The greater glider

Come back to the gum forest after dark and, if you are lucky, you will catch it in the torch beam: the slow, cat-sized, wide-eyed shape of a greater glider, drifting from a high branch in a long swooping arc on the membrane it stretches between wrist and ankle. The animal here is the southern greater glider, Petauroides volans — the species split out in 2020 from what was long treated as one wide-ranging glider — and it has made almost exactly the koala's wager. It eats little but eucalypt leaves, browses only a favoured handful of species, and pays for that thrifty, low-grade diet by living slowly: a quiet, folivorous night-drifter running the same low-energy economy as the koala asleep in its fork.

But it carries a second dependence the koala does not, and this is the glider's particular bind. Through the day it dens in the hollow of an old tree, and not just one — an individual may rotate through several hollows across its range. So it needs the two slowest things the forest makes, and it needs them side by side: the right feed trees, and the century-old hollow (see hollow-dependence). A hollow big enough for a glider is not built by the glider; it is excavated by decades of fungi, termites, fire and rot working through the heartwood of an old tree, a process that in this region's eucalypts only begins at roughly a century and yields large hollows only at something like two-and-a-half centuries (Wormington & Lamb 1999; Gibbons & Lindenmayer 2002). And because a greater glider cannot cross open ground — it must launch from height and glide — it cannot shop around: the leaf and the hollow have to be within gliding reach of each other, in unbroken forest. Clear between the trees and you strand it as surely as felling them.

That double dependence has made the greater glider one of the quiet casualties of the age. Once common down the eastern forests, it has fallen away sharply — to land-clearing, to the loss of the old hollow-bearing veterans, and above all to a hotter, drier climate its thick fur and meagre diet leave it badly equipped for. It is peculiarly undone by heat: with only evaporative cooling to fall back on, a greater glider on a night that stays warm struggles to shed enough body heat, and a severe heatwave — or a hot fire running through the very canopy it cannot flee — can empty a whole forest of them. In 2022 it was uplisted to Endangered under national environmental law. Where the koala's plight is famous, the greater glider's is barely known, and it may be the more precarious of the two.

Here is the calibration that matters, and the one a tidy-minded manager gets wrong. You can keep every hectare of a forest as forest, full of green leaf, and still lose the greater glider — if what you have logged out of it is the old, half-dead, hollow-bearing trees. The leaf is rarely the limit. The century-old hollow is, and no amount of young regrowth replaces it within the animal's lifetime (see species-dependence). That is the gliders' double bind in one line: leaf and hollow, both slow, both needed, and the slower of the two the first to be cut.