Adaptations for Flight
Adaptations for flight
Adaptation for flight in bats
Bats, and an uneasy creeping in one's scalp As the bats swoop overhead! Flying madly. Pipistrello! Black piper on an infinitesimal pipe. Little lumps that fly in air and have voices indefinite, wildly vindictive; Wings like bits of umbrella. Bats!
The poet, D. H. Lawrence, seemed to find bats disgusting, but these creatures of the night are the only mammals to have evolved powered flight. Occupying the nocturnal flier niche has been extremely successful—so successful that one out of every four mammal species is a bat.
Three vertebrate taxa have evolved lineages capable of powered flight: the pterosaurs (Reptilia), birds (Aves), and bats (Mammalia). In all three cases, the forelimbs of these vertebrates were modified over time to form wings. This is an example of convergence, the independent evolution of a common structure that performs a similar function among unrelated species. The pterosaurs, the only reptiles to evolve true flight, were the first vertebrates to develop powered flight. Pterosaurs (order Pterosauria) appeared about 225 million years ago and lasted about 130 million years until they became extinct at the end of the Mesozoic era. The most diverse lineage of flying vertebrates is the birds (class Aves), which underwent extremely rapid evolution during the Cretaceous period, approximately 150 million years ago (mya). Bats (order Chiroptera) appear to be the most recent flying lineage among vertebrates, although precisely how recent is uncertain because only a few examples are represented in the fossil record. The oldest unquestioned fossil bat dates back to the early Eocene (about 50 mya) and is already a well-developed bat. Fossils from the early Paleocene (65–60 mya) attributable to bats consist mainly of teeth and jaws. They are often disputed as belonging to the order Primates.
Advantages to flight
Flight in a vertebrate provides several advantages. First, the flying animal has access to food sources unavailable to terrestrial species. This includes insects flying above the ground level that cannot be reached by earthbound animals as well as fruits and flowers on the terminal ends of thin branches. Second, the flier has a ready means of escape from non-flying (or non-volant) predators and can rest in places that are not accessible to earthbound predators. Third, flight gives a species great mobility and the ability to cover large expanses rapidly and cheaply. Although the amount of energy required to initiate flight is great, once the animal is airborne, flying is the most economical form of locomotion per distance traveled in a terrestrial environment. In addition to daily foraging advantages, flight provides the means to compensate for seasonal changes in climate and food availability. A fourth advantage is at the evolutionary level. Fliers can overcome geographic barriers such as large bodies of water and, consequently, can disperse to locations not easily traversed by non-volant terrestrial animals. For example, bats are the only mammals native to New Zealand, to many remote Pacific Islands, and to the Azores in the Atlantic. Before humans arrived on Australia with dogs, bats and a few rodents (apparently arriving from New Guinea) were the only eutherian mammals among all the terrestrial fauna on the continent.
Nocturnal flight adaptations
The focus of this entry will be the adaptation for flight among bats, the only mammals to evolve structures for powered flight. Bats are not just fliers, they are mammalian, nocturnal fliers. Consequently, their adaptation to flight involves more than just the evolution of wings, but also requires solutions to nocturnal navigation, thermoregulatory problems, and energy considerations.
Over a span of 65 million years of evolutionary history, natural selection acted to balance several physical considerations to accommodate demands of flight: body mass and shape, wing morphology, flying style (i.e., control of wing shape, orientation, and motion), and physiology (to meet the energy requirements for flight). To understand flight adaptation, it is useful to gain an understanding of the forces exerted on the animal in powered flight. Adaptation for flight of bats is guided by the need to generate and withstand, or minimize, these forces during flight. However, before looking at flight it is imperative to look at a prerequisite for nocturnal flight: some way to navigate in darkened space. Before flight could evolve in bats, a bat ancestor must have developed
echolocation. There is more to being a flying bat than just having wings.
Before there could be nighttime fliers, there had to a way to navigate in the dark. Bats are active at night and they often inhabit darkened areas such as caves or the inside of hollow trees.
