Spot a bright orange mark on a sloth's upper back and your first thought might be that the animal is hurt. It is an easy mistake to make. On an adult male three-fingered sloth, however, that striking patch is usually a perfectly natural feature called a speculum.
The marking is one of the rainforest's loveliest puzzles. Sloths survive partly by being difficult to notice, yet these males carry a yellow or orange patch crossed by a dark stripe. A 2026 study has now measured the feature in wild brown-throated sloths and offered new clues about what it might mean. The honest answer is still not settled, which makes the science even more interesting.
What does a sloth speculum look like?
The speculum sits in the middle of the back, between the shoulders. It appears when male three-fingered sloths mature and can range from pale yellow to deep orange, often with a brown or black stripe through the centre. The size, shape and extra dark markings vary from one individual to another.
It is not simply ordinary fur in a different colour. Field observations described by the Sloth Conservation Foundation suggest that the patch lacks much of the long, coarse guard hair found elsewhere on the coat. It is made largely of shorter, denser underfur and appears less hospitable to the algae that can tint the rest of a sloth green.
The feature belongs to male Bradypus, the three-fingered sloth group, with an important exception: maned sloths do not develop it. Two-fingered sloths belong to a separate family and do not have this orange-backed marking either. A World Land Trust species profile also notes the yellow or orange patch with a black stripe as a feature of adult male brown-throated sloths.
Why would a camouflaged animal carry a bright patch?
That is the evolutionary contradiction. Brown-throated sloths move quietly through foliage, their fur can take on a greenish cast, and their slow behaviour helps them avoid attracting attention. A conspicuous patch seems to work against that careful camouflage.
There is another twist. Sloths have rod monochromacy, a form of colour blindness linked to the absence of working cone-based colour vision. An orange badge might be obvious to a person, but not look orange to another sloth. Visually sharp predators may be better equipped to notice the contrast.
Scientists have proposed several explanations. The patch might identify a mature male, advertise something about his body, help individuals recognise one another, carry a useful scent, or combine several kinds of information. It could even be an inherited feature whose original purpose has changed over evolutionary time. Until recently, though, nobody had systematically measured how speculums differ across many wild sloths.
What did the new study find?
The 2026 paper, A spectrum of speculums, analysed photographs and body measurements from 60 wild male brown-throated sloths in Costa Rica. Fifty-seven were adults and three were juveniles.
The clearest result was a relationship between body size and patch size. Larger males had larger speculums. The researchers also found weaker links between body length and the balance of dark and lighter pigmentation. Juveniles tended to have narrower, darker patches with little orange, suggesting that the feature develops as a male matures.
Those patterns are consistent with the idea that the speculum is a signal connected with sexual selection. A larger patch could potentially reveal size, maturity or condition. But consistency is not proof. The study did not show females choosing males because of their patches, nor did it demonstrate that males use the marking to judge rivals.
The scent may matter more than the colour
The speculum is reportedly coated with an oily secretion that has a distinctive musty smell and can leave yellow-orange residue when touched. For an animal with limited colour vision and a useful sense of smell, that raises a fascinating possibility: perhaps the patch works partly as a chemical message.
It might help another sloth recognise a sexually mature male, assess reproductive status or identify an individual. The visual contrast may still matter as an achromatic pattern, even without colour perception, so scent and sight do not have to be rival explanations. The researchers describe this as possible multimodal communication, where one feature carries information through more than one sense.
Testing that idea will require analysing the chemistry of the secretion and carefully observing how wild sloths respond to it. Researchers also need to discover whether the scent changes with hormones, health, age or breeding condition.
A normal feature, not a wound
The most useful takeaway for everyday sloth lovers is simple. A tidy orange or yellow patch with a dark central stripe on the upper back of a male three-fingered sloth is normal anatomy. It is not automatically a burn, a machete injury or missing skin.
Real wildlife injuries do happen, of course, and should be assessed by trained local professionals rather than handled by visitors. But recognising the speculum can prevent an alarming natural feature from being mistaken for trauma.
The deeper lesson is that familiar animals can still hold unanswered questions. We know the patch changes as males grow and that its size tracks body size. We have good reasons to investigate both sexual and scent signalling. We do not yet know exactly who receives the message or what it says. Somewhere in the canopy, male sloths are carrying a clue that science has only just begun to read.
