nutrient cycling by depositing concentrated organic matter directly at the root base of a preferred tree, functioning as a form of reciprocal mutualism with the tree itself (Pauli et al., 2014). A secondary hypothesis involves the moth community living in sloth fur — moths use the ground descent to lay eggs in the feces, completing their life cycle. Both mechanisms may operate simultaneously.

This descent is also the moment of maximum predation risk. Eagles (Harpia harpyja, Morphnus guianensis) and large felids (Panthera onca, Puma concolor) take the majority of sloth predation events during or immediately after ground contact. Awareness of this risk may explain the nocturnal timing of Choloepus descents.

3. The Living Ecosystem: Sloth Fur as Habitat

The fur of Bradypus is uniquely grooved — each hair shaft has transverse cracks running along its length, microscopic channels that collect moisture and support the growth of algae, cyanobacteria, and fungi. The most studied resident is Trichophilus welckeri, a green algae found in no other habitat on Earth.

The ecological value of this algae is debated. The camouflage hypothesis is well-supported visually — a Bradypus with full algal coverage in a Cecropia canopy is remarkably cryptic. A more recent hypothesis (Pauli et al., 2014) proposed that sloths may actively ingest algae from their fur while grooming, deriving lipid nutrition from a source invisible to other animals — an internal food supply attached to the body. Isotopic analysis of the study, while suggestive, remains contested.

The arthropod community associated with sloth fur has been sampled and documented. A single individual of Bradypus can harbor up to 980 individual moths (primarily Cryptoses spp.), multiple species of pyralid moths, mites, ticks, beetles, and cockroaches. This community represents a stable microhabitat with consistent temperature, humidity, and food sources (algae, skin cells, exudates) — an ecosystem maintained by the sloth's slow movement and infrequent disturbance.

4. Reproduction, Dispersal, and Home Range

Breeding in Bradypus is not strictly seasonal in Costa Rica, though births peak in the dry season (December–April), allowing juveniles to develop during periods of maximum leaf availability. Single offspring per year is the rule; twins are exceedingly rare and associated with high juvenile mortality. The young clings to the mother's ventral surface for 6–9 months and remains in the natal home range for up to 2–3 years.

Home range in Bradypus is small — typically 0.5 to 2.0 ha — but varies significantly by habitat quality and food availability. In secondary forests with abundant Cecropia, home ranges compress; in degraded habitat, they expand as animals must travel farther between food trees.

Dispersal is primarily female-philopatric: juvenile females tend to establish territories adjacent to or overlapping with the mother's range, while juvenile males disperse farther. This pattern has implications for population connectivity in fragmented landscapes like the Arenal corridor, where isolated forest patches may not sustain viable long-term populations without connectivity to larger forest blocks.

5. Conservation Status and Threats in the Arenal Region

Both Bradypus variegatus and Choloepus hoffmanni are currently listed as Least Concern on the IUCN Red List, but regional trends and specific threat pressures warrant local attention.

Habitat fragmentation is the primary long-term threat in the Arenal lowlands. Sloths are poor dispersers across open terrain — they avoid descending into pasture or crossing roads. Electrocution on uninsulated power lines is documented as a significant source of mortality in La Fortuna specifically; organizations such as Proyecto Asis have recorded dozens of cases annually in the greater Fortuna area.

Handling by tourists and operators remains an unregulated pressure. Sloths that are repeatedly removed from their natural substrate for photographs experience chronic physiological stress (elevated cortisol, disrupted thermoregulation, predation risk on the ground) that is not visible in the moment of the interaction. A sloth that appears calm in a human's hands is not expressing comfort — it is expressing tonic immobility, a passive fear response common across prey species.

Respiratory infections and internal parasites increase significantly in animals subjected to repeated ground-level contact with humans.

The appropriate observation protocol — from a distance of at least 10 meters, without touching, without flash photography, without forest sounds played to attract or hold the animal — is not only ethical but produces the most authentic behavioral observations.

6. How to Find and Observe Sloths in La Fortuna — Field Notes from NWE

After hundreds of hours in the forests around Arenal, these are the patterns that most reliably lead to quality sloth encounters:

Two-toed sloths in active foraging mode move with surprising speed relative to their reputation; a Choloepus descending a liana to reach a new feeding tree is genuinely impressive.

Habitat cues: Focus attention on Cecropia stands — a clump of Cecropia at forest edge with visible scratch marks on the trunk, or sparse leaf damage in the mid-canopy, is a strong indicator of Bradypus use. Scan with binoculars from a distance rather than approaching the base; sloths at height are best located by the silhouette of a rounded shape against the sky.

