Flying lemurs are neither lemurs nor capable of true flight, making them one of the most misleadingly named mammals on Earth. Properly called colugos, these cat-sized animals belong to the order Dermoptera and are found only in the tropical forests of Southeast Asia. They are the most accomplished gliding mammals alive, stretching a broad membrane of skin between their limbs and even their fingers and toes to sail distances of over a hundred meters between trees. What makes them especially interesting to biologists, though, is not just the gliding but their evolutionary position: colugos are the closest living relatives of primates, sitting right next to our own branch on the mammalian family tree.
The Name Problem
The common name “flying lemur” has confused people for centuries. Lemurs are primates found only in Madagascar. Colugos are not primates and live thousands of kilometers away, in the rainforests of the Philippines, Malaysia, Indonesia, Thailand, and surrounding islands. The name seems to have stuck because early European naturalists noticed some superficial resemblance to lemurs in the colugos’ large eyes and nocturnal habits. The “flying” part is equally wrong: colugos cannot generate lift the way bats or birds do. They are gliders, launching themselves from high points and descending along a curved path to land on a lower trunk. Still, the name persists in field guides and casual usage, so researchers generally refer to the animals as colugos to avoid confusion.
How Colugos Glide
The membrane that allows colugos to glide is called a patagium, and it is the most extensive of any gliding mammal. It runs from the neck to the tips of the fingers, continues between the fingers, stretches along the sides of the body to the toes, and then connects between the toes all the way to the tail. When a colugo leaps from a tree and extends its limbs, the patagium unfurls into something resembling a large, furry kite. Other gliding mammals, such as flying squirrels and sugar gliders, have patagia too, but none cover as much of the body surface as a colugo’s. This gives colugos exceptional glide ratios, meaning they lose relatively little altitude for the horizontal distance they cover.
Glide paths are not straight lines. As a colugo moves through the air, the aerodynamic forces on its body change, producing a curved trajectory. The animal can adjust its posture and limb position to modulate lift and drag, giving it a surprising degree of control over its flight path. Research on gliding animals broadly has shown that gliders share no single anatomical or behavioral feature; some species are relatively unspecialized, producing aerodynamic forces mainly through posture and body orientation.1Canadian Journal of Zoology. How animals glide: from trajectory to morphology Colugos, by contrast, represent one of the most anatomically specialized endpoints of this spectrum, with elongated limb bones and webbed digits devoted almost entirely to maximizing their gliding surface.
But there is an important catch: gliding saves time, not energy. A study tracking Malayan colugos found that while gliding lets them cross horizontal distances much faster than if they had to climb down one tree and up another, the total energy expenditure is actually higher than you might expect. The reason is the cost of climbing. Before a colugo can glide, it has to haul itself up the trunk of a tall tree, and climbing with that big membrane draped around the body is hard work. So per unit of horizontal distance, gliding turns out to be more energetically expensive than simply traveling through the canopy would be for a comparably sized animal that could walk or leap between branches.2PubMed. Gliding saves time but not energy in Malayan colugos The payoff is speed and the ability to cross large gaps between trees, not caloric efficiency.
Life on the Trunk
On a tree trunk, colugos look nothing like the graceful gliders they are in the air. They cling to bark with sharp, curved claws, their limbs splayed outward, the patagium hanging loosely around them like an oversized poncho. Moving upward is slow and effortful. On the ground, they are even more awkward, essentially helpless, shuffling along in a way that would make them easy prey. This extreme specialization for an arboreal gliding lifestyle means colugos are almost entirely dependent on tall, continuous forest. They cannot cross open ground or survive in fragmented habitats where the canopy gaps exceed their glide range.
Colugos are nocturnal and spend their days resting in tree hollows, pressed flat against trunks, or curled inside their patagium like a hanging pouch. Mothers carry their young clinging to their bellies, and the patagium doubles as a kind of hammock for the infant. Their diet consists of young leaves, flowers, buds, and sap, which they access by climbing slowly to the outer branches of trees. They are folivores with a relatively low-energy diet, which may partly explain why they invest so much of their anatomy in the ability to move efficiently over long distances between food sources without descending to the forest floor.
