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My principal experience with bats comes from summer nights at my Grandmother’s house in South India as a child. Swarms of fruit bats would circle our villa, swooping down on the surrounding vegetation to – we supposed – forage for fruits and nectar. We caught sight of one long-snouted bat drinking from the open lips of a banana flower. I wouldn’t hazard to guess what Taxon I was looking at, since India is home to over a hundred species of bats and I am far from being an expert. Several years have passed and the city I presently live in is home to the largest urban bat colony on the planet. I plan to make a visit to it in the near future – and host at least a few of the resultant photographs here!
The bat holds a unique position among the mammals. They are the only mammals capable of powered flight. They comprise a whopping 20% of all known mammal species. They spend most of their lives in an upside down posture. They rank among the most widely distributed mammals on the planet. However, the most remarkable aspect of bat biology, for me at least, is echolocation – the biological sonar that bats use to navigate and hunt by nightfall. The idea of “seeing” the world through rebounding sound waves is fantastically alien to our own sensory experience – and I reckon that it is well beyond the limits of human cognition to ever truly understand what it is “like” to experience the world as a bat. Nevertheless, understanding the physical and physiological underpinnings of bat sonar will help us appreciate what a beautiful evolutionary innovation it really is.
A large part of the business of echolocation rests on a simple fact: It is possible to determine the distance between two points by measuring the time it takes for a sound wave to travel from one point to the other and back. The calculation involved in figuring out the distance between two points from a given time delay (that is, the time that passes between sound production and echo) and a known value of sound velocity is trivial provided that neither point is moving.
But consider a bat’s situation, weaving through a cluttered environment at a considerable speed, with sound waves bouncing off several objects of varying size, closeness and texture. Think of the variety and complexity of the variables involved – and yet bats can accomplish incredible feats of aerial agility in pitch black conditions. Lazzaro Spallanzani, an 18th century bishop and experimentalist, was surprised to discover that blinded bats could fly confidently around his study without disturbing the wires he had suspended from the ceiling as obstacles. He also discovered that blocking off their ear canals with closed brass tubes drastically diminished their ability to avoid the wires. Bats produce high frequency sounds that lie outside the range of audible frequencies for the human ear – and ultrasound was unknown to 18th century science. So Spallanzani could only go so far as to say that object perception in bats (or echolocating bats anyway) was related to hearing. The true nature of bat echolocation was only uncovered in the 1940s.
A mental soundscape
The sound source for bat echolocation is expired air. The stroke of the wing and the contraction of the thoracic muscles produces a forceful exhalation. Air rushes past the larynx out through the open mouth or nostrils. Bats have evolved a fairly grotesque complement of noses with various flaps and folds to modify the emergent sound in various ways. Bat calls are ultrasonic and very loud . So loud, in fact, that many bat species find it necessary to close their ears at the moment of sound generation in order to avoid being deafened by their own calls. Some bats are known to produce vocalizations of around 130 db (louder that a rock concert), the very loudest sounds produced in all the animal kingdom. The sound energy is emitted as a directional cone. The call rate changes depending on how close to a target the bat is – from 10-15 pulses per second during normal flight to a continuous buzz just before snagging a winged insect.
Now bat calls are far more structurally complex than one might expect. Calls can sweep through a wide range of frequencies (FM or Frequency Modulated) or hold a single frequency over an extended period of time. CF and FM calls are used in different contexts on account of their different frequency-time profiles. They can also have multiple harmonics. Different species use CF and FM calls for different purposes.
Echo: The sound bounces off a target – say, a particularly unfortunate moth or a tree looming ahead. This reflected sound, or echo, can be orders of magnitude less intense than the emitted call – because of the dissipation of sound energy when it travels through air and when it strikes an object – and the bat ear has evolved to be appropriately sensitive to these quieter echoes. The bat receives the echo response and processes the information in its auditory cortex. Bats do not have an especially high brain-to-body ratio (they lie somewhere between primitive insectivores and other mammals on this scale). But they do have a series of specialized neural pathways and auditory nuclei that act to measure the time delay between echo and call. Bats process various pieces of time-delay and echo frequency information to help create an echo-image of the world. There is evidence to show that, apart from telling the distances to objects, bats can make amazing determinations of size, shape, movement and surface structure from the properties of the received echo.
Constant Frequency calls are used in open spaces because they have a greater operational range. This is because bat ears are most sensitive to the frequencies in their CF calls. Bats also make use of the Doppler effect to detect motion with their CF calls. The Doppler Effect refers to the phenomenon where the frequency of a sound changes depending on the relative motion of the observer and the sound source. The classic example used to illustrate the Doppler Effect is this: a vehicle approaching you produces a sound with a higher pitch (i.e. frequency) than it does when it moves away from you or when it is stationary. The beats of a insect’s wings produces fluctuations in echo intensity that a bat can detect. Thanks to the Doppler effect, the sound returned from a moving target also has a broader range of frequencies than the original CF call. The bat brain can use this information to compute general direction and distance to moving prey. In cases where the frequency of an echo is actually raised above of the audible range for a bat by the doppler effect, they merely reduce the frequency of the call itself (Doppler shit compensation).
