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Borneo's Gliding Giants

Just as the sun sets a whiskered nose pokes out of a hole 15 stories above the forest floor. As the light dims a furry head, body, and massive tail follow. Crawling on a branch it isn’t unstable, just a bit awkward and perhaps overburdened, like when you have to delicately shuffle your way over to the coffee maker in the morning because your whole comforter is still cloaked around you. After carefully scanning the surroundings, this arboreal acrobat crouches down, takes aim, and launches itself into the warm night air.

This is how the flying squirrel begins its night, and not just any flying squirrel – this is the giant flying squirrel.

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There are more flying squirrels found in Borneo than anywhere else on earth. Out of the 49 species of flying squirrels throughout the world, 14 are found in Borneo, including four giant flying squirrel species. You can see below two different species of giant flying squirrels using the same tree at the Rainforest Discovery Center near the Sepilok Forest Reserve in Sabah, Malaysia. On the left is a Red Giant Flying Squirrel (Petaurista petaurista), easily identified by the dark tip on its tail. The on the right is the Black Giant Flying Squirrel (Aeromys tephromelas). There are two other giant flying squirrel species found in Borneo, the Spotted Giant Flying Squirrel (Petaurista elegans) and the Thomas’s Giant Flying Squirrel (Aeromys thomasi).

The Red Giant Flying Squirrel (left) and Black Giant Flying Squirrel (right) using the same tree.

Flying squirrels don’t actually fly; they glide using a membrane of skin that connects their front and back limbs called patagium. The name is derived from the Greek word “patageion,” which referred to the gold lining at the edge of a woman’s tunic. By stretching out this membrane as they leap from some of the tallest trees in the tropics, giant flying squirrels can travel well over 150 meters. The trajectories of flying squirrels in the air were once thought to be something of a gamble, but research on Northern Flying Squirrels in North America has shown that they are actually quite agile in their gliding. They are able to steer themselves in the air and even weave around trees by tensing muscles on either side of their patagium.

A Red Giant Flying Squirrel leaps off a branch, stretching its limbs. The membrane that allows it to glide is called the patagium.

While the “flying” part of their name is a bit of a misnomer, the “giant” part is spot on. The largest ones reach a head-and-body length of 40 centimeters with a tail just as long (if that means nothing to you, that’s about the same size as a house cat) and can weigh up to 2 kilograms (well over 4 lbs).

All flying squirrels are completely nocturnal, and giant flying squirrels are known for their regularity. They tend to emerge from their nest holes around the same time each day and follow similar routes as they forage throughout the treetops. The small- and medium-bodied flying squirrels have a somewhat diverse diet of vegetation, fruit, seeds, nuts, and insects, but the giant flying squirrels rely almost exclusively on leaves.

The diversity of flying squirrels and gliding animals in general in Southeast Asia presents an additional challenge for surveying canopy wildlife. I started conducting canopy camera trap surveys in the Neotropical forests of Panama, which tend to have a more enclosed canopy with more species that walk along branches rather than glide from tree to tree. As a result, I tend to position cameras only along branches that extend laterally to nearby trees. Considering the diversity of gliding species in Borneo and the paths that they take from tree crown to tree trunk, I took a page from the Tim Laman playbook and pointed a few of my cameras straight down the trunk.

A pair of Black Giant Flying Squirrels land on the trunk and crawl up to their next launching point.

The field of view is always a limiting factor with any camera trap, particularly in the 360-degree canopy environment. My experiment with cameras facing down the trunk and along branches in the same tree yielded similar detection success, though I am somewhat more intrigued by the photos from trunk-facing cameras like the one below that show a pair of Black Flying Squirrels scrambling their way straight up the tree en route to their next leap.

While gliding mammals certainly utilize negative space (the open area between trees) to move through their habitat, they also spend plenty of time on the branches and trunks of trees like other non-volant (non-flying and non-gliding) mammals. I was worried that placing camera traps along branches would not be sufficient to detect and monitor flying squirrels in these canopies (keep in mind, these photos are meant to be data points for a survey), but my success in capturing photos of the giant flying squirrels as well as a couple other species (below) are promising.

The Hose's Pygmy Flying Squirrel, the smallest flying squirrel in the world, lands in front of the camera in Danum Valley, Malaysia.

A Temminck's Flying Squirrel lands in a fruiting Ficus tree in Sepilok.

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