some advanced flight manoeuvres of bats - CiteSeerX

J. exp. Biol. (1976), 64, 489-49S With 3 figuret

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SOME ADVANCED FLIGHT MANOEUVRES OF BATS BY ULLA M. NORBERG Department of Zoology, University of Gdteborg, Fack, 5-400 33 Gdteborg 33, Sweden (Received 22 September 1975)

SUMMARY

Two manoeuvres in bats are described: rolls through 1800, in Nyctalus noctula, and a series of sideslips in Otomops martiensseni. These manoeuvres cause a rapid loss of height. They are initiated by pronation of one wing and supination of the other. After the roll, when the bat is in an upside down position, the lift force of the wings is directed downwards, causing a tight turn downwards (apparently for insect catching). During sideslip the body drag of the bat is increased. This reduces the total lift/drag ratio, thus steepening the equilibrium gliding angle. INTRODUCTION

Flying animals perform aerial manoeuvres in different ways which depend upon their purposes. Prey catching, avoidance of obstacles, landing, for example, require different types of manoeuvres. Two different manoeuvres in bats are described here, both causing a quick loss of height. The manoeuvres are (1) a roll through 1800, in Nyctalus noctula (Vespertilionidae), and (2) a rapid sequence of sideslips, altematingly to the left and right, in Otomops martiensseni (Molossidae). The half roll in Nyctalus was not part of flight display, but was presumably performed to catch an insect. The manoeuvre in Otomops was made when the bat descended towards the entrance to the roosting cave, a hole in the ground. Half rolls in Nyctalus and sideslips in Otomops were observed in several specimens. Filming techniques

Since most insect-eating bats are nocturnal, they are difficult to observe when hunting insects. The present descriptions are based on films that were made of flying bats in the field during the day. In the autumn in southern Sweden, swarms of newly hatched insects entice some bats out into the daylight, and they can easily be studied when they catch insects. Specimens of the African molossid bat were caught in their roosting cave and were released outside during the day. The films were taken with a Pathe 16 mm film camera with a film speed of 87-88 frames s"1. Kodak Tri-X film was used. Body measurements

The body measurements of the two species are listed in Table 1.

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Table i. Body measurements Nyctalut noctula Mass (kg) Wing area (m1) Wing span (m) Aspect ratio Wing loading (N m"1)

Otomopt martienssem

0-027* 0-0120 0-322 8-53 22-1

O-O35

00218 0467 10-04 156

Data from Siivonen (1968).

12:0126

1:0-000

3: 0023

13:0138

7: 0069

8: 0080

11:0115 Fig. 1. The noctule bat Nyctaha noctula performing a roll through 1800. The first number indicates the frame number of the film, and the second number the time in seconds. The time between each frame is -j^ s. MANOEUVRES

Half roll Before rolling, Nyctahis flew horizontally with ca. 7-0 wing strokes 8~1. The half roll was preceded by slight flexion of the wings at the elbows and wrists (Fig. 1:7). This reduced wing area increased the wing loading and decreased the lift force. The half roll was performed in the following way.

Some advanced flight manoeuvres of bats

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'i) Pronation (nose-down rotation of the wing about its long axis) of the right pvrxng and supination (nose-up rotation) of the left wing (Fig. 1:9). The pronation decreased the angle of attack, perhaps below zero, and hence decreased the lift coefficient. If the angle of attack was negative (i.e. if the relative air flow met the wing from a direction dorsal to its zero-lift line) the lift force was directed ventrally. A supination increased the angle of attack and hence increased the lift coefficient. From Fig. 1 (10, 13) it is obvious that there was a large difference in pitching angle between the left and right wing. (2) Decreased lift force on the right wing and increased lift force on the left wing caused a roll to the right (right wing going down). There was no rotation in the yaw plane, indicating that the drag forces were about equal on both wings. (3) After half a roll (1800) the two wings were held outstretched almost symmetrically (Fig. 1:16). The concave ventral side of the wings was then turned upwards. The angle of attack of the wings was positive, which resulted in a downwardly directed lift force. This downward force and the force of gravity gave a downward acceleration which resulted in a rapid descent. (4) When the bat had descended in this upside down position, for ca. 0-08 s, the tail membrane was pouched by forward flexion of the hind legs and tail (Fig. 1:24, 26). It is presumed that the bat was intercepting an insect although it was impossible to see this on the film. The roll through 1800 was performed in o-o8-c-ios, which corresponds to an average angular velocity of 39-2-31-4 radians s - 1 during rolling. Sideslip