Frequency modulated calls can be used in more cluttered environments where it is necessary to clearly distinguish prey from background noise. The broad sweep of frequencies used in FM results in a complex echo structure (a higher resolution echo image) and allows for more precise timing of delay, but it has a smaller physical range. Changes in the spectrum of frequencies of this echo image could indicate a change in distance between the prey and the background. And thus, the bat is able to detect moving prey even in forested areas.
This description is intended to show what a remarkable affair echo-imaging really is. It has allowed bats become “independent of sunlight as a medium for perceiving their world” (“The Biology of Bats”, Gerhard Neuweiler, pg 141). But echolocation has its drawbacks: it involves a serious expenditure of energy and is limited in range.
Bat wings are structurally very different from bird wings. For one, all the digits in the bird forelimb are fused. The digits of the bat forelimb are unfused and the 2nd to 5th digits are greatly elongated. The wing consists of a membrane of skin stretched out between the digits and between the fifth digit and the sides of the body. The rigid wings of bird are a better suited for generating lift, but bat wings provide a greater degree of maneuverability on account of how adjustable and flexible they are. The adjustments are performed by the fine action of several separate muscle groups. Bats do however, as a general rule, have slower flight speeds – the fastest known bat clocks in at about 55.92 mph, while the fastest bird can manage level flight speeds of over 150 mph. Bats can brake very effectively by spreading out their hind limbs mid-flight, opening up the uropatagium – a membrane of skin that joins the legs and often encloses the tail – like a drag parachute.
The outspread digits of a bat are light-weight and highly bendable – this is the part of the wing that actually flaps during flight (rather than the entire forelimb, as in birds). Bats need about 8-15 wingbeats per second to stay airborne. The shape of the wing varies depending on the species – fast-flying bats have short, narrow wings, while large bats that eat fruit or pick prey off the ground have large, broad wings.
Bat anatomy and physiology is clearly adapted for life on the wing. The skeleton is light and fragile. The heart is large and muscular – accounting for more of the animal’s mass as a percentage than any other mammalian heart – to provide the rapid circulation required for powered flight. The delicate wing membrane can heal after sustaining tears and wounds. The wing is also provided with sensory receptors that can assess the flow of air over the membrane.
Bats are awkward animals on the ground, however. Their knees are bent backwards and outwards and they lack grasping hand claws. They crawl along surfaces like spiders. Bats have evolved a kind of locking mechanism where the muscles and ligaments of the leg are linked up in such a way that, in a relaxed posture, the sharp claws of the foot are clenched together. While it takes energy for us to close our hands, a bat needs to make an effort to open its foot. This allows bats to hang upside down from the ceiling of a cave without expending any energy!
Unfortunately, flight-adapted bat bones are thin and do not fossilize easily – and thus the bat fossil record gives scant clues as to the early evolutionary history of the chiroptera. We face similar problems with understanding the early history of the Pterosaurs, a group of flying reptiles that are often mistaken for dinosaurs. Onychonycteris, the very oldest known Chiropteran genus, had longer hindlimbs, more clawed digits and shorter forelimbs than the modern bat. It is unclear whether or not Onychonycteris was capable of echolocation. The first bats appeared in the Eocene (as far as we can reliably tell, anyway), about 40-55 million years ago and, for the most part, appear to be fully differentiated, with a complex auditory apparatus and a nearly modern wing profile.
We observe two major taxonomical divisions in modern bats – the Megachiroptera and the Microchiroptera (megabats and microbats). Megabats have long snouts, big eyes, a claw on the second finger and are primarily found in the tropics and subtropics. Apart from bats of the genus Rousette (they generate ultrasonic calls by clicking their tongues), the megabats are incapable of echolocation. Microbats have small eyes, short snouts with strange noses and are capable of echolocation.
The most recent shared ancestor of microbats and megabats was most likely capable of echolocation. The evolution of flight (powered or otherwise) probably preceded the evolution of echolocation. There are no examples of echolocating ground-based insectivores. There are, however, examples of cave-dwelling birds that have developed a relatively crude form of echolocation. Bats may have developed the ability to echolocate to navigate through caves. Bats invaded a hitherto unoccupied nocturnal ecological niche when they took to the air and the associated selection pressures may well have driven the development of echolocation. Echolocation and flight may have evolved concurrently.
It is suggested that the ability to echolocate was secondarily lost in megabats The microbats retained it and, indeed, it is remarkable to see how closely the echolocative systems of different bat orders separated by many millions of years of evolution resemble one another.
At any rate, bats are a highly successful and well-researched group of mammals – I only wish we had a more robust fossil record to seal the deal on their evolutionary history.
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