The photo below is of three bats roosting on the ceiling of a passage in Clearwater Cave, Mulu.
Apparently, bats roost like this, upside down, because it puts them in the ideal position for takeoff. Their wings are do not generate enough lift for them to fly up from the ground and their legs are not strong enough to give them a running start. So instead, they claw their way up to a ceiling of some sort and just hang out. This also places them out of sight from most night predators and hard to get at for those predators that do see them. The bat doesn't have to exert any energy to hang upside down; its weight stresses its tendons which causes its talons to clench. Interestingly, if a bat dies in its sleep, it keeps hanging there till something else knocks it down.
This was a fairly low ceiling. According to the manual for my Canon 300TL Speedlite, the unit had a guide number of 131 feet at ISO 100 when zoomed to 85mm. So if I shot this at f2.8 then the ceiling is considerably less than 47 feet above me given the exposure of the ceiling. There were other areas of Clearwater Cave further into the walk where our flashlights and my speedlite lacked the power to illuminate the ceiling in any way – even a glimmer. We could only guess from the mounds of guano on the floor how many bats were hanging above our heads.
Quite why these three bats chose this spot in Clearwater Cave I've no idea. But apparently, every evening they'd head out to feast in the jungle and then return here again at dawn. Home, sweet home!
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