Sleeping is a pleasure, but, in a certain sense, it is also a duty for all living beings they need sleep to rest the brain, reorganize memories and process experiences. And to do this we need a safe and quiet place. While most mammals prefer a den and a comfortable horizontal surface, bats sleep vertically and even upside downraising the questions: couldn’t they have found a more comfortable position? Why exactly hanging with the risk of falling? The answer it’s not simple and depends on physiological, developmental, biomechanical, and behavioral factors. In short, the specific shape of the lower limbs allows bats to hook upside down without expending much energy; furthermore, in this position they are safe from predators and can take flight quickly.
Bats are not rodents and not all bats sleep upside down
Let’s get one thing clear: even if it seems strange, bats are not rodents! In fact, they belong to the order of “bats”from the Greek “wing-shaped hands”, an order related to but distinct from that of rodents. There are many species of them (they are second in number only to their rodent cousins) and they are the only mammals capable of flying. Furthermore, they present peculiar physical and behavioral traits, such asecholocationi.e. the ability to identify objects in space through the processing of the echoes of the sound waves they emit and, indeed, the habit of sleeping upside down. However, not all bats sleep upside down, for example Thyroptera tricolor it is a South American species that uses real suckers on its legs to sleep, head up, adhered to rolled-up banana leaves.
Sleeping upside down is an evolutionary choice
The upside down position allows bats to sleep safely and be ready to take flight in no time. The choice to sleep in this strange (at least from our perspective) position depends on anatomical, evolutionary and adaptation factors.
In terms physiological and anatomicalwe can explain the bat’s upside-down sleep with the conformation of the bones and tendons of the lower limbs. In fact, once attached to the branches or to a spur of a cave, these assume a “hook” position without expending any energy and without using muscles: once there is the right pressure on the toes of the hind legs, these close and they spontaneously freeze in that position, thanks to the force of gravity.
Sleeping hanging from the ceiling of a cave has some great advantages adaptive. First of all, being difficult to reach by any predators, it guarantees a remarkable safety. Plus, it makes extremely easy to take flightbecause the bat will only need to open its hind legs to find itself in free fall and fly. Being already at the top when you wake up, it also makes the passage faster from sleep to hunting activity, which is thus facilitated.
From one point of view evolutionarybats have adapted by developing large wings made of a membrane of skin stretched between the third, fourth and fifth toes of the forelimbs. It is unclear why and how these long wings evolved: of bats and their ancestors very few fossils exist! The few fossils available already have characteristics quite similar to those of bats as we know them today, so it is difficult to understand how wings evolved. One of the most accredited hypotheses is that they evolved from insectivorous ancestors who hunted insects on trunks by clinging with their bodies parallel to the trunk, ready to run and fall downwards to grab any prey: hunting flying insects on trunks is easier if you can sprint leaving yourself falling and also taking advantage of gravitational acceleration!
A question of perspective
There are many ways in which animals can sleep and each species sleeps following habits that are the result of evolution and the means that its own physiology makes available to it: there are even birds that sleep in flight. The case of Thyroptera tricolor it makes us understand, for example, that even within the same order, some members can develop different evolutionary strategies based on different means and needs.
The problem of bat sleep teaches us how important it is, in science, the perspective from which a phenomenon is studiedallowing us to recognize the bias inherent in our gaze. Although the habit of bats sleeping upside down may seem “strange” to us humans, this strangeness is the result of a perspective anthropocentric. What is spontaneous and natural for us, almonst never it is for other living beings, which deserve to be studied in the most complete and broad way possible, with a single glance free from prejudice and open to investigating from multiple perspectives.
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