4.1 The colugo
An animal that is not closely related to the flying squirrel but shares common features is the colugo. The colugo is a bit of a mystery and the historical confusion is evident from its common name – the flying lemur. It neither flies (in that it doesn’t flap its limbs) nor is it a lemur.
The colugo is not a monkey either, despite the fact that its main predator is the monkey-eating eagle. Having once been placed with insectivores and then with bats, it’s now in a mammalian order of its own (the Dermoptera, i.e. ‘skinwings’), recognising its ancient and distinct evolutionary beginnings. This ancient origin is why it is such an interesting animal as it early on became adapted to a tree-dwelling life. As you read about the colugo, think about adaptations that have hidden ‘costs’ to the animal.
One particular evolutionary development associated with tree dwelling is taking to the air. The gliding habit evolved independently in different mammalian and reptilian lineages and yet the anatomical modifications that allow it are similar in, for example, flying squirrels and the colugo. In particular, the ‘sail of skin’, technically termed a patagium, stretches between the limbs – and a good deal further in the colugo, acting as an effective (and to some degree manoeuvrable) gliding membrane.
Colugos are sizeable mammals (about the size of a domestic cat) and entirely arboreal. Their record-breaking glides (in excess of 70 m) are achieved without great loss of height. But in trees, they move about rather awkwardly. The patagium is then an encumbrance and there’s a limited ability to grasp effectively – the colugo lacks the opposable thumb of primates. So the benefits of a gliding lifestyle are achieved at a ‘cost’. The resulting vulnerability – especially to the Philippine monkey-eating eagle (a species under threat, as are colugos) – may help explain why the colugo is nocturnal.