Echolocation is an adaptation for navigating in visually limited environments. There are other mammals that employ echolocation, including various marine mammals and possibly members of the order Insectivora such as shrews. There is also some suggestive evidence that the colugo, a nocturnal glider, has some form of echolocation. Marine mammals such as whales and dolphins move through a medium that transmits light very poorly. The water they swim in is often obscured by murkiness from plankton and other suspended particles. At depths greater than 656 ft (200 m), a routine diving depth for marine mammals, the surroundings become completely dark. The shrew is a terrestrial mammal with tiny eyes and presumably poor vision. They are active at night. Shrews are fossorial, i.e., they burrow, dig, and forage in the leaf litter of wooded areas. Consequently, they also occupy a visually limited environment. It should also be pointed out that only two bird species are known to echolocate. Oilbirds are nocturnal and inhabit caves. The other, the Asiatic cave swiftlets, frequently fly in dark caves.
Birds have been part of the terrestrial fauna for at least 150 million years. They fill the available diurnal (daytime) flying niches. By the time bats appeared in the Eocene, birds were completely developed and no latecomer mammal would have been able to out-compete them in the daytime. Bats most likely descended from small nocturnal insectivorous mammals. Therefore, protobats were already in the nocturnal niche when there was an opening for a nocturnal flier. However, flying in the dark can be dangerous. In addition to the open nighttime environment, most bats roost in caves or inside hollow trees, which are darker environments than outside. There is also the danger of mid-air collisions with other bats. Consequently, before nocturnal flying could be feasible, some way of avoiding obstacles had to evolve. Of course, there is no way to know when echolocation actually evolved in bats. However, it had to be very early in their development. As mentioned previously, shrews have a crude form of echolocation, and shrews and other insectivores are often cited as a mammalian rootstock. If so, it is not unreasonable to suggest that echolocation developed sometime before flight.
It was the Italian physiologist Lazzaro Spallanzani who first experimented with obstacle avoidance in bats and owls in the eighteenth century. He discovered that owls would not fly in complete darkness, but this did not deter bats. He hung wires from his ceiling with small bells attached. Bats could fly throughout his study and never jingle the bells. When he blinded the bats, again they did not touch the wires. He finally inserted brass tubes into their ear canals. This was observed to impair the bats ability to avoid the wires. Spallanzani was still baffled. No sound came from the wires while they were simply hanging. Nevertheless, he attributed the bats' ability to avoid the wires to keen hearing. Of course, he was not able to hear the high-pitched sounds that the bats were actually emitting.
In the 1930s, the first microphone capable of detecting ultrasound (beyond the hearing of humans) was produced. American zoology and comparative psychology student, Donald Griffin, prominent in the 1980s for his work on animal cognition, found that placing one of these microphones in the middle of a group of quiet bats suddenly changed these relatively quiet animals into loud chatterboxes. At about the same time, the Dutch zoologist, Sven Dijkgraf, who had very keen hearing, discovered that he could hear sounds coming from Geoffroy's bat. When he placed muzzles over their jaws, preventing the emission of the sounds, these bats became disoriented and crashed into objects. From these discoveries, early researchers were able to gain some understanding of the mechanisms of echolocation. However, to date, the details of detection and interpretation of these signals by the bat are still a very active area of research (e.g., the bat project at the Auditory Neuroethology Lab at the University of Maryland, College Park).
Echolocation in bats results from the production of a high-pitched sound by the larynx and emitted through either the mouth or the nostrils. Often, the nose has been modified into a nose leaf, a fleshy process on the upper snout, which helps direct these sounds. Sound waves travel until hitting an object and bouncing back. The pinnae (external ears) of bats are large, highly modified structures designed to receive the returning signal of the bounced sound. The tragus is a small flap located in front of the ear canal. It acts as an antenna and allows the bat to discern the direction from which the sound is coming. Different species of bats utilize different frequencies. Individuals of the same species will alter their frequencies slightly to prevent confusion of signals that could lead to mid-air collisions.
