Gliding mammals rely primarily on extensive skin membranes or ‘patagia’ that stretch between fore- and hind-limbs, creating a wing-like structure.

Gliding is defined as descent through the air at an angle of less than 45° to the horizontal, and this mode of locomotion has been achieved in a surprising diversity of animals through a number of impressive morphological and behavioural adaptations that generate the required aerodynamic forces (upward ‘lift’ exceeding air resistance, or ‘drag’). Among the distinct types of gliding that are known, several provide excellent case studies in convergent evolution, as they have evolved repeatedly in very distantly related groups, including reptiles, mammals, frogs, ants, fish and even in some species of squid.

Gliding mammals rely primarily on extensive skin membranes or ‘patagia’ that stretch between fore- and hind-limbs, creating a wing-like structure. Such large patagial membranes are perhaps most famously known from the so-called ‘flying’ squirrels, and they permit gliding as air pressure during descent from trees passively extends the membrane and generates lift to support the animal’s body mass. Other mammalian adaptations to aerodynamic locomotion include a feather-like tuft or broad expansion at the end of the tail (e.g. in scaly-tailed possums), a dorso-ventrally flattened and light, slender body, skin membranes between fingers and toes (‘interdigital webbing’) and very accurate vision (e.g. in colugos). Each of these features is shared to a degree with adaptations of certain non-mammalian gliders, from several modern lizards and amphibians, to some unusual extinct forms of reptile.

1. Mammalian gliding: the ‘wing’ patagium

Mammalian gliders are remarkably diverse, and typically have a patagium extending from the forelimb ‘elbow’ to the hindlimb ‘ankle’ or tail region, making an effective wing structure for maximising lift and minimising drag. Gliding has evolved independently at least seven times in the mammals (as described below), making them an excellent example of convergent evolution in their own right!

In the rodents both ‘flying’ squirrels (Sciuridae: Pteromyini) and anomalures, or scaly-tailed ‘flying squirrels’ (Anomaluridae) have independently acquired a patagium that reaches from the wrist to the ankle, is controlled by cartilage spurs (these affect membrane tautness), and is supported as a gliding device by a flattened, broad tail. In the anomalures, two rows of scales are also present under the tail, possibly providing extra lift during gliding. Gliding squirrels are widespread in Eurasia and North America, and comprise 15 genera, including Petaurista (giant flying squirrels) in S.E. Asia, Glaucomys in N. America and Petaurillus (pygmy gliding squirrel) in Borneo-Malasia. They can glide over 200m, but usually glide from tree to tree, pursuing prey or escaping predators. Anomalures inhabit forests of Central Africa, and include the genera Anomalurus, Idiurus and Zenkerella. In spite of certain common names – e.g. Beecroft’s flying squirrel (Anomalurus beecrofti) and the long-eared flying mouse (Idiurus macrotis) – they are not related to squirrels or mice, but represent convergence on a flying squirrel-like form.

The most stunning example of mammalian adaptation to gliding is found in the colugos (Dermoptera: Cynocephalidae), which are nocturnal tree-dwellers capable of gliding up to 70m between trees without any significant loss of height. Cynocephalus volans and Galeopterus variegatus inhabit S.E. Asian rainforests, and two fossil colugo species (Dermotherium major and D. chimaera) are known from Eocene rocks <50 million years old. The colugo patagium covers as great a surface area as geometrically possible, its skeletal structure is adapted for maximising lift, and it possesses excellent binocular vision for precision landing. The patagium extends from the shoulder blade to the tip of the outer forelimb, then back to the tip of the toes and the tail; inter-digital webbing is present, and the limbs are long and thin, giving a light, aerodynamic build. Within the closely related primates we find limited gliding ability in lemurs (family Indriiidae) of the genus Propithecus. These lemurs are diurnal, arboreal herbivores from Madagascar, and are commonly known as sifaka. Propithecus diadema and its allies have thick hair on the forearm and a small membrane on inner forelimb, both of which provide limited aerofoil properties and degree of lift when jumping between the trees. The only other known gliding placental mammal is an extinct species, Volaticotherium antiquus, from the Lower Cretaceous (~125Ma) of Inner Mongolia. It was a small insectivore (12-14cm long), similar in mass to the flying squirrel Glaucomys volans, and had a furry patagium stretched between fore- and hind-limbs, a smaller membrane (‘uropatagium’) between the hindlimbs and tail, and elongated limbs (as in colugo) for support. The tail was long, stiff and dorso-ventrally flattened, providing an excellent means of achieving control and stability during gliding. The relationship of Volaticotherium to other eutherian mammals is uncertain.