The Strangest Teeth in the Mammal World
If colugos were only remarkable for gliding, they would still merit attention. But their teeth are genuinely unlike anything else in mammals. The lower front incisors are modified into flat, comb-like structures, each tooth bearing as many as fifteen tiny prongs, or tines, arranged side by side. This gives them what biologists call a dental comb, though it is fundamentally different from the tooth combs seen in certain primates like lemurs and lorises.3PubMed Central. Function of the mandibular tooth comb in living and extinct mammals
For a long time, people assumed the colugo tooth comb served the same purpose as a lemur’s: grooming fur. It seemed intuitive. After all, the teeth look like tiny combs, and the animals have fur. But research into the functional morphology of these teeth has challenged that assumption. The evidence suggests that the pectinate incisors of colugos probably do not function primarily as fur combs.4PubMed Central. Function of the mandibular tooth comb in living and extinct mammals Instead, the teeth may play a role in scraping or straining food, helping to process leaves or extract nutrients from plant material. The precise function remains debated, which is itself a reminder of how little we know about colugo biology despite decades of study.
Two Known Species, Possibly More
Traditionally, colugos have been split into just two species. The Philippine flying lemur, Cynocephalus volans, lives in the Philippines, while the Sunda flying lemur, Galeopterus variegatus, ranges across mainland Southeast Asia and the large islands of Borneo, Java, and Sumatra. These two are the only living members of the entire order Dermoptera. Despite looking broadly similar, their skull and dental anatomy differ enough that some taxonomists argue the two should be placed in separate genera, not just separate species. The level of morphological distinction between them is dramatic enough that if they were known only from fossils, few experts would question their placement in different genera.5Journal of Mammalogy. Craniodental Functional Morphology and Taxonomy of Dermopterans
But the story does not end with two species. Genetic analysis of Sunda colugos across their range has revealed deep divergences within what was long treated as a single species. Populations on Java, Borneo, and the Malay Peninsula mainland appear to have separated from each other millions of years ago. Using the roughly 20-million-year-old split between Philippine and Sunda colugos as a reference point, researchers estimated that Javan colugos diverged from mainland populations around four to five million years ago, and Bornean colugos split off at a similar time.6Current Biology. Evidence for multiple species of Sunda colugo Even within the Malay Peninsula, populations in Thailand and Singapore show significant genetic separation. Bornean colugos are generally smaller, and mainland Malay colugos tend to have wider skulls relative to skull length compared to those from Java.7Current Biology. Evidence for multiple species of Sunda colugo
These findings suggest that what we call the “Sunda flying lemur” may actually be three or more distinct species. Recognizing that hidden diversity matters for conservation. If a population you thought was part of a widespread, reasonably secure species turns out to be its own species confined to a single island, its conservation status could change overnight from “least concern” to something much more urgent.
Closest Relatives of Primates
The evolutionary position of colugos has been argued over for decades, and for a while it looked like the debate might upend our understanding of primate evolution entirely. One provocative finding, based on mitochondrial genome comparisons, seemed to place colugos inside the primate family tree rather than next to it. Specifically, colugos grouped with anthropoid primates (monkeys, apes, and humans), with prosimians like tarsiers and lemurs branching off earlier. If true, this would have meant that “primates” as traditionally defined were not a natural, unified group at all.
Closer investigation, however, revealed a methodological pitfall. The DNA composition of colugo mitochondria happens to be unusually similar to that of anthropoid primates in ways that reflect shared biochemical tendencies rather than shared ancestry. When researchers turned to a completely different type of evidence, looking at the presence or absence of transposable elements (short segments of DNA that insert themselves into the genome and stay put across generations), the picture cleared up. Certain transposable elements were found in all living primate groups but were absent in colugos, confirming that primates are a unified group and that colugos sit outside of it.8Molecular Biology and Evolution. The Colugo (Cynocephalus variegatus, Dermoptera): The Primates’ Gliding Sister? Colugos are the primates’ closest living relatives, a sister group, but not primates themselves. This makes them uniquely valuable to biologists studying what the common ancestor of primates might have looked like and how it lived.