Echolocation is also used for foraging. In fact, echolocation may have originally developed in a bat ancestor that was foraging in the forest litter. Bats can catch insects "on the fly,"
part of what makes them successful as nocturnal fliers. However, an evolutionary arms race exists because some moths have developed a defense against bat echolocation. They possess sound sensors on their thorax that enable them to detect the ultrasonic pulses being aimed at them by the bats. They then engage in erratic flight patterns in an attempt to evade the foraging bats. Some moths have even developed countermeasures. They produce sounds directed at the bats that seem to deter them. It is possible that these sounds are jamming the bats' echolocation in the way that aluminum foil was used to jam radar signals during World War II.
One group of bats is notable for not having echolocation. These are the large flying foxes and fruit bats (Megachiroptera). These bats depend on vision during activity under low-light conditions at dusk and dawn, a cycle referred to as a crepuscular activity cycle. They are also active during all moon phases, except the new moon when there is no moonlight. Megachiroptera lack the large pinnae and elaborate nose leafs found on the echolocating insectivorous bats (Microchiroptera). There is one exception: rousette bats that roost in dark caves (which is unusual for a megachiropteran) use a form of echolocation in which they produce sounds by slowly clicking their tongue. This is different from microchiropteran echolocation. Although echolocation is not necessary for flight in itself, it is a required adaptation for fliers who travel in pitch-black darkness.
The physics of powered flight
Once a means for detecting and avoiding obstacles was developed in a bat ancestor, the lineage was free to expand into the nocturnal flier niche. Powered flight allows access to flying insects. Because gliders do not have the maneuverability to pursue flying insects, this feeding niche was wide open during early bat evolution. The difference between powered flight and other modes of traveling through the air is maneuverability. Gliders such as the colugo have extra skin at the body's sides, which can both stretch out and change angle during flight to control both the rate and the angle of descent. Therefore, gliding has both a downward and a horizontal component of motion. However, the starting point is always higher than the final position of the animal. This is because gravitational potential (the energy determined by a body's position in a gravity field) is the only source of kinetic energy (energy of motion) in this mode of traveling through the air. To obtain a greater height above the starting position, gliders must utilize other means (e.g., tree climbing). Power flyers can oppose the force of gravity and increase their height above the ground by using wings and the power generated by their own muscles. They are also capable of controlling the magnitude and direction of their forward speed without depending on gravity or air currents.
Powered flight is possible because air is a fluid. In everyday usage, the word "fluid" brings to mind a liquid such as water or gasoline. But technically, a fluid obeys the law that the faster an object moves through it, the greater the force exerted on the object. In the terminology of fluid mechanics, the force exerted on an object in a direction perpendicular to the direction in which the object moves through a fluid is called dynamic lift, which is generated when an object moving through a fluid changes the direction of the fluid flow.
Another fluid force exerted on an object is dependent on the shape of the object. This is called Bernoulli lift, which may be involved in natural selection pressure for the wing and body shape of the bat. The Bernoulli principle in fluid mechanics states that the faster a fluid flows over a surface, the lower the pressure on that surface perpendicular to the fluid flow. Therefore, the pressure is lower on the top than it is on the bottom. This pressure difference results in Bernoulli lift upward. Experimentally it has been determined that Bernoulli lift alone is not sufficient for power flying, but most likely provides a selection pressure favoring a particular wing shape, body streamlining, and flight style.
In summary, the forces that must be overcome in powered flight are inertia (the resistance to change in motion that is a property of all masses), weight (the force exerted on the mass by gravity), and drag (the fluid force exerted by air on any object moving through it). To change the height from the ground and the speed and direction of forward motion, the bat has to use its wings to manipulate the airflow to generate the forces of lift and thrust. The wing structures themselves must also be able to withstand the stresses of moving through the air.
Bat wing morphology and its role in powered flight
Chief among the many adaptations of the bat for powered flight is the bat wing, and the flapping flight style that uses muscle power to generate lift and thrust. The bat wing evolved from the forelimbs of a terrestrial mammalian ancestor. The mammalian forelimb is exceedingly mobile because the shoulder joint between the scapula (shoulder bone) and the humerus (upper forelimb bone) is loosely held together with muscles. This allows for actual rotation of the arm around the shoulder joint in many species. Primates have this mobility, and so do bats.
