Colugos: The fossil record

The origins of the colugos and their relationship to both living and extinct mammals is still the subject of scientific debate. The two species of colugos living today differ greatly from all other living mammals and are placed in their own group known as the Order Dermoptera. When the Philippine Colugo was originally described in 1758 it was given the name Lemur volans, after the lemurs of Madagascar, and the relationship of colugos to primates has remained controversial ever since. In 1910 the colugos were placed in a group called the Archonta that also included bats, primates, tree shrews and elephant shrews. Although various reviews support this conclusion, other studies do not. In particular, the question of whether colugos share a closer relationship with bats, primates, or tree shrews has remained particularly contentious.

Fossils of species thought to be related to modern colugos existed as long ago as the Eocene (some 50 mya) in Thailand and Burma, and the early Oligocene (30 mya) in Pakistan, but questions remain over the relationship of these fossils to living species and whether they were able to glide.

One fossil group which has been proposed to be related to the Dermoptera is the extinct mammal Family Paromomyidae from North America and Europe, which are traditionally included within the group known as the Plesiadapiformes. The Plesiadapiformes have in turn been considered Primates or at least closely related to them. The paramonyids, along with another similar group the micromomyids, share a number of morphological characters with living dermopterans. These shared features have been cited as evidence that micromomyids and paromomyid plesiadapiforms were ‘mitten gliders’, having webbing between the fingers like living colugos. However, more recent detailed studies have provided evidence that the paromomyids did not glide like the living colugos, but were committed arborealists adapted for locomotion on large vertical supports, similar to the only modern primates with clawed hands and feet — the marmosets and tamarins of South America. It thus appears that all plesiadapiform fossils so far attributed to Dermoptera are indeed not related to the Dermoptera at all.

Another fossil group frequently related to the Dermoptera are from the extinct Family Plagiomenidae. Much less is known of the anatomy of these animals. One known skull does not reveal any distinctive similarities, but evidence from the jaws and teeth of these fossils offers some evidence of relationship to dermopterans. Confirmation or otherwise of the dermopteran-plagiomenid hypothesis awaits more complete fossil material.

Several genetic studies have suggested that the colugos are most closely related to the tree shrews of the Order Scandentia. Another study found the colugos to be less related (an out-group) than a pairing of tree shrews and primates. Alternatively, other genetic studies link the modern colugos to primates as a sister group of Anthropoidea, therefore being more closely related to the higher primates than the prosimians (which include the lemurs, bush babies, lorises and tarsiers). Yet other studies suggest that the colugos are most closely related to bats of the Order Chiroptera. However, despite all of the studies, the question of the relationships of modern-day colugos remains unresolved.

An intriguing aside to the evolution of gliding in the colugos and their potential primate relationships is why gliding has not evolved in primates, which are almost an exclusively arboreal radiation of mammals. Arboreal primates may have avoided the selective pressures for gliding niches because they lack claws, which appear to be a critical morphological feature present in gliding mammals. In order to respond to a pressure to cross large gaps in the canopy by evolving adaptations for gliding, almost any given species of primate must re-evolve claws. Because primates rely on their nails and padded finger tips for many things in their foraging, locomotor and social repertoires, it seems extremely unlikely that evolution would sacrifice this morphology to allow gliding, when there is an easier way to do it: just lengthen the legs to allow extreme leaping.

Long distance jumping is known to occur in various species of primates including the Verreaux’s Sifaka (Propithecus verreauxi) and the Diademed Sifaka (Propithecus diadema), both of which occur on Madagascar. These species make dramatic leaps of 10 metres or more from tree to tree. When its limbs are spread out during these jumps, skin folds between the upper limbs and the trunk of the body are exposed. This has raised the idea that they make use of aerodynamic forces for gliding, steering or braking before they land.

Another group of prosimian primates that make long leaps of up to five metres are from the Family Galagonidae, including the Southern Needle-clawed Bushbaby (Euoticus elegantulus) and the Senegal Bushbaby (Galago senegalensis) from the rainforests of Central Africa. The new world primates of the Family Cebidae from central and South America are yet another group that make long distance jumps which seem somewhat functionally equivalent to gliding. This family includes the Monk Saki (Pithecia monachus) which is known locally as ‘volador’ (flier). Living in the treetops 10–35 metres above the ground, it has a long body with moderately long arms and legs and makes long leaps between trees. A second genus within this family, the squirrel monkeys of the genus Saimiri, are known to jump long distances through the tree canopy.

Interestingly, the new world primates of the Family Callitrichidae, which includes the marmosets and tamarins, have claws instead of nails and are potential targets of selective pressures that could possibly lead to gliding. These small primates show the most similarity to the plesiadapiformes.