Why do flying lemurs glide?

By Ed Yong | July 28, 2011 9:00 am

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The flying lemur must be one of the most inaccurately named animals in the world, for it cannot fly and it isn’t a lemur. This is why most biologists prefer to refer to it by its other name – the colugo. It lives in the forests of South-East Asia, where it glides (not flies) from tree to tree. From a standing start, it launches itself into the air with a powerful jump and spreads the massive membrane that runs from its chin to its hands, feet and tail.

The glide looks effortless, but Greg Byrnes from the University of California, Berkeley has found that it’s a surprisingly inefficient means of travel. Contrary to expectations, gliding actually takes up more energy than travelling the same distance by running and jumping through the canopy. So why do it? Byrnes has the answer – it saves time. For the busy modern colugo, gliding saves precious minutes that could be better spent on eating, mating or whatever it is that colugos do.

There are many gliding mammals, but colugos have the most extensive gliding membranes, and they are among the most skilled in the air. We also know very little about them. This combination of traits had an irresistible allure for Brynes, who had already been studying North America’s small flying squirrels. But he was frustrated with the limits of studying gliders in a lab. He wanted to work with wild colugos.

Together with Andrew Spence, Byrnes built a small data-logging backpack that he could fix to a colugo to record its natural movements, without any interference from watching humans. The device weighed in at 25 grams, hardly an encumbrance to an animal that weighs around a kilogram, and can sometimes carry a 400 gram baby on its back.

Now, all Byrnes had to do was catch colugos. He tried nets and umbrellas of netting, but nothing worked. In the end, the simplest approach was the best one. “I would go out during the day and search for any animals that were roosting low enough to capture,” says Byrnes. “The basic plan was to sneak up on them and grab them.  Believe it or not, I had 100% success with this very sophisticated plan.”

Byrnes captured thirteen animals in this way, and fixed the data-loggers to four of them. He tracked these animals until the loggers fell off and recovered their data within them, including almost 400 hours and over 250 glides.

Surprisingly, the data showed that gliding takes up 50 percent more energy than running across the same distance. It’s probably more economical than running down to the ground and back up again, but then again, colugos never do this. They can jump gaps of 5 metres or more, easily enough to clear the distance between treetops. And doing so, it turns out, takes up far less energy than gliding.

The problem is that colugos can’t fly upwards like a bird. They lose height the further they go, so they need to climb before every glide in order to land at the right level. And every vertical metre costs 10 times more energy to cover than every horizontal one. Byrnes thinks that it’s just as energetically inefficient for smaller gliders as it is for colugos. After all, the colugo has quite a shallow glide, losing less height over a given distance than other species, which would need to climb even higher to compensate.

This is next on Byrnes’ agenda. “One of our next goals is to do similar work in a couple of other species – probably a small North American flying squirrel and one of the giant flying squirrels,” he says. “The other big goal is to compare gliding to non-gliding species to see if there is actually an energetic benefit of being a glider.”

Rather than saving energy, Byrnes thinks that gliding saves time. Plotting a route through the canopy is slow and meandering; no one knows how quickly colugos do it, but similarly sized mammals can’t run through treetops any faster than one metre per second. By contrast, gliding colugos can easily cover 10 metres per second, giving them more time to forage when they get to their destination. Gliding could have other advantages too, including a quick escape from predators, and access to patches of food that are otherwise inaccessible.

Whatever the answer, it’s clear that gliding is a successful lifestyle. It has evolved six times in mammals and it’s an ancient habit, dating back to creatures like Volacotherium that took to the air on leathery membranes some 130 million years ago. Today, there are over 60 species of gliding mammals, including flying squirrels, phalangers and two species of colugos.

Reference: Byrnes, Libby, Lim & Spence. 2011. Gliding saves time but not energy in Malayan colugos. Journal of Experimental Biology http://dx.doi.org/10.1242/jeb.052993

Photos from Greg Byrnes, Lip Kee and Wild Singapore

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