The taxonomic name of the bats, order Chiroptera (meaning, "hand-wing"), perfectly describes the morphology of the bat wing. The skeletal structure of the bat wing consists of the humerus, a well-developed radius, and a much-reduced ulna. In humans, the ulna is a major bone of the forearm and forms a hinge joint in the elbow region with the humerus. The highly elongated hand (metacarpal) and finger (phalanges) bones form the rest of the bat wing skeleton. Only the pollex (or thumb) retains the claw of the mammal ancestor, although on fruit bats and flying foxes the second digit also retains a claw. The bones of the wing provide a segmented skeletal frame for support and control of the flight membrane.
The flight membrane (called a patagium) is a flexible double-layered structure consisting of skin, muscle, and connective tissue. It is richly supplied with blood vessels. The region of the patagium that stretches from the sides of the body and the hind limbs to the arm and the fifth digit is called the plagiopatagium. Other portions of membrane extend from the shoulder to the pollex (first digit) along the anterior portion of the wing (propatagium), between the fingers (the chiropatagium), and from the hind limbs to the tail (the uropatagium, also called the interfemoral membrane). The wing operates on an airfoil design, with the flexible membrane segments changing shape to produce variable pressure gradients along the wing surface that results in variable amounts of lift and thrust. The bats' fine control of the shape of the patagium gives them a maneuverability that cannot be matched by birds.
Bat flight is controlled by seventeen different pairs of muscles. Three different muscles provide power for the down-stroke. Another three muscles execute the upstroke. This is very unlike birds, where two pairs of muscles provide the power for the depression and elevation of the wings. The sternum (breastbone) in bats is not particularly well developed, while in birds it is very prominent with a well-developed keel. The pectoralis muscle that originates from the sternum is the largest bat flight muscle and it has the richest supply of blood vessels known for any mammal. Other muscles that originate from along the vertebral column (backbone) and the scapula help to provide tension to the membrane and adjust the position of the wing. Muscles fibers embedded in the membrane assist in regulation of the tension of the patagium. Many muscles that exist in terrestrial mammals have been slightly repositioned, while others unique to bats assist in keeping the patagium taut. The wing operates on an airfoil design, with the flexible patagium segments changing shape to provide variable amounts of lift and thrust.
The hind limb possesses a bony spur unique to bats called a calcar that projects inwardly from the tibia. This bone attaches to the uropatagium and functions to keeps the tail portion from flapping during flight. The legs can also form a pouch out of the uropatagium used for catching insects. In most bats, the hind limbs have rotated 90° outwardly and assumed a reptilian-like position. The legs are used to control the uropatagium during flight. Another important adaptation of the hind legs is as a hook, an adaptation for hanging upside down. Bats are able to hook the claws of their hind paws onto horizontal supports or rough edges on walls or on ceilings of caves. The claws have developed a locking mechanism that allows them to hold without any muscular involvement. Hanging upside down allows bats to occupy areas unavailable to birds and allows a bat to use gravity to initiate flight by dropping. It is often believed that bats are completely helpless on the ground because of the arrangement of their legs; this is not true. Some species hop while others move quadrupedally. If a bat falls in water, it can swim to land. However, they do not use these forms of locomotion habitually. The arrangement of the bat hind limbs has probably constrained the bat lineage to being flyers. There are no flightless bats nor are there swimming bats comparable to those found among the birds (e.g., ostriches and penguins, respectively).
The superior aerobatic ability of the bat in flight is due to the wing segmentation provided by the skeletal frame, the flexibility of the membrane segments, and the very fine controls provided by the wing musculature. To date, the best analytical theory of animal flight is the vortex theory first introduced by Ellington in 1978 and further developed by Rayner in 1979. According to vortex theory, bats fly by generating volumes of circulating air (called vortices, singular vortex) that create pressure differences on different parts of the bat's wing. The resulting fluid forces push the animal in the direction it wants to go, at the speed it wants to go. The bats' flight motions are similar to the motions of a human swimmer doing the butterfly stroke. During the downstroke, the wing is fully extended. It envelops the maximum possible volume of air and pushes it down, generating a region of high pressure beneath the wing and low pressure above the wing. The pressure differences add up to a resultant force that has two components: a thrust component that opposes the drag exerted on the animal by its motion through the air, and a lift component perpendicular to the drag that opposes the action of gravity on the mass of the animal (the animal's weight). The numerical value of each component depends on the angle of attack. The steeper the angle, the higher the lift and lower the thrust. During steep-angle ascent, the bat increases the curvature of the propatagium to prevent stalling. To maximize lift, the uropatagium is curved downward. During the upstroke, the bat flexes the wing and extends the legs to decrease drag by decreasing the surface area perpendicular to the airflow. Each wing segment contributes different relative amounts of lift and thrust. The wing segments closest to the sides of the body, the plagiopatagium, generate mostly lift, and the distal wing segments (the chiropatagium) provide most of the thrust. The exact pattern of airflow over different wing segments is not yet known, however, computer simulations of the aerodynamics of the bat in flight are an area of very vigorous research. For example, a project to simulate the airflow around the changing geometry of the bat wing in flight is under way at Brown University. Preliminary results were published by Watts in 2001.