Before starting the sequence of sideslips, which alternated to the left and right, Otojnopsflewhorizontally at an airspeed of ca. 5 ms" 1 with a stroke frequency of ca. 8-8 s -1 . Rough estimate of the flight speed was obtained with the use of the known body length and by reference to features in the background. Filming was done under calm conditions. The manoeuvre was performed as follows. (1) The sideslip was preceded by an extremely short gliding phase {ca. 0-03 s), which started when the wings had come to the later part of the upstroke (Figs. 2:1 and 3:5). The wings were then held in a strongly flexed position, which increased the wing loading and decreased the lift force. (2) Pronation of the right wing (Figs. 2:11 and 3:5) caused a decrease of the lift and drag coefficients of the right wing. Supination of the left wing caused an increase of the lift and drag coefficients of the left wing. (3) The result of this was that the body rotated to the right in the roll plane and the nose rotated to the left in the yaw plane. (4) The bat then began to sideslip down to the right. (5) The projection of the body, on a plane normal to the incident air flow, then became larger than the greatest frontal area of the body in ordinary forward flight. Hence the body (parasite) drag was increased. (6) Increased parasite drag and the fact that the bat was also less streamlined, thus giving more separated flow, resulted in a lower lift/drag ratio, and hence in a steeper equilibrium gliding angle. The bat thus descended more steeply (i.e. lost height faster). 31

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U . M . NORBERG

40: 0-443

48:0-534

56: 0-625

Fig. a. The giant mastiff bat Otomopt martienueni performing sideslips to the right (8—23) and left (35-48). The first number indicates the frame number of the film, and the second number the time in seconds. The time between each frame is -j^ s.

Fig. 3. Sideslip of the giant mastiff bat Otomopt martienueni. The first number indicates the frame number of the film, and the second number the time in seconds. The time between each frame is -^j s.


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The duration of each sideslip period was only ca. 0*24 s. However, the bat was "sideslipping alternately to the left and right down to the cave. Each sideslip involved a roll through ca. 500 and a rotation of ca. 250 in the yaw plane. The wings were held at this 500 position in the roll plane for ca. 0-15 8 before rolling back again. This manoeuvre is known to pilots as ' fishtailing', and is an extremely efficient way of losing height rapidly. DISCUSSION

Nyctalus, 1800 roll A bat can perceive an insect by echolocation only at distances of less than a few metres. Therefore it is necessary for the bat to be able to make rapid manoeuvres in order to catch an insect. Towards the end of the film sequence Nyctalus seemed to use the tail membrane for intercepting an insect while descending upside down (Fig. 1:26). The bat descended so fast that it was lost from the field of view of the camera shortly after the half roll. Bats often use their wings and their tail membrane for insect catching. Webster & Griffin (1962) studied insect catching in bats by throwing insects in the air and filming the bats when catching them. They found that the tail membrane usually was formed into a pouch by forward flexion of the legs and tail just before an insect was caught. They also found that the bats sometimes used the outer part of their wings to convey insects to the mouth, usually by way of the pouched tail membrane. When making turns in the looping plane, the centre of the turning curve is approximately in the direction of the lift force (i.e. in the direction of the centripetal acceleration). The radius of the turn is inversely proportional to the magnitude of the centripetal force. When the bat intended to turn downwards in pursuit of an insect, it could pronate its wings sufficiently to obtain negative angles of attack (the air meeting the dorsal side of the wing), and hence ventrally directed lift forces. However, there are two reasons why a sharp turn downwards is preceded by a 1800 roll. (1) The bat wing has a chordwise camber that gives high lift coefficients at positive angles of attack (i.e. when the air meets the ventral, concave side) .The wing camber is due mainly to a ventral angling of the membrane in front of the arm (propatagium), in front of the second digit, and between the second and third digits (dactylopatagium minus), and a ventral flexion of the distal phalanges of the fourth and fifth digits (Norberg, 1972). For structural reasons this camber cannot be fully reversed. If a turn downwards is brought about by a wing pronation, the air would meet the dorsal, convex surface of the wing. Although the wing membrane would then tend to bulge the other way (ventrally), the wing would certainly give lower lift coefficients than at positive angles of attack and, hence, a turn with a correspondingly larger radius. However, by rolling through 1800, the bat can make use of the high lift coefficients associated with the wing camber and positive angles of attack. This results in the tightest downward turn possible. (2) If a turn downwards is brought about by wing pronation, the wing torque due to the inverted lift force would have to be counteracted by the wing elevator muscles. These are very weak relative to the wing depressor muscles and are not designed 32-2