Genomic Clues to the Evolution of Gliding
Gliding has evolved independently in mammals at least three times: in colugos, in flying squirrels (rodents), and in sugar gliders and related marsupials. This kind of repeated evolution of the same ability in unrelated lineages is called convergent evolution, and it raises a tantalizing question: do different lineages use the same genetic toolkit to arrive at the same solution, or do they get there by completely different molecular routes?
A recent comparative genomics study tackled this by assembling high-quality genomes of a flying squirrel and a sugar glider and comparing them against the Sunda colugo genome and fourteen non-gliding mammal species. The protein-coding genes of the three gliding lineages showed relatively limited convergent changes. Where convergence was much more striking was in the regulatory regions of the genome, the stretches of DNA that do not code for proteins themselves but control when, where, and how much a gene is expressed. Regions that had evolved unusually fast in all three gliding lineages clustered near genes with overlapping functions, and certain hotspot regions showed high divergence yet functional convergence. The researchers identified dozens of key transcription factors potentially involved in multiple aspects of gliding adaptation, including factors linked to limb development and body proportions.9PubMed Central / Science China Life Sciences. Functional convergence of regulatory regions provides vital insights into mammalian gliding adaptation In other words, the three gliding mammal groups do not share the same gene mutations so much as they share changes in the switches that control those genes.
This fits a broader pattern in evolutionary biology: when distantly related animals evolve similar traits, the convergence often shows up not in the genes themselves but in the regulatory machinery surrounding them. For colugos, whose entire body plan is organized around gliding to an extreme degree, understanding these regulatory changes could eventually explain how an entire order of mammals became so thoroughly committed to a single mode of locomotion.
Ancient Parasites and the Colugo Connection
One of the odder footnotes in colugo biology comes from parasitology. Among mammals, the blood parasites responsible for malaria and related diseases (the Plasmodium group and their relatives) infect a surprisingly narrow range of hosts. Mammalian malaria parasites are confined to primates, bats, rodents, artiodactyls, and Dermoptera, the order containing colugos.10Oxford Academic. The palaeogeography of mosquito-borne disease Related parasites called haemoproteids appear in all of those groups plus elephant shrews. This distribution pattern has led researchers to propose that these parasites originated in the early ancestors of the Archonta, the superordinal grouping that historically included primates, tree shrews, colugos, and bats.
The reasoning goes like this: if a parasite infects a specific set of host lineages, it may have been present in the common ancestor of those lineages and co-evolved with them as they diversified. The fact that colugos carry these parasites at all, despite being a tiny, obscure order with just a handful of species, points to the deep antiquity of the colugo lineage and its long intertwined history with the other arboreal mammal groups that eventually gave rise to primates. It is a reminder that colugos are not some marginal evolutionary curiosity but a lineage with roots going back tens of millions of years, carrying biological baggage from a time before primates, bats, and rodents had fully gone their separate ways.
Why Colugos Remain So Poorly Understood
Given their unusual anatomy, evolutionary significance, and ecological specialization, it is striking how little fieldwork has been done on colugos compared to other mammals of similar scientific interest. Several factors contribute to this. They are strictly nocturnal and spend most of their time high in the canopy, making them difficult to observe. They are solitary and quiet, lacking the loud vocalizations that make some nocturnal animals easier to locate. Their cryptic coloring, a mottled gray-brown pattern that blends perfectly with tree bark, means you can be staring right at a resting colugo and not see it.
There is also a practical barrier. The tropical forests of Southeast Asia where colugos live are among the most rapidly disappearing habitats on the planet. Palm oil plantations, logging, and urbanization have fragmented huge swaths of colugo habitat, particularly in Borneo and Sumatra. Because colugos cannot cross open ground or survive in degraded forest, habitat fragmentation effectively isolates populations, making each remnant population smaller and more vulnerable. The potential reclassification of “Sunda colugo” populations into multiple distinct species makes this even more pressing. A species restricted to the remaining forests of Borneo, already losing canopy at an alarming rate, faces a fundamentally different conservation calculus than a single widespread species distributed across all of Southeast Asia. For an animal that cannot walk on the ground, forest continuity is not a luxury but a prerequisite for survival.