Wing form and flying strategies
The wing forms of bats are highly variable from species to species. A particular form (e.g., either long and narrow or short and broad) may reveal a relationship between flight style and foraging habits because it is likely that selection pressure favors the evolution of the best wing form for a particular feeding style. The two primary quantities used for comparing wing morphology to flight style are wing loading (WL) and aspect ratio (AR). WL is the ratio of body weight to the surface area of the wing, which demonstrates the size of the wing relative to the size of the bat. In general, the higher the WL, the faster the bat has to fly to generate sufficient lift with relatively small wings. One calculates AR by squaring the wingspan and dividing that number by the wing's surface area. AR measures the broadness of the wing. The higher the AR, the narrower and more aerodynamically efficient (lower drag)
is the wing. Bats with high-AR wing morphology are faster flyers, but lack the agility of bats with low AR. The surface areas of the uropatagium and the plagiopatagium are large in slow, agile flyers because these areas provide most of the lift during flight. The propatagium alters the leading edge curvature of the wing, and prevents stalling during steep-angle flying. If the surface areas of these regions are large compared to the wingspan, giving a low AR, the agility of the bat is very high. Examining the wing form can provide clues about the bat's specialization in foraging. There are no exact correlations because bats are very adaptable and highly flexible in their foraging habits. Also, the wing form suitable for a certain foraging style may be a disadvantage in other aspects of bat behavior. Generalizations must be made with caution. With that in mind, observers have noted that some tendencies do emerge. In general, bats with wings having high WL and high AR are bats that fly fast and forage in open air above vegetation. These bats regularly fly long distances in a short amount of time, feeding on insects while in flight. Bats with wings having low WL and low AR are able to fly slowly without stalling and can make tight maneuvers. They are gleaners and hoverers, able to navigate in heavy vegetation and to take off from the ground while carrying heavy prey. Fruit-eating bats that forage among vegetation and carnivorous bats that catch prey from the ground both fit in this category. High-WL and low-AR wings tend to belong to bats that fly fast, but have short, broad wings and are capable of maneuvering in cluttered spaces. They tend to be expert hoverers, and their flight speed allows them to visit among separated patches of vegetation in a minimum amount of time. They also tend to specialize in nectar or pollen feeding. Low-WL and high-AR wings are found among fishing bats that fly slowly over open water with very little tight maneuvering required. The low body weight allow these fishers to carry off the day's catch for later consumption.
To understand bat body adaptations for flight, it may be instructive to examine bird bodies. Bird bodies are designed for mass reduction. They do this in a number of ways. They have lost teeth and the accompanying heavy jaws and jaw musculature over evolutionary time. They have thin, hollow, and strong bones. Many bones are fused or reduced in size. The long bony tail of their ancestors has been greatly reduced to the small vestigial pygostyle. Birds have a series of air sacs in the body that serve to reduce weight. They do not have a urinary bladder to store urine nor do they have a urethra. The kidneys excrete uric acid into the cloaca where it is mixed with intestinal contents to produce the white guano associated with birds. Birds have lost one ovary, and lay eggs so they do not have to carry a fetus. The most distinctive feature of birds is their feathers, which provide lift, insulate them against heat or cold, streamline the body, and reduce mass.
