Whenever we’ve heard of flying mammals, bats are the first animals that come to our minds. Although bats have imprinted the image of blood-sucking monsters in our brain, the truth is there are only a few species, such as the vampire bats, that consume blood, while the rest of the species are either insectivorous or frugivorous. They are nocturnal animals that are important for pollination and seed dispersal.
So, are bats the only mammals that can fly? Are there other mammals that can fly too? If not, then why? We’ve discussed these and much more below, but to take it slow and informatively, let us begin with the different types of flights found in animals.
Unpowered flight is not flight in its true sense but manipulation of how air mechanisms work. Unpowered flight works by converting potential energy into kinetic energy wherein the organism starts from an already raised position and uses aerodynamic forces to control the path of flight and the angle of the trajectory.
Unpowered flight has a very short duration since energy is continuously lost, and the trajectory can only be controlled upto a certain point before which they have to start over the whole process again.
Types of Unpowered Flight
Gliding (also grouped as falling)
Gliding is a form of parachuting in which the angle of descent is less than 45 degrees; it allows the gliders to go longer horizontal distances. The gilding airfoil design helps the animals to stay in the air for a long period by generating a lift force. The streamlined structure is also an important factor that reduces the drag forces in gliders. It is very common among vertebrates, reptiles, mammals, and also ray-finned fish.
People get confused by the animal’s name, like flying squirrel, because as the name suggests, they don’t actually fly but glide, and gliding is actually flying, but it is the second type of flight.
The flying squirrel has a fold of skin on either side of the body which extends to increase the surface area of the squirrel, and it launches itself from one tree to another at a gliding angle which is accurate enough to land them safely on the target tree, climbing back or to a safe height above the ground.
Basically, the definition of parachuting says that falling is parachuting, and any animal can parachute. Technically a fall is considered parachuting when the descent angle between the parachuting direction and the horizontal axis is greater than 45 degrees. In parachuting, the surface area is more relative to the parachuter’s weight because of which the drag forces are maximized, and the object’s descent is slowed down.
Not only humans and animals but plants also use this technique to disperse their seeds. Animals parachute by launching themselves in the air and control their direction by spreading toes, limbs, and membranes and land on the ground or the branches of trees.
Soaring is a specific physiological and morphological adaptation in which the animals don’t flap their wings and seem to be only gliding. The animals use the energy of the surrounding air to keep themselves at a constant altitude.
This type of flight is achieved by very few large birds and pterosaurs over the course of evolution. It doesn’t require any sort of power input, and the birds soar by gliding into a column of air rising faster than the animal is descending.
Powered (True) Flight
Powered flight or true flight is adopted by most of the birds, insects, and mammals like bats; in this flight, the animals keep flapping their wings in order to stay aloft. The evolutionary adaptations like wings, shortening of their torso, enlargement of the heart and thorax, light bones, and improved vision have been significantly helping these animals to fly. Pterosaurs, birds, and bats are the three vertebrae that have the capability of true or powered flight.
How is an unpowered flight different from a powered flight?
For starters, an unpowered flight is not a true flight at all. In other words, we can put unpowered flight as manipulation of science and air along with bodily mechanisms to fly, which in other words can be considered gliding, parachuting, soaring, etc. Gliding and parachuting can not exactly be considered evolutionary steps that allow gliding and parachuting animals to ‘fly.’ Instead, these methods are adaptations used mostly by animals living in forests, generally for transportation, more than for hunting.
Powered flight in animals may be considered (no strong evidence so far) as an extension to gliding and hopping. This theory is strongly strengthened by the running and gliding of roadrunners who usually tend to glide and run and are only capable of flying over short distances.
Which mammals can fly?
This might come as a shocker (or may not at all), but bats are the only mammals that can fly – by flying, we mean true, powered flight, an ability most common in birds. Bats belonging to the order Chiroptera are highly evolutionized with multiple adaptations, from wings to echolocation, that enables them to gain the advantage of true flight.
Below we have discussed the adaptations that allow these mammals to be the only ones that can fly.
What adaptations in bats allow them to fly?
When it comes to adaptations that aid true flight in bats, they can be divided into two:
- Physical and Physiological Adaptations
- Behavioral Adaptations
We’ve discussed them in detail below:
Physical and Physiological Adaptations:
Bats have a variety of skeletal adaptations that allow them to fly. Like birds, they have reduced and shortened bones so that they’re light enough to take to the air. As mammals, their distant ancestors would have been flightless. Bats’ wing membranes are supported by long bones, which actually are highly elongated fingers, which evolved over a long period of time to allow them to fly. In fact, Chiroptera, the scientific name for bat, means “hand-wing.”
Below we’ve discussed the adaptations in bats:
The wings of the bats are a unique adaptation – they are thin as paper and have elastic membranes. Their wings are basically extensions of their leathery skin from either side of the body. Bats are able to fly with less drag and more lift because of their thin specialized wings. They have elongated forearms with flexible finger bones with low calcium levels towards the tip.
Along with this, they also have five elongated fingers with one finger clawed while the other four fingers are clawless and embedded in the web of the wings and support it. A bat’s wing also acts as a parachute of skin which helps it in gliding and parachuting.
The fingers open and close wings like the ribs of an umbrella, and their bones undergo small bending stress during a flight. Their skin membrane is quite delicate and can tear easily, but these membranes have excellent healing power.
