How gliding mammals avoid crash landings
The development of the aeroplane owes much to its humbler relative, the glider. The Wright brothers explicitly acknowledged the efforts of nineteenth century enthusiasts such as Otto Lilienthal and Sir George Cayley. However, other mammals first evolved the ability to glide many millions of years ago. For a tree-dwelling creature, gliding offers considerable advantages in terms of conserving energy, avoiding injury and evading predators.
An example is the colugo. One of the closest evolutionary relatives to primates, colugos are also known as flying lemurs although they are neither lemurs, which are primates, nor do they fly. These squirrel-sized beasts are one of the more successful mammalian gliders, using membranes of skin between their outstretched limbs to cover distances of 150 metres between trees (sometimes even carrying young).
However, in order to enjoy the benefits of gliding, mammals such as the colugo must actively manipulate forces during take-off and landing to reach their target and avoid injury due to high-impact landings. To date, most research has been conducted under artificial conditions in the laboratory and, consequently, has failed to shed light on how they achieve these feats. First, there are discrepancies in the relative magnitude of take-off and landing forces observed depending on the rigidity and orientation of the artificial platforms. No-one knows what colugos use in their natural environment. Second, take-off and landing forces increase with distance travelled. However, this may be due to the small range of short distances examined since aerodynamic theory predicts that gliders reach an equilibrium velocity (where weight is balanced by drag) beyond which such forces no longer increase.
The fact that gliding mammals are nocturnal has hampered previous efforts to study them in the wild using videography. However, in a paper published in the Proceedings of the Royal Society B, Greg Byrnes from the University of California, Berkeley, and his colleagues made novel use of technology to discover how colugos glide in their natural habitat. The researchers captured colugos in Singapore and, before returning them to the wild, glued tiny rucksacks to their backs. These contained an electronic accelerometer, similar to those used in the Wii computer game controllers, which records 3-d acceleration. Also included was a radio tag, allowing Byrnes to track the whereabouts of each animal as well as recover the device when it eventually fell off (between one and four weeks later).
In contrast to some laboratory studies, the data retrieved from these gadgets indicated that landing forces (up to 17 times the animal’s body weight) exceed take-off forces (up to 13 times thereof) suggesting that colugos mainly encounter rigid landing surfaces. Furthermore, over the wider range of natural distances studied, take-off forces did not vary with glide duration. Take-off speed varied widely, probably reflecting natural variation in the stiffness and orientation of the branches colugos glide from.
However, the most surprising outcome was that landing forces show an inverse relationship with glide duration: longer glides result in lower landing forces, in contrast both with previous laboratory research and aerodynamic theory. It turns out that this counterintuitive relationship is due to aerial braking: colugos change their body posture dynamically in order to slow down throughout a glide. This is particularly significant just before landing and allows the colugo to reduce its speed by 60% on average. Short glides simply do not provide enough time to change body posture and result in high impact landings.
Byrnes and his colleagues suggest that this ability to brake forms a key step in the evolution from leaping to gliding. By using the accelerometer to study and compare other species in their natural habitats, they hope to discover more about the evolution of mammalian gliding.
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