Bats, as mammals, must address these weight reduction issues differently. In general, bats are much smaller in size than birds. Most bats belong to the suborder Microchiroptera (the insectivorous bats or microbats, also called the "true bats") and range from 0.07 oz (2 g) (Kitti's hog-nosed bat, perhaps the smallest mammal) to 8.1 oz (230 g), but fewer than 50 species weigh more than 1.8 oz (50 g). The larger flying foxes (Megachiroptera) may reach 56.4 oz (1,600 g) with wingspans of 6.5 ft (2 m), but they are never as large as the largest birds. Bat bones are thinner and lighter than those of most mammals, but not as light as bird bones. Bat bones have marrow in the shafts, whereas bird bones are hollow. Several bones in the bat skeleton (ulna, caudal [or tail] vertebrae) have been reduced, while several have been lost altogether (fibula, caudal vertebrae in fruit bats). The distal phalanges have less mineralization and a flatter cross-section than normally found in mammal bones, which provides more flexibility in the wing frame. Birds, on the other hand, have more mineralized bones that are somewhat more brittle. If present in the bat wing, these could actually break under the stresses on the wing frame during flight. Bats have not lost any organs as birds have. Bats still retain teeth. To compensate for the extra skull mass, they have a short neck that helps to keep the center of gravity in the middle of the torso. The bat body as a whole has been shortened and some of the vertebrae have fused, making for a stiff backbone. The diets of bats are high-energy foods, such as insects, fruit, or nectar, that pass through the gut quickly so as not to load the animal down with bulky fiber. This high-energy diet also meets the energy requirements for flight. Bats, because they are mammals, have fur instead of feathers. Fur has some limited lifting properties, produces rough surfaces that change airflow, and has some malleability for streamlining, but it is inferior in those properties to feathers. Fur does insulate, but not as efficiently as feathers.
The most important difference between bats and birds is that birds are daytime flyers and bats are nighttime flyers. As nocturnal flyers, bats face problems not faced by birds. The first problem they had to solve is navigation in a visually limited environment. Other problems bats must solve are getting sufficient oxygen and nutrients to tissues and thermoregulation. Bats have dealt with these problems very successfully.
Powered flight has enormous energy costs. Flight is energetically cheaper than walking or running once the bat is up in the air. However, it takes a considerable amount of calories to get airborne. Flying is very demanding on bat physiology. In some species, the heart rate may rise to approximately 1,000 beats per minute in order to supply oxygen to the tissues during flight. Because of these demands, the heart and lungs are larger in bats than in comparably sized mammals.
Bats do not consume fibrous plant material. Such a diet simply would not supply enough calories. Also, the gut passage time and the gut modifications needed to digest high-fiber material would increase the weight of the animal. Bats consume easily digestible, high-calorie items such as insects, fruit, or nectar. Some species also eat small vertebrates like fish, frogs, mice, or even other smaller bats.
Associated with the metabolic costs of flight is thermoregulation. Bats have unusual problems to solve in this regard. Bats probably have the most complex thermoregulatory problems to solve of any mammal. Most bats are small. Small mammals must overcome heat loss problems because they have a greater proportion of surface area in relation to their volume (or equivalently, their body mass). Heat is lost through surface area. The higher the surface area–to-mass ratio, the greater is the rate of heat loss. For this reason, small mammals have higher rates of heat loss than larger mammals. Fur helps to insulate a small mammal to some degree, but often it is not enough to prevent the high rate of heat loss. Small
mammals need to obtain more calories per unit time to produce heat (via muscles) by continuously eating foods that are quickly digested and absorbed. This is quite the opposite of larger mammals. As mammals get larger, their surface area– to-mass ratio decreases as the mass increases. Elephants have a heat load problem, not a heat conservation problem. Besides being a small mammal, bats also have additional surface area from their wing membranes. Therefore, this flight adaptation results in about six times greater surface area than that present in non-volant mammals of comparable size, which increases the heat loss rate many fold.