Their body structure is designed in such a way that it can provide support and help them to operate their wings. The bats have lower breaking stress points than the other flying animals like birds. Although their arms and hands are converted into wings, they walk, climb, hold food and kill their prey with their hands and arms.
The knee joints in bats are directed backward, which allows the wing membranes to spread to their maximum potential. Apart from this, the hindlegs in bats are specially designed to help bats sustain and live on trees, which is important for any flying animal. The hindlegs are small with claws that are sharply curved, allowing the bat to hang down from a tree branch or perch while resting.
Well-developed tails in bats support the interfemoral membrane that serves multiple purposes when it comes to acting as a flight aid. Interfemoral membranes can act as a brake to flight. However, it also serves as a pouch for holding food for the captured prey. The interfemoral membrane also acts as cradles for receiving the newly born bat offspring.
The milk dentition allows the offspring to hold onto the maternal teats while the mothers are in a state of flight. The teeth in the young bats are often shaped like a hook or needle or fork that gives them a firm grip on the mother’s teat.
Bats have highly modified sense organs such as the nose leaf, Merkel cells, and many nerves that make nocturnal flying possible. Since bats are nocturnal animals, flying in the dark poses certain threats, including obstacles. Also, bats need to be able to perceive their prey and food in general, if not see it, which requires them to have a sense of their surroundings, made possible by their sense organs.
Apart from this, the wing webs of the bats are adequately supplied with nerves and blood vessels that help perceiving the surroundings and thus avoid obstacles as well as colliding against each other when flying in swarms.
Echolocation: Echolocation or bio-sonar is what bats use to navigate and hunt for food even in the dark. Other than the frugivorous who depend largely on their sight in order to navigate, the others use echolocation. In the dark, the bats produce a series of high-pitched sounds from their mouths, and when these sound waves hit an object, they bounce back, which allows the bats to locate objects and prey in the dark.
These sounds produced by the bats are inaudible to human ears. The bats have special adaptations in their inner ear and auditory cortex of the brain, which helps them to understand the sounds in the ultrasonic range. In most insectivorous bats, there are sensitive accessory lobes on the ears, which help them to hear the warning echoes.
Nose leaf: Not all, but certain species of bats have skin flaps that encircle the nose area, which is believed to act as antennas to perceive air vibrations. Although the exact function of these nose leaves is not yet known, they are believed to act as receptors.
Merkel Cells: The touch receptor known as the Merkel Cells allow bats to detect changes in airflow, therefore allowing them to change their trajectory and decide on the most efficient speed that can help them avoid hitting potential impediments. The Merkel cells are present on the surface of the wings of the bat as tiny hairs.
Flying is a mechanism that burns a lot of energy and is very tiresome, especially during the day, which is one of the reasons that even mammals that glide and parachute can only do so within a short range and for a shorter span of time. When it comes to birds, the energy consumption is not as big a deal as it is for mammals since birds are oviparous animals who lay eggs (that is, reproduction requires lesser energy for these animals).
However, for mammals, reproduction is a much more energy-consuming process in comparison to birds, and if they do not store this energy, burning it all away flying during the day, their population will stagger. In fact, flying consumes so much energy that even though bats fly at night, they are only able to produce one offspring in a year’s span.
Apart from this, bats have very thin wing membranes, which are used for flight during the day, will absorb excess heat, leading to dehydration of the bat and maybe even death. Therefore, being nocturnal allows them to be able to fly without any physical complications in the first place. Also, bats mostly fly in the night to avoid competition with insectivorous birds and predators such as kestrels and sparrowhawks.
Although this is something that has happened over a long period of time, bats are certainly more aware of their surroundings and quick at their reflexes during the night than in the day, which gives them an advantage of changing directions quickly to avoid dangers.
Why are bats the only flying mammals?
When it comes to evolutionary biology, the question of ‘why’ starts to sound absurd, unlike the other verticals of science. Evolution does not have a way of doing something for a reason; however, if we are to give a more informed answer, we are likely to say this: Bats are the only flying mammals because they are possibly the only ones who were highly successful at adapting to the diversity of their surroundings, which came in the form of true flight.
This is one reason why we see a number of mammals, such as flying squirrels and flying foxes, that are only able to replicate true flight in forms such as gliding and parachuting.
It should also be kept in mind that flying for mammals would need them to adapt to the physical requirements for flight, such as wings and claws, apart from a completely different metabolism and bone structure highly. It is possible that not all mammals were able to evolve to fly as successfully as bats.
Why do bats fly low?
One must have noticed that the bats are flying low and sometimes just over your head. It’s mainly because they get their prey, such as insects generally at that level, and it becomes easier for them to chase and hunt them. So, whenever you see a bat flying low near you, then it’s actually after its prey or you (the possibilities of which are quite low).
Can bats walk on the ground?
There are species of bats that move around while walking on the ground. They use their small-sized clawed fingers, which are like a human thumb, to climb and walk. Their strong forelimbs and hind legs help them to crawl freely on the ground. Most of the insectivorous bats are found walking on the ground because they tend to find a number of flying and ground insects on land. This is the reason why species of bats like vampire bats are found often walking on the ground.
Can bats take off from the ground?
Some species of bats can easily take off from the ground level, but a majority of the bat’s species are unable to do so. The bats which can’t take off from the ground hang to a cliff of a tree from where take-off is easy, and they drop from their hanging position when it’s time to fly. At the same time, species like horseshoe or vampire bats can easily take off from ground level. In order to land, these species slow down their speed until they stall and then hold onto a suitable surface like a branch before dropping lower.