Dermoptera

On wings of skin
Published 6 October 2022

Among the more distinctive inhabitants of the rainforests of southeast Asia are the flying lemurs or colugos of the Cynocephalidae. These medium-sized browsers have the most extensive gliding membrane of any mammal, extending right to the ends of the toes and tail. As such, they are able to glide distances of over one hundred metres between trees with ease (Macdonald 1984).

Sunda colugo Galeopterus variegatus with offspring, copyright Lip Kee.

Modern flying lemurs are divided between two geographically distinct species. The Philippine colugo Cynocephalus volans is found on islands of the southern Philippines. The Sunda colugo Galeopterus (or Cynocephalus) variegatus is found in continental southeast Asia from Malaysia to southern Vietnam and on the adjacent Sunda islands including Sumatra, Java and Borneo. The two species are similar in superficial appearance. They are about the size of a cat with the largest individuals growing close to a foot and a half in body length, with nearly another foot of tail. In colour they are brown or grey dorsally, more or less mottled with pale spots. The eyes are large. The Philippine colugo averages slightly smaller than the Sunda colugo but has a broader, deeper skull with larger teeth (Stafford & Szalay 2000). The most remarkable feature of the dentition is the incisors which are comb-like (the most divided tooth, the lower second incisor, has up to thirteen tines in C. volans) and project forwards in the lower jaw.

Colugo while gliding, from Nasir (2013).

Both species of colugo are mostly nocturnal. As a result, though they may be abundant within their ranges, their habits seem to be poorly known. They are herbivores, feeding mostly on leaves and new growth; fruit may be taken but does not seem to form a significant part of the diet (Macdonald 1984). The more robust jaws of Cynocephalus volans suggest that it may somehow has a tougher diet than Galeopterus variegatus but the nature of this difference (if any) remains unknown. The exact function of the pectinate incisors remains similarly speculative, whether they somehow function in feeding or are used in grooming.

Anterior part of lower jaw of Cynocephalus volans, showing the pectinate incisors, from Stafford & Szalay (2000).

As mentioned above, colugos are the only gliding mammals in which the patagium incorporates the hands, leading to their common description as “mitten-gliders”. Though this makes them ideally suited for aerial travel—colugos have been recorded gliding over 130 metres in distance while losing only about twelve metres in height—it restricts their abilities otherwise. A colugo on the ground is pretty much helpless. They are not even particularly agile climbers, clambering up tree trunks after landing by gripping the bark with their sharp claws. Infant young remain clinging to their mother as she moves between trees, with part of the patagium near the tail curled up to form a sort of pouch in which the infant can rest. A number of sources refer to this behaviour resulting from the infant being borne at an early stage of development but I personally wonder how much it relates to the simple point that newborns would be otherwise unable to keep pace with their parent. Young are usually born singly but may be produced in quick succession as females have been found to be pregnant while still carrying unweaned offspring.

Because of their distinctiveness, colugos have long been assigned to their own order Dermoptera within modern mammals. Most authors have considered dermopterans related to primates and tree shrews though the exact relationships between the three orders have been more disputed. A long-standing proposal that bats were also part of this complex, with bats and dermopterans together forming a clade of aerial mammals, has been rejected with the advent of molecular data. Perhaps the most widely supported arrangement today sees colugos as the closest relatives of primates.

Because primates are known to have an extensive fossil record going back to the early Cenozoic, this implies a similarly long history for dermopterans. Unfortunately, direct evidence is currently minimal. This may indicate that the ancestors of modern flying lemurs have always been associated with tropical rainforests, a habitat largely inamicable to fossilisation. Alternatively, it could be that the derived morphology of modern species may make it difficult to recognize connections with more generalized ancestors. Fossil teeth and jaws from the Palaeogene (Eocene and Oligocene) of southern Asia (Pakistan, Burma and Thailand) have been assigned to the Cynocephalidae as the genus Dermotherium (Marivaux et al. 2006). Multiple authors have also supported inclusion in the order of the Plagiomenidae, a family also known from dental fossils from the early Palaeogene of western North America. Plagiomenid teeth resemble those of cynocephalids in multiple ways, most notable in the presence of divided incisors albeit those of plagiomenids have fewer, coarser divisions than modern taxa. Conversely, other authors suggest that similarities between plagiomenids and cynocephalids could be the result of convergence from independent stocks. Even if they are correctly placed, the fact that fossil dermopterans are known from jaws and teeth only makes it impossible to know if they were gliders. Just when the colugos attained their remarkable specialisations remains a mystery.

References

Macdonald, D. (ed.) 1984. All the World’s Animals: Primates. Torstar Books: New York.

Marivaux, L., L. Bocat, Y. Chaimanee, J.-J. Jaeger, B. Marandat, P. Srisuk, P. Tafforeau, C. Yamee & J.-L. Welcomme. 2006. Cynocephalid dermopterans from the Palaeogene of South Asia (Thailand, Myanmar and Pakistan): systematic, evolutionary and palaeobiogeographic implications. Zoologica Scripta 35: 395–420.

Stafford, B. J., & F. S. Szalay. 2000. Craniodental functional morphology and taxonomy of dermopterans. Journal of Mammalogy 81 (2): 360–385.

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