Bats solve these extreme heat loss problems through heterothermy, a temporary reduction in body temperature to conserve calories. Mammals are endothermic ("warm-blooded"), i.e., they generate internal heat. Most mammals are also homeothermic, which means that they regulate their body temperature within a particular range (generally, 95–102°F [35–39°C]). Bats, however, can reduce their body temperature to conserve energy. This strategy is called heterothermy and results in torpor. Bats can lower their body temperature to the environmental (or ambient) temperature and therefore do not have to devote calories to produce heat, much of which would be lost to the environment. Additionally, bats are able to reduce blood flow to the extremities and to the wing membrane that reduces heat loss through these surface areas.
During flight, the bat's thermoregulatory problems are reversed. The problem becomes how to dissipate the heat generated from the flight muscles. Bat wings have a rich supply of blood vessels. Heat is transferred from the blood to the wing membrane and is radiated off the surface. Bats do not have sweat glands, but a small amount of water vapor passes through the skin onto the membrane surface. As water evaporates off the surface of the wing, it also carries away some heat. Another area where the echolocating bats lose heat is from the blood vessels of the large external ear. Breathing also helps remove heat. Water vapor is one of the byproducts of respiration and, when the animal exhales, more heat is dissipated.
Some species can build up a heat load while they are resting during the day. This is more likely to occur among the larger bats, but some smaller bats that roost in sunny locations face this problem as well. In these situations, temperature can be regulated through behavior by moving to a shadier location. Some bats will also use their wings to fan themselves. Sometimes, they also lick themselves to promote evaporative cooling from the saliva.
Cardiovascular and respiratory adaptations
Like birds, bats have hearts that are about three times bigger than those of comparably sized mammals. The heart muscle fibers (or cells) in bats possess higher concentrations of ATP (the molecule that is utilized for energy by cells) than observed in any other mammal. These adaptations enables bats to pump more blood during a flight, a period of peak demand for oxygen. Resting bats may have heart rates as low as 20 beats per minute. Within minutes of initiating flight, the heart rate may rise to between 400 and 1,000 beats per minute. Bats also have relatively larger lungs than most mammals, providing a larger respiratory membrane for gas exchange. This is in response to the demands for oxygen required for muscle metabolism during flying.
Bats have highly vascularized wings (i.e., rich in blood vessels) that supply the wing membrane with oxygen and other nutrients. Because of this circulation, damage to the wing membrane can heal very quickly. An unusual feature of the bat wing circulation is sphincters (muscular valves) that can close off blood flow to the capillaries and shunt blood directly from the arteries to the veins. It is not exactly known when and why this is done. Some biologists believe that the sphincters are closed and blood flows through the shunts during flight. The sphincters may open during rest to allow blood to flow into the capillaries and nourish the wing membrane. A problem that exists for wing circulation is that the flapping of the wings creates a centrifugal force that impedes the flow of blood back to the heart, causing pooling in the extreme ends of the wings. To compensate for this, the veins of the wings have regions in between venous valves that contract rhythmically. These have been referred to as "venous hearts." When venous hearts contract, the vein is constricted and pushes venous blood back towards the heart. The valves in mammalian veins prevent back flow, ensuring that blood will only travel in one direction. Bat blood is capable of carrying more oxygen per fl oz (ml) than other mammals. In fact, it carries more oxygen than bird blood. It appears that this is accomplished by increasing the concentration of red blood cells (RBC), which contain the iron pigment heme that binds to oxygen. Bat blood has smaller individual RBCs than normally found in mammals and a larger number of RBCs within the same circulating blood volume. These smaller cells also provide a relatively larger surface area for gas exchange to occur. The actual mechanism of bat circulation is still not completely understood. For budding bat biologists, the bat circulatory system offers many research possibilities because little experimentation has been done on many aspects of this system.
Bat lungs are larger than the lungs of terrestrial mammals, but they do not contain the respiratory volume found in birds. The alveoli, the tiny sacs that help form the respiratory membrane, are smaller in the bat lungs than in the lungs of other mammals. The smaller the alveoli are, the greater the functional surface area for gas exchange. In addition, the alveoli are richly endowed with capillaries that bring a rich flow of blood for gas exchange. Bats are superior to other mammals at extracting oxygen from the environment, approaching the capability of birds. Bats do not have the lung volume of birds, but they have high respiratory rates that facilitate aeration. The high respiratory rates are also believed to be associated with heat removal via water vapor.
