The world’s gliding mammals are an extraordinary group of animals that have the ability to glide from tree to tree with seemingly effortless grace. There are more than 60 species of gliding mammals including the flying squirrels from Europe and North America, the scaly-tailed flying squirrels from central Africa and the gliding possums of Australia and New Guinea.
There are five basic stages to any glide: preparation, launch, glide, braking and landing.
In preparation for a glide, the animal usually climbs towards the end of a branch in a position that may be anything from horizontal to vertical, although a horizontal position seems to be preferred. Once in the launch position, the glider generally sways or weaves from side to side and often bobs its head up and down seemingly to assess the landing point. This has been interpreted as a method of triangulation to estimate the distance from the launch point to the landing point. This hypothesis is supported by observations that arboreal and gliding rodents have more widely spaced eyes than ground-dwelling forms, thus improving their perception of depth. Similarly, an examination of the interorbital widths of the marsupial gliders compared with other non-gliding possums has revealed that, with the exception of the Greater Glider, marsupial gliders have a wider interorbital width than the majority of non-gliding possums.
On the other hand, it has been proposed that the eyes of flying squirrels are placed far to the sides of the head in order to provide a wider field of vision to detect predators. This eye placement restricts the field of visual overlap to the front and therefore limits depth perception.
Most species of gliding mammals hold their head low in preparation for a launch, crouching down to allow a greater spring off. The giant flying squirrels curl their tails tightly like a watch spring and move it up and down quickly three or four times before jumping. The marsupial Yellow-bellied Glider has been seen to run along a slender branch and leap without pause to an adjacent tree. Similarly Hodgson’s Giant Flying Squirrel may take a short, hobbling run before launching itself into the air to gain momentum for the glide.
During the launch phase, the glider typically raises its tail and kicks into the air with its hind limbs to provide additional momentum. Most gliders spread their forelimbs and hind limbs out at right angles to the body quickly after taking off. The scaly-tailed flying squirrels, however, appear to wait until they have dropped a metre or more and gained momentum, before stretching out their limbs. After take-off, the animal is subject mainly to the force of gravity until its patagium is deployed and begins to generate aerodynamic lift. Once its patagium is spread out, the gliding phase begins as aerodynamic forces come into play.
Although most gliders launch from a stationary position on a roughly horizontal surface, the colugos typically launch by jumping backwards from a tree trunk. Once in the air they rotate their body and spread their patagia. When feeding at the end of a branch, the Malayan Colugo has been seen to initiate a glide without leaping; it just lets go and rotates its body into the glide.
There are three main types of glide: the most common is the ‘S-shaped’ glide in which the glider leaps from the tree, gains a bit of elevation before it assumes a downward path; the little used ‘J-shaped’ glide in which the animal dives from the launching place, loses elevation quickly, and then pulls out of the glide to a more horizontal angle of descent; and the ‘straight-shaped’ glide which involves launching at approximately the angle of descent of most of the glide.
During the glide, the animal must exert some control over the orientation and stability of its body in order to maintain or adjust its flight direction and angle of descent so that it will reach a particular destination. To maintain a steady trajectory and avoid spinning or tumbling out of control, it must correct any inadvertent perturbations that cause its body to rotate. At the same time it must initiate limb movements in order to execute deliberate manoeuvres. The control over the orientation of its body occurs around three axes — in aerodynamic terms, the roll, the yaw and the pitch.
The gliding possums and flying squirrels glide with their forelimbs and hind limbs fully extended at right angles to the rest of the body, and their forefeet flexed slightly upward. In contrast, the Greater Glider completes its glides with its forefeet tucked under its chin and its elbows extended out to the sides. A disadvantage of having the limbs in this position is that the glider appears to be less manoeuvrable.
Most gliding mammals, especially the smaller species, have a remarkable ability to steer during a glide, allowing them to land accurately at the desired location. They do this by changing the position of the limbs and the tension of the muscular gliding membrane. A left turn is accomplished by lowering the left forelimb below the right. This increases drag against the left membrane and the glider is spun into a turn. It has been suggested that the tail of gliding mammals helps steering by acting as a rudder, which may be partly the case for the smaller species with feather-like or flattened tails. It is more likely, especially in the larger species of gliding mammals (except for the colugos), that the tail trails behind the body and either acts to create drag, helping the animal to balance by acting as a stabiliser (similar to the tail on a kite) or that the tail acts to reduce turbulence and drag at the posterior margins of the animal.
During long glides the animal sometimes needs to steer around obstacles such as non-target trees and branches. Most species of gliding mammals are able to ‘bank’ (make turns of 90° or less) and even make a U-turn. For example, the Mahogany Glider, Sugar Glider, Greater Glider, Yellow-bellied Glider, giant flying squirrels, Southern Flying Squirrel and the scaly-tailed flying squirrels are all able to make acute turns. Due to the increased drag required to make a turn, the animal has to trade the total glide distance for each turn (depending on the angle) it makes.
Remarkably, it has been claimed that an Arrow-tailed Flying Squirrel was seen to gain 1 metre in elevation over a distance of 6 metres by using vigorous flapping movements of the skin between the fore and hind feet, leading to the idea that active flight developed from gliding flight. However, there is considerable doubt about the validity of this observation and it is likely that such movements would dramatically reduce the glide distance. Gliders are structurally and aerodynamically different from active fliers. A flapping motion by a glider would modify the shape of the patagium causing major shifts in the location of the centre of lift relative to the animal’s centre of mass, thus creating serious problems with stability and lift.
When approaching the landing point the glider moves its forelimbs and hind limbs down and forward, which traps air and creates maximum air resistance. This movement allows the patagium to billow like a parachute until the angle of attack increases from an approximately horizontal position to over 60° and causes drag, via induced turbulence. This results in deceleration and allows the glider to make a slight swoop upwards several metres before landing.
As the upward swoop continues, the drag increases until the glider stalls and loses height due to gravity. Just before landing, the angle of the body of the glider increases further to approximately 90° to the horizontal so the glider is roughly parallel with the tree trunk on which it lands. Sometimes the transition and braking phases of the glide are eliminated so there is no upward swoop. This typically occurs during a shorter glide which has a higher angle of descent and results in a relatively harder impact when the animal lands.
On landing, the glider usually makes initial contact with the landing point with its forelimbs as its claws grab hold of the tree. This causes the mass of the animal to rotate downward and its hind limbs make contact shortly afterwards. Making use of all four limbs further reduces the landing forces by spreading the impact more evenly over the body. The Feathertail Glider, and probably the similar sized smaller gliders, brings its tail well forward before landing, making its body into somewhat of a parachute.
Apart from the sound of the strong claws gripping the bark, the landing — like the rest of the glide — is typically almost silent. The scaly-tailed flying squirrels, however, have been reported to land relatively noisily due to the scales on the tail, and the Greater Glider typically lands with a ‘clop’.
When leaping from tree to tree Lord Derby’s Scaly-tailed Flying Squirrel assumes the shape of a small umbrella. The animal prefers to land on a tree trunk rather than on the branches of a tree. Immediately before reaching the trunk, its head is at a lower level than the tail, but at the last moment, it throws its forelegs back over its shoulders, its head comes up, and its tail sweeps up to meet its head over its back. The result is that the whole animal assumes a vertical position in