A Bat’s Inverted Sleep Position

by Anupum Pant

I have written about sloths in the past. In that post, we appreciated the way their bodies are engineered to stay inverted for most of their lives. It turns out, a bat’s body is designed (rather evolved) in a similar way, which enables them to relax and sleep upside down. In this post, I would like to discuss – why did they evolve this way and how do they do it?

If you are interested to know more about bats, you will definitely like this post from the archives. [Bats can See]

How can bats manage to sleep like this?

Humans sleep in a horizontal position, cows sleep with their eyes open, horses sleep in a standing position, and of course, bats sleep in an inverted position. What makes an animal sleep in the position they do, is basically their anatomy – the way their bodies are designed. While sleeping upside down might seem as an anomalous behavior to us, it is a normal position for the bat’s body. Like we don’t exert energy when we are lying down, bats’ bodies don’t consume extra energy for hanging down like that.

Firstly, a bat’s claw is like a hook. A better way to understand why this helps is, to look at a converse behavior – the way a human hand works. We use up energy to contract tens of muscles and make up a hook with our fingers; this is not a normal state of our hand. Also, our relaxed hands are open where we don’t exert any energy and we sleep with our hands in that position. A bat’s claws are designed in a completely inverse fashion; they are hooked in the normal position. They don’t take up energy to make them into hooks, they are like that. And they sleep like that – which enables them to hang without using energy.
So, unlike our hands, a bats’ closed fist is their relaxed position. They have to contract tendons and use energy to open them up. This anomalous talon design allows them to hang in a relaxed position.

Bat's Talons - Normal position

Secondly, unlike every other bird, a bat can’t take off from an upright position, or from the ground. They have to be inverted to start flying. This is because they have relatively weaker wings which can’t make them fly from a stationary position. Think of an X-51A Waverider, which has to be carried on a B-52 plane and dropped down to start a flight. They drop down for a very small amount of time and beat their wings vigorously to start a flight. Since, they have to wake up inverted to go flying and catch a meal, they go to sleep like that.

Why did they evolve this way?

They’ve evolved this way to simply stay away from the predators:

  1. By hiding up in a place where not many predators would look – under a bridge, roof of the cave and dark tree canopies. Also, at places like these, they don’t have to compete with other birds for a place.
  2. And by escaping quickly in case of an attack by attaining instant flight [see above].

The Purpose of Sleep – Theories

by Anupum Pant

Did you know?

  • You can live longer without food than you can live without sleep.
  • On an average, a person sleeps for 1/3rd of his life. That means if you are 30 years old, you’ve slept for 10 years already.
  • The world record for the longest span remaining awake is 11 days. The record was set by 17-year-old Randy Gardner in 1964 when he was awake for 264 hours and 12 minutes. (Don’t try this)
  • If it takes you less than five minutes to fall asleep then very likely you are sleep deprived. It should take about 10 minutes normally.
  • You can lose up to 2 grams every minute while sleeping. That comes to about a lost Kilogram at night.

What is Sleep?

Everybody sleeps. On an average, everyday, a bat sleeps for 15 hours while just 3 hours are enough for a Horse to feel rested [How much do animals sleep – chart]. Dolphins can sleep with half of their brains active and even the smallest of worms need sleep. But, till date, scientists haven’t been able to reach a consensus on why we actually do it?

Sleep can be understood as a life sustaining activity. It can be compared to another of our important activity – eating. These strongest of the human urges – eating and sleeping – serve a nearly common purpose – quell the urges; just as eating relieves hunger, sleeping relives sleepiness.

While it is true that we may have not been able to spot the primary purpose of sleep, years of research hasn’t gone wasted. With time, we have learnt a lot more about sleep than we used to know 50 years back. There have been hundreds of practical and impractical theories on why we sleep. I’ve mentioned a few of the realistic ones here.

Purpose of Sleep – Theories

Inactivity Theory:

According to this old theory, sleep evolved out of the need to be safe. It suggests that inactivity at night is an adaptation that serves as a survival mechanism. It functions as an activity which would keep an animal safe during the time it is most vulnerable e.g. Humans are vulnerable at night because they don’t see well in dark.

This theory is easily countered using the argument: Being conscious and still, is much better way to stay safe than lying unconscious and still.

Energy Conversation Theory:

It suggests, the main function of sleep is to reduce the demand and consumption rate of energy at times when searching for food isn’t an efficient option. Some believe, that this theory is a part of the inactivity theory. But, unlike the inactivity theory, this theory has been bolstered by various experiments which prove that the consumption of energy is reduced when an organism sleeps.

Restorative theory:

It explains sleep as an opportunity for a body to rejuvenate in terms of muscles, tissues, proteins, cells and growth hormones. Also, during sleep, the body clears an accumulated molecule called adenosine from the system, and makes us feel alert when we wake.

recent study also seems to support the restorative theory in a fresh manner. It says, brain accumulates toxins [like beta amyloid] while it is awake. The concentration of such toxins keeps increasing during waking state. The study observed an increased activity of spinal fluid being pumped into the brain in rodents (hasn’t been studied on humans yet). This spinal fluid functions as a medium to pump out toxins from the brain. This theory could be a breakthrough in the study of causes and prevention of Alzheimer’s as it is known that toxins like beta amyloid cause the disease in humans [How beta amyloid causes Alzheimer’s].

Brain Plasticity Theory:

According to this, sleep plays a critical role in brain development. Infants (age when the brain develops the most), for example, sleep for about 13 – 14 hours every day. On the other hand, the poor ability to learn and perform tasks due to the lack of sleep explains the role of sleep in adults. Since, It has been proven that brain is highly active during sleep, this theory remains the most plausible one in being able to state one of the functions of sleep.