Bats own the night sky
Bats are extremely successful nocturnal mammal fliers. Their anatomy, physiology, and ecology are a finely tuned integration of many different body organs and organ systems that enable these animals to dominate the night sky. Bat adaptations for flight include more than just wings. The diet consists of high-calorie food that is easy to digest, assimilate, and pass quickly through the gut. They have solved thermoregulatory problems ranging from the heat loss due to small size to the heat load of flight metabolism. The cardiovascular and respiratory systems are highly adapted for efficient distribution. All of these adaptations work together efficiently to make the bat a well-integrated nighttime flying machine.
Anderson, D. F., and S. Eberhardt. Understanding Flight. New York: McGraw-Hill, 2001.
Fenton, M. Bats. New York: Checkmark Books, 2001.
Fenton, M. Just Bats. Toronto: University of Toronto Press, 1983.
Hall, L., and G. Richards. Flying Foxes: Fruit and Blossom Bats of Australia. Malabar, FL: Krieger Publishing Company, 2000.
Hildebrand, M. Analysis of Vertebrate Structure, 4th edition. New York: John Wiley & Sons, Inc., 1995.
Hill, J. E., and J. D. Smith. Bats: A Natural History, 1st edition. Austin: University of Texas Press, 1984.
Neuweiler, G. The Biology of Bats. New York: Oxford University Press, 2002.
Norberg, U. "Flying, Gliding and Soaring." In Functional Vertebrate Morphology, edited by M. Hildebrand, D. Bramble, K. Liem, and D. Wake. Cambridge, MA: Belknap Press, 1985.
Norberg, U. "Wing Form and Flight Mode in Bats." In Recent Advances in the Study of Bats, edited by M. B. Fenton, P. Racey, and J. Rayner. London: Cambridge University Press, 1987.
Vaughan, T., J. Ryan, and N. Czaplewski. Mammalogy, 5th edition. Philadelphia: Saunders College Publishing, 1999.
Welty, J., L. Baptista, and C. Welty. The Life of Birds, 5th edition. New York: Saunders College Publishing, 1997.
Carpenter, R. E. "Flight Physiology of Australian Flying Foxes, Pteropus poliocephalus." Journal of Experimental Biology, 114 (1984): 619–647.
Maina, J. N. "What It Takes to Fly: The Structural and Functional Respiratory Refinements in Birds and Bats." Journal of Experimental Biology, 203 (2000): 3045–3064.
Morris, S., A. Curtin, and M. Thompson. "Heterothermy, Torpor, Respiratory Gas Exchange, Water Balance and the Effect of Feeding in Gould's Long-eared Bat (Nyctophilus gouldi)." Journal of Experimental Biology, 197 (1994): 309–335.
Padian, K. "The Origins and Aerodynamics of Flight in Extinct Vertebrates." Palaeontology 28 (1985): 4132–433.
van Aardt, J., G. Bronner, and M. de Necker. "Oxygen Dissociation Curves of Whole Blood from the Egyptian Free-tailed Bat, Tadarida aegyptiaca E. Geoffroy, Using a Thin-layer Optical Cell." African Zoology, 37, no. 1 (April 2002): 109–113.
Watts, P., E. Mitchell, and S. Swartz. "A Computational Model for Estimating the Mechanics of Horizontal Flapping Flight in Bats: Model Description and Validation." Journal of Experimental Biology, 204 (2001): 2873–2898.
Auditory Neuroethology Lab Webpage. University of Maryland, College Park. 2002. <http://www.bsos.umd.edu/psych/batlab>.
Swartz Lab Webpage. Brown University, Providence, RI. 2003. <http://www.brown.edu/Departments/EEB/EML/about/about.html>.
Vertebrate Flight Exhibit Webpage. University of California, Berkeley. January 11, 1996. <http://www.ucmp.berkeley.edu/vertebrates/flight/enter.html>.
Weinstein, R. Simulation and Visualization of Airflow around Chiroptera Wings during Flight. Brown University, Providence, RI. May 1, 2002. <http://www.cs.brown.edu/rlweinst/bat.pdf>.
Marcus Young Owl, PhD