Late one night, I was walking down a dark eerie street. The street lights flickering cast shadows of unknown monstrous objects on the ground all around me. There was not a glimpse of a single soul as far as my eyes could see. With hastened steps, occasionally looking over my shoulder, I wanted to reach my destination fast. My eyes fell on a huge old barren tree by the side of the road that I was walking on and suddenly had loomed into sight. The branches were filled with numerous bats hanging upside down from leafless branches, making the whole scene very creepy. With my eyes fixated on the scary creatures, I happened to step on some dried leaves, that created a deafening sound in the stillness of the night. The sound woke up the entire colony of bats and got them swarming towards me.
Scared out of my wits, I woke up with a chill. I was dreaming! Heaving a sigh of relief, I picked up my glass of water to wet my parched throat. The image of bats hanging upside down from the tree branches was still hovering in front of my eyes. Feeling uneasy and a trifle scared to go back to sleep, I decided to find out how these creepy creatures hang upside down while they snooze. Why is it that they don’t they fall off in their sleep?
Why do they hang upside down in the first place ?
Hanging upside down even for a minute sends all the blood rushing to our head, forcing us to get down immediately. Then why do these creatures do it?
Well, the bats actually have very weak legs and cannot run and take flight like birds. They hang upside down mainly for two reasons-:
i. For flight – The weak legs are not suitable for the run-ups as their legs won’t be able to take up the weight of the body and will snap. Thus just releasing their clench and opening their wings to soar, seems to be a good option.
ii. For safety – Being inverted helps them stay safe. Hanging upside-down is a great way for bats to hide from predators and danger. It helps them to stay out of the sight and the reach of a predator and allows them to enjoy a safe snooze.
The Strong Grip
Do the bats have – Super strong muscles? No, the muscles actually have no role in all this. Moreover, the muscles instead of being tensed and active during the sleep are actually relaxed.
You would be surprised if I say that bats spend minimal energy while their inverted sleeping position. They do not involve any active clenching mechanism.
A special mechanism called a Passive Digital Lock is involved. The bats actually use the unusual mechanical trick involving their tendons. Bats have a unique physiological adaptation that lets them hang around this way without exerting any energy.
A tendon connects a muscle to the bone. Unlike human tendons, which are connected to a muscle, bats have a special tendon that connects their legs to the upper body. These are pulled tight when the bats hang upside down. The weight of the bats keeps their talons closed, and the talon joint locks them into position The tendon is surrounded by a smooth, slippery coating called a tendon sheath that keeps everything sliding along nicely. Besides these, they have unique rough bumps like the teeth of a saw on their outer side.
The ribbing on the sheath nearly fits in the spaces between these bumps. As soon as a bat settles in the upside down position and grabs a tree branch with its feet, the tendon pulls down and the saw teeth slide along the ribbings like a ratchet, holding the feet clenched in position.
The alignment of the bumps and the angle are such that once the ribbing of the sheath slides in, they get locked in place. All this requires no energy from the bat and thus is called a Passive mechanism.
Consequently, the bat doesn't have to do anything to hang upside down. It only has to exert energy to release its grip, flexing muscles that pull its talons open. The lock is so passive that even dead bats can keep hanging from the tree trunk.
Not all bats can do this, though. Vampire bats, for instance, have much stronger legs and do not need to hang from the trees.
While this wealth of new found knowledge made me happy, I still could not shake off the creepiness feeling of the image in my dreams. Knowing that I was not going to get any more sleep tonight, I slipped off my covers and headed down to the kitchen to make myself a nice cup of hot chocolate.
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