[intro plays]

[text: QQS: Why don't sleeping bats fall down?]

Let's say I just really wanted to sleep in a tree tonight, but I didn't bring a hammock. If I really wanted to get in touch with my animal side, I could make like an orangutan and build a nest out of branches and leaves, or copy the koala and wedge my butt into a joint or crevice and hope for the best.

One mammal I'm not going to take inspiration from is the bat. I can barely hang with my arms for more than a minute, so I don't think my feet are up to the job. But bats, bats can hang upside-down for hours while they snooze because of a special mechanism in their feet. They don't have super strong gripping muscles or anything, active clenching in any mammal critically fatigues muscles and bats simple don't have that energy to spare, they have flying to do. Besides, muscles tend to relax during sleep, something you've probably noticed if you've ever woken up with that book you were reading flat on your face. The bat's secret is to not use muscles at all while they hang. Instead, they've evolved a nifty mechanical trick with their tendons.

Generally speaking, the job of a tendon is to connect muscles to bone, but bats have another use for it. Special tendons connect their legs and feet to their upper body, and are pulled tight by the bat's weight as it hangs. Like most tendons, they're surrounded by a smooth, slippery coating called a tendon sheath that keep everything sliding along nicely.

What's special about these tendons is they have rough bumps like saw teeth on the outer side. They face the sheath, which has ribbing on part of the outside that neatly fits the size and shape of the spaces between the bumps. When a bat finds a good spot to hang out and grabs on with its feet, the tendon pulls down and the saw teeth slide along the ribbings like a ratchet, holding the feet clenched. The bumps on the tendons are angled so that once the ribbings slide in, they lock in place. This takes almost no effort from the bat, so it's called "passive digital lock." The bat can sleep soundly without tiring itself out or falling to the ground.

It's so passive that even dead bats have been known to stay hanging. Their grips are so tight that smaller bat species can cling on to even the tiniest ridge on a cave ceiling. Birds might have a similar locking mechanism to keep them perched on top of branches, but scientists still aren't sure how they use it, and certain mice use a system like this to help them climb.

Not all bats can do this, though. Vampire bats, for instance, don't, but they have much stronger legs. They eat on the ground or on the backs of their victims, so they need to stay quick and agile on their feet.

So what's the advantage for those bats that do hang? Well for one thing, you don't find too many predators on the underside of branches or on the roof of a cave, plus taking off is a lot easier if you can just release your feet, open your wings and soar, no energy-wasting run-ups needed. As long as you won't fall to your death, sleeping upside down sounds kinda fun.

Thanks for asking and thanks to all of our Patrons on Patreon who keep these answers coming. If you'd like to submit questions to be answered, or get these quick questions a few days before everyone else, go to Patreon.com/SciShow, and don't forget to go to YouTube.com/SciShow and subscribe