Among the marsupial mammals gliding has evolved independently on three separate occasions; in the gliding possums, greater glider and feather-tailed possums. Gliding possums or ‘phalangers’ comprise six species in the genus Petaurus (family Petauridae); they inhabit forests of Australia, New Guinea and Borneo, and are able to glide between trees for over 50m in search of sap, nectar or small insects to eat. In petaurids  (e.g. Petaurus biacensis – the Biak glider, and P. breviceps – the sugar glider) the patagium stretches from the fifth finger to the first toe and is under a degree of muscular control, very similar to the eutherian group Pteromyini (flying squirrels). Stabilisation during gliding depends on a broad, flattened tail, as in most other flying mammals, and also in gliding lizards to a degree. The greater glider Petauroides volans is the single species of the family Pseudochieridae, and resembles Petaurus in its habits and form, except that the patagium is more restricted, stretching from the fifth finger to the hindlimb ‘knee’. Feather-tailed possums refer to two species of the family Acrobatidae, including the smallest known gliding mammal, the Australian ‘feather-tailed glider’ Acrobates pygmaeus, and the feather-tailed possum of New Guinea, Distoechurus pennatus. Both have a patagium between the fore- and hind-limbs, but are characterised by a tail that has long, stiffened hairs running down both sides, giving it a feather-like appearance and providing good steering ability when gliding.

In addition to the convergent use of a large patagium in placental gliders (e.g. flying squirrels, anomalurids, colugos, sifaka, Volaticotherium) and marsupial gliders (e.g. phalangers, Petauroides, acrobatids), an extinct reptile called Sharovipteryx mirabilis also evolved a limb-associated ‘wing’ patagium, thus developing a similar adaptation to gliding in spite of millions of years of independent evolution from the last shared ancestor of both mammals and reptiles. Sharovipteryx is a small reptile from the Upper Triassic (225 million years ago), and was evidently tree-dwelling (‘arboreal’). Presumably it was able to run up trees aided by claws on its two back legs, and then launch itself into a powerful, controlled glide. Its gliding mechanism is unique among the reptiles: a wide, triangular membrane (termed a ‘delta wing’) spreads between its outstretched hindlimbs at the posterior and joins the body just behind the shoulders to form an anterior point. In addition to the primary ‘delta wing’ Sharovipteryx is hypothesised to have had small membranes between the forelimbs and base of the neck, providing enhanced maneuverability. Sharovipteryx is an enigmatic reptile, and its unusual ‘delta wing gliding’ mechanism is most similar to that seen in gliding mammals; most other gliding reptiles using skin membranes spread between digits (e.g. geckkonid lizards Ptychozoon) or as thoracic wings, supported on elongated ribs or dermal rods (e.g. agamid lizard Draco)

2. Mammalian gliding: inter-digital membranes

Well-developed patagial membranes between fingers and toes (interdigital webbing) are an important feature of the most extremely specialised of gliding mammals, the colugos. Colugos are tree-dwelling mammals that inhabit the South-East Asian rainforests, and the two species known, Cynocephalus volans and Galeopterus varieagatus, may be the closest relatives of primates alive today. Their interdigital webbing and expansive wing patagium permits them to glide between trees in search of food and mates.

Outside the colugos, webbed digits have evolved convergently in two types of lizard (geckos and lacertids) and also in two independent lineages of ‘flying’ frogs. Web-footed lizards include gliding geckos, for example the six species of Ptychozoon from South-East Asia and Indo-Australia, and the distantly related West African lacertid, Holaspis guentheri. Flying frogs glide and maneuver in the air using extensively webbed digits, glide to reach mating sites on the rainforest floor and to escape predation. Such frogs live in tropical and neotropical forests, and include species of New World ‘Hylidae’ (e.g. Hyla miliaria) and Old World ‘Rhacophoridae’ (e.g. Rhacophorus nigropalmatus).

3. Mammalian gliding: tail plumage

A laterally expanded tail plume is hypothesised to provide stability when gliding between trees, and as a morphological and functional adaptation, shows convergence in form both within the mammals and also between mammals and several gliding reptiles.

Gliding in scaly-tailed anomalures (African rodents, e.g. Anomalurus, Idiurus) and feather-tailed possums (marsupials of the family Acrobatidae – e.g. Acrobates, Distoechurus) is dependent on projecting rows of scales or stiff, feather-like hairs respectively, projecting from either side of a flattened, broad tail.  Anomalures and acrobatids are very distantly related – one being placental, the other marsupial, and yet some reptiles from within an extinct dinosaur group called the theropods provide an even more distantly related comparison and so a further example of convergence.

Caudipteryx skeleton” width=”218″ height=”218″>Theropods include ‘oviraptorosaurs’, which are slightly less evolutionarily advanced than the ‘Eumaniraptoria’, the latter group containing notable species such as Microraptor and Sinovenator, capable of gliding, and also Archaeopteryx and Sapeornis, primitive ‘birds’ capable of powered flight. The oviraptorosaur Protarchaeopteryx robusta had indisputably bird-like feathers in a tuft on the end of its tail, and Caudipteryx zoui had a well feathered tail and forelimbs; both species are Late Jurassic-Early Cretaceous (150-140 Ma) in age. Caudipteryx and Protarchaeopteryx  evolved a ‘tail-wing’ independently from one another, as a gliding adaptation that appears to be only a small step behind the specialised ‘four-wings plus tail-fan’ of Microraptor and other eumaniraptors. As such, these broad-tailed reptiles share remarkable elements of convergence with dominant tail-fan bearing mammals such as Anomalurus and Acrobates.