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to carry heavy loads. The elevators have a mass of only ca. 20 % of that of the depress sors in Plecotus auritus (Betz, 1958). However, by rolling through 1800 before turning downwards, the bat can make use of its powerful wing depressor muscles to oppose the torque of the centripetal lift force. The load on the wings then becomes oriented in the direction that the wing skeleton and muscles are designed to carry it. The most important reason for the roll certainly is that given in (2) above. Also, with aircraft in aerobatic manoeuvres sharp turns downwards are preceded by a 1800 roll to maintain the lift force of the wings in the direction they are designed to carry the heaviest loads. Half rolls can often be seen in ducks and geese during landing descent. This is obviously done when they want to steepen the descent. Ravens use a similar manoeuvre in flight display. Oehme (1968) described 1800 rolls and flight 'on the back' in the swift, Apus apus. The rolling in Nyctalus and Apus (as described here for Nyctalus and by Oehme for Apus) is performed in the same way, but with the difference that Apus made some wing beats when flying in the upside down position before rolling back again to the normal position. On three occasions Pennycuick (1972) observed, from a motor-glider, tawny eagles, Aquila rapax, carry out a complete roll through 3600. Otomops, sideslip The manoeuvres in Otomops were not used in insect hunting but were employed to make a steep descent to its roosting cave. As with Nyctalus, the bat descended so rapidly that it was lost from the field of view of the camera soon after the start of the manoeuvre. Sideslip is an extremely efficient method of steepening the descent angle without necessarily increasing the flight speed. Other ways to steepen the gliding angle in gliding birds are to lower their feet or their feet and tail. The purpose of this, as with sideslip, is to produce additional drag but no lift. This steepens the equilibrium gliding angle without excessively increasing the flight speed. The use of the feet for gliding-angle control is especially effective in water birds which have webbed feet (Pennycuick & Webbe, 1959; Pennycuick, i960). Pennycuick (1968, 1971) also found that the feet of the pigeon, Columba livia, and the feet and tail of vultures are highly effective as airbrakes. I am indebted to Dr F. Mutere, Department of Zoology, University of Nairobi, for catching specimens of Otomops for me, and to Dr C. J. Pennycuick, Department of Zoology, University of Bristol, for commenting upon the manuscript.


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Zool.Jb (Abt. Anat.) 77, 491-526. NORBERO, U. M. (1972). Bat wing structures important for aerodynamics and rigidity (Mammalia, Chiroptera). Z. Morph. Tiere 73, 45-61. OEHME, H. (1968). Ober besondere FlugmanOver des Mauerseglers (Apiu apus). BeitrSge zur Vogelkunde 13, 393-6. PHNNYCUICK, C. J. (i960). Gliding flight of the fulmar petrel. J. exp. Biol. 37, 330-8. PENNYCUICK, C. J. (1968). A wind-tunnel study of gliding flight in the pigeon CoUtmba livia. J. exp. Biol. 49, 509-26. PENNYCUICK, C. J. (1971). Control of gliding angle in RUppell's griffon vulture Gyps rUppellii.J. exp. Biol. s s , 39-46. PENNYCUICK, C. J. (1972). Soaring behaviour and performance of some East African birds, observed from a motor-glider. Ibis 114, 178-218. PBNNYCUICK, C. J. & WEBBB, D. (1959). Observations on the fulmar in Spitsbergen. Br. Birds 5a,

321-32SIIVONEN, L. (1968). Nordeuropas ddggdjitr [Mammals of Nortliern Europe]: Stockholm: P. A. Nordstedt & SCners fflrlag. WEBSTER, F. & GRIFFIN, D. (196a). The role of the flight membranes in insect capture by bats. Amm. Behav. 10, 332-40