At descent time: If you locate an individual repeatedly in the same tree, the weekly ground descent is predictable. The animal typically descends to the same root zone of the same tree. Observing this behavior from a respectful distance — without intervention — is one of the most remarkable field experiences in Neotropical wildlife watching.

What you're looking for: Not the whole animal. A single curved claw around a branch; a rounded shape where branches diverge; a patch of greenish-brown fur against bark. Sloth detection is a skill, not luck — it improves with hours in the field and with learning to read the canopy rather than scan it.

7. Summary: Two Species, One Forest, Infinite Complexity

	Bradypus variegatus	Choloepus hoffmanni
Common name	Three-toed sloth	Two-toed sloth
Forelimb claws	3	2
Activity	Diurnal	Nocturnal
Diet	Specialist folivore	Generalist
Tail	Present (short)	Absent
Cervical vertebrae	8–10	6–7
Fur color	Brown-gray + algal green	Cream to pale brown
IUCN status	Least Concern	Least Concern

The sloths of Arenal are not mere backdrop to the forest — they are active agents in nutrient cycling, arthropod community structure, and the phenology of their preferred tree species. Every encounter with a sloth in the wild, observed correctly, is a window into one of the most elegantly adapted life strategies in the Neotropics.

Bibliography

1. McNab, B.K. (1978). Energetics of arboreal folivores: physiological problems and ecological consequences of feeding on an ubiquitous food supply. In The Ecology of Arboreal Folivores (ed. G.G. Montgomery), pp. 153–162. Smithsonian Institution Press.

2. Gillooly, J.F., Brown, J.H., West, G.B., Savage, V.M., & Charnov, E.L. (2001). Effects of size and temperature on metabolic rate. Science, 293(5538), 2248–2251.

3. Pauli, J.N., Mendoza, J.E., Steffan, S.A., Carey, C.C., Weimer, P.J., & Peery, M.Z. (2014). A syndrome of mutualism reinforces the lifestyle of a sloth. Proceedings of the Royal Society B, 281(1778), 20133006.

4. Sunquist, M.E., & Montgomery, G.G. (1973). Activity patterns and rates of movement of two-toed and three-toed sloths (Choloepus hoffmanni and Bradypus infuscatus). Journal of Mammalogy, 54(4), 946–954.

5. Hayssen, V. (2010). Bradypus variegatus (Pilosa: Bradypodidae). Mammalian Species, 42(1), 19–32.

6. Cliffe, R.N., Hauber, M.E., & Avery, R.A. (2015). The spatial ecology of the brown-throated three-toed sloth (Bradypus variegatus): home range, activity patterns and habitat preferences. Journal of Zoology, 295(1), 46–55.

7. Voirin, B., Kays, R., Lowman, M., & Wikelski, M. (2009). Evidence for three-toed sloth (Bradypus variegatus) predation by spectacled owl (Pulsatrix perspicillata). Edentata, 8–10, 15–16.

8. Peery, M.Z., & Pauli, J.N. (2014). Shade-grown cacao supports a self-sustaining population of two-toed but not three-toed sloths. Journal of Applied Ecology, 51(1), 162–170.

9. Lara-Ruiz, P., & Chiarello, A.G. (2005). Life-history traits and sexual dimorphism of the Atlantic Forest maned sloth Bradypus torquatus. Journal of Zoology, 267(1), 63–73.

10. Montgomery, G.G., & Sunquist, M.E. (1978). Habitat selection and use by two-toed and three-toed sloths. In The Ecology of Arboreal Folivores (ed. G.G. Montgomery), pp. 329–359. Smithsonian Institution Press.

Want to Observe Sloths in Their Natural Habitat?

At Neotropic Wildlife Expeditions, our day and night tours in La Fortuna give you the opportunity to encounter both sloth species in the wild — observed correctly, at distance, with the ecological context that transforms a sighting into a real understanding of the forest.

Bryan guides private groups with a focus on natural history interpretation, field identification, and responsible wildlife observation. No animals are handled or removed from their substrate.

→ Book your tour with NWE

Tours available year-round. Night walks depart weekly. Private and small group options available.

© Neotropic Wildlife Expeditions · La Fortuna de San Carlos, Costa Rica · All original photographs by Bryan Ramírez Castro.

Sloths in the Arenal region aren’t just slow — they’re perfectly adapted survivors, thriving in the canopy where energy conservation and camouflage are key. Around Fortuna, Alajuela, you can observe both two-toed and three-toed sloths in their natural habitat, often hanging motionless yet deeply connected to the forest’s rhythm. If you want a guided experience that reveals their biology, behavior, and ecological role, message us on WhatsApp: https://wa.me/message/N2EQJK4Q7RXHH1 and let’s find these gentle icons of Costa Rica’s wild side — quietly, respectfully, and with purpose.