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.
Why did gliding evolve in the first place? There are three primary theories to explain the evolution of gliding: (a) predator avoidance, (b) optimal foraging, and (c) cost of foraging.
The ‘predator avoidance’ model suggests that gliding locomotion, nesting in cavities and nocturnal behaviour reduce the chance of predation. Some evidence supports this as gliding mammals appear to live significantly longer than non-gliding species. Other evidence suggests that predation by birds may have limited gliding mammal diversity; all species of gliding mammals are active at night which may be due to the lower number of nocturnal predators. However, it has been proposed that although gliding may provide flying squirrels with an escape from mammalian and reptilian predators, they may actually suffer increased levels of predation from owls. This is because, despite being able to make turns during their glides, they are relatively clumsy in the air compared to the high manoeuvrability of night predators such as owls.
Gliding mammals often appear to choose their landing site cautiously before launching and do not appear to be readily adapted to quickly launching in order to escape predators. Observations that support this include predation of an estimated 500 Northern Flying Squirrels by Spotted Owls (Strix occidentalis) in a single year and predation of approximately 120 Sugar Gliders by seven breeding pairs of Powerful Owls (Ninox strenua) in a single breeding season. The gliding marsupials may differ from the colugos, flying squirrels and scaly-tailed flying squirrels, as their ancestors were almost certainly already nocturnal, then developed a gliding membrane. Whereas at least some of the eutherian gliding mammals may well have been diurnal, then developed a gliding membrane and subsequently become nocturnal. Either that or the ancestral squirrels were all nocturnal and some of them subsequently became diurnal.
The ‘optimal foraging’ hypothesis argues that gliding locomotion reduces travel time between patches of food. Some food types such as nectar, pollen and fruit are typically located in patches which are spread out within the forest, while other food types such as foliage are more ubiquitously available and more clumped together. Many gliding mammals use patchily distributed food resources and gliding locomotion allows these animals to forage more quickly and over a wider area than would otherwise be possible by climbing alone. In habitats where food is limited and widely spread, adaptations such as gliding may increase mobility and therefore food harvesting rates of the different dietary food items.
The ‘cost of foraging’ hypothesis proposes that gliding locomotion is energetically less expensive than quadrupedal locomotion and therefore allows an increased foraging radius. The most energetically expensive part of transport is climbing up tree trunks or vertical branches, so gliders save energy by gliding from tree to tree instead of running down the tree and climbing or running to the next tree. During gliding, the animal makes use of potential energy gained during previous climbing up the tree and uses little energy to control and manoeuvre during gliding. Thus gliding is theoretically an energetically cheaper way of moving from one place to another, and also much quicker than running or climbing.
Apart from allowing the ability to glide, gliding membranes can have other advantages. For example, a trapped Mahogany Glider has been seen to pull its patagium over its head, somewhat like an umbrella, when it was raining. And some flying squirrels have been observed using their patagia apparently as a blanket, wrapped around their bodies, presumably to protect themselves from heat loss. Similar observations have been made on the Greater Glider. During hot weather, the large surface area of the patagium could also allow more effective heat dissipation, enabling the glider to cool down more quickly.
Gliding membranes thus bring a number of advantages to these animals; however, it remains that their forelimbs are not greatly specialised and remain useful for climbing and the manipulation of food, unlike the forelimbs of true flying animals.