Measuring anything about a nocturnal mammal as it glides from tree to tree through the steamy, cluttered Singaporean forest is quite a daunting prospect,best tackled by a team of dedicated field biologists, electronic wizardry and superglue. But this was the challenge that Greg Byrnes, Norman Lim and Andrew Spence were well prepared to rise to. They pooled their skills, and succeeded in attaching backpacks containing accelerometers, data-loggers and radio telemetry tags to six wild colugos (aka `flying lemurs'), allowing them to study gliding under truly wild conditions.
Colugos are not mere squirrels with small flaps of skin between the legs. In fact, they are closely related to primates (diverging only 87 million years ago), and have extremely elongate, slender limbs, supporting a huge area of skin – they truly are dermopterans (literally `skin wings'). But how often do our close relatives actually glide in the wild? How good a glide can they perform? (And how bad a glide do they habitually survive?) Impossible questions for traditional laboratory or field studies to answer, but now approachable with advances in lightweight sensors and data-loggers.
In some respects, the project sounds simple enough: catch six animals, glue on a backpack, let the animals do their thing for 2 to 6 days, then use the telemetry tags to find where the backpacks fell off so that the data loggers can be recovered. However, the logistics required to achieve this goal are impressive: the whole backpack weighed less than 30 g, less than 4% of the animal's body weight and far less than the load they carry when gliding with their young (Greg has observed gliding culogos carrying young weighing 400 g!); the data loggers held a total of 430 h of data, recording 222 glides; all but one logger was recovered. And afterwards, the animals are at liberty to roam free unharmed.
So, what can be deduced from all these 3D accelerometry data? For a start,the team measured glides lasting up to 15 s. Take a moment to consider just how impressive a 15 s glide is. I would hit the ground within 5 s if I fell out of the tallest tree in the world, so these mammals' ability to remain airborne is quite remarkable. The team also analysed the accelerations during the leap and landing to determine peak take-off and landing forces, and the velocity when coming in to land. Unsurprisingly, glides of longer duration were initiated with bigger leaping impulses. More interestingly, landing after brief glides included the highest forces. It appears that, once the animal has been gliding for more than 4ish seconds, it can use aerodynamics to control its landing speed and forces. Just as for parachutists, it may be the jump from a low height that presents the greatest risk of injury, as the aerodynamic surfaces take time to be deployed and develop lift and drag.
The next step – or should that be giant leap? – is to incorporate miniature gyroscopes or magnetometers in order to get a dynamic measure of orientation throughout the glides. When and how much do they pitch to control their landing? How much can they steer? How stable – or manoeuvrable – are they? Further extending this work to trustworthy measurements of velocity and displacement for glide periods exceeding 15 s will present a considerable technical challenge – and another significant milestone. And at this stage it will be possible to really understand the performance envelope of these extreme gliders.
