Download GLIDING AND FLIGHT IN THE VERTEBRATES

A.M. ZOOLOCIST, 2:161-166(1962).
GLIDING AND FLIGHT IN THE VERTEBRATES
D. B. O. SAVILK
Plant. Research Institute, Ottawa
In this paper I shall try to review the evolution of gliding and true flight, indicate
some refinements of flight in birds, and present some problems that reveal our ignorance.
Although engineers and aerodynamicists
have made some significant contributions
to our understanding of animal flight, their
work has largely been unavailable to biologists. Since flight is the most distinctive attribute of birds as a class, the small amount
of serious attention given to it by biologists
is a matter of concern. Without some understanding of flight it is hard to see how
the most can be made of studies in comparative anatomy, physiology, ethology,
ecology, evolution, and taxonomy. Some
action to remedy this situation is surely
overdue.
Aerial locomotion is conveniently treated
under parachuting, gliding, and flying, although these categories intergrade freely.
Soaring might be treated separately, but is
legitimately regarded as gliding with the
downward gradient balanced by thermal or
mechanical updrafts.
In the flying fishes (Exocoetus) we find
what Gray (1953) calls velocity gliding;
they depend on the kinetic energy of their
moving bodies. Glides of'50 yards are not
uncommon and this distance may be greatly exceeded in turbulent conditions. It has
been estimated that such glides require a
take-off speed of 30-35 mph, a speed unbelievably high for normal swimming. However, photographs show that, after the body
breaks out of the water, the tail continues
to drive the fish forward. Thus we have an
analogy to a hydroplane rising on to the
step.
I wish to acknowledge a grant from the Frank
M. Chapman Fund of the American Museum of
Natural History to defray the cost of attending the
A.A.A.S. meeting at Denver to present this paper.
All other aerial vertebrates seem to have
started as parachuting jumpers. The case
for what Heilmann (1926) called the cursorial pro-avis seems scarcely tenable; for a
substantial wing development would be
needed before it could become adaptive,
whereas the slightest fringe of feathers
would be adaptive in checking the fall of a
parachuter. Parachuting is here used in the
sense of Oliver (1951) for a reduced rate of
fall with a gradient steeper than 45°. It
involves balance but generally little or no
steering. It is clearly adaptive in protecting
from injury in accidental falls, as an escape
mechanism, and in pursuit of prey. A few
amphibians have reached this stage but
none have progressed further. It was first
reported by Wallace in a Malayan frog,
Rhacophorus nigropalmatus, to a somewhat sceptical world. This species has recently been studied and filmed by Professor
John Hendrickson of Kuala Lumpur, who
has shown that it is a competent parachuter, coming down in a stable attitude at
about 60° gradient. He states (in litt.) that
it seems to have little power of steering, but
that as it is evidently very short-sighted
such ability might have little value.
Cott (1926) made the first important
study of an aerial frog. He showed that a
Brazilian tree frog, Hyla venulosa, fell slowly, always belly down, at a gradient of
about 60°, with the legs spread laterally.
The European Hyla arborea and Rana
temporaria, of about the same size and
structurally no worse equipped to check
their fall, fell vertically, gyrating wildly and
landing heavily. Thus the first requirement
in parachuting seems to be behavioral,
through the development of attitude control, rather than structural; but it may be
noted that in Rhacophorus we do find long,
fully webbed toes and lateral flaps of skin
on the limbs.
(161)
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D. B. O. SAVILE
Among the reptiles there are several parachuting lizards and some tree snakes
(Chrysoplea and Dendrolaphis). Oliver
(1951) noted the same distinction between
parachuting Anolis carolinensis and the
fence lizard (Scleoporus undulatus) that
Cott did between his frogs. The latter fell
heavily straight down with constant thrashing. The" tree snakes check their descent by
pulling in the abdomen and flattening the
body, obviously a highly unstable aerodynamic form demanding considerable nervous and muscular control.
The only true gliders among living reptiles seem to be the so-called flying dragons
(Draco) of Indo-Malaya. Here we have a
flattened body with a wing-like membranous extension, supported by the last six or
seven ribs and capable of being folded like
a fan. Draco volans is a skilled glider that
can steer well enough to land on a selected
tree; and it must be highly maneuverable,
for Hendrickson (in litt.) reports a pair in
a courtship chase performing a full roll—a
difficult trick even under power.
The only reptiles to achieve true flight
were the Jurassic and Cretaceous pterosaurs. The wing was mainly supported by
an enormously elongated fourth digit. The
hind limbs may have been partly included
in the patagium, which would have simplified flight control. Smith (1952) has pointed
out that the relatively primitive Rhamphorhynchus had a long tail with a horizontal paddle at the tip, which was clearly
a stabilizer with a long moment arm to control pitching movements. In the advanced
pterosaurs, culminating in Pteranodon, the
tail was almost completely suppressed, as in
modern birds; this makes it clear that an
improved nervous system had allowed inherent stability to be sacrificed for improved maneuverability. A parallel change
took place early in birds. Archaeopteryx
had a long tail that was typically reptilian
except for the paired feathers along the
sides; but the Cretaceous birds had an essentially modern tail structure.
The pterosaurs were actually perhaps
only what the aeronautical engineer would
call powered gliders—sailplanes with just
enough power for brief use. Whether, as
reptiles, they had adequate energy sources
for sustained flight cannot be decided. The
lack of evidence for adequate flight muscles
is more serious. Little Rhamphorhynchus,
with a span of perhaps 4 ft, had a flimsy
pectoral girdle and a very small sternum.
Pteranodon had a larger sternum and a
strengthened girdle with a new structure,
the notarium, between the scapulae. But
Pteranodon had a body about the size of a
swan's and a span estimated at 25 ft. I suspect that most of the muscle capacity was
needed just to lift the wings as the animal
launched itself from a cliff. The notarium
perhaps served to keep the wings from lifting too far above the horizontal under the
action of air pressure as well as to anchor
lifting and trimming muscles. I cannot conceive an animal capable of beating such
wings continuously and being light enough
to fly at all. If, as their slender beaks suggest, these animals were fish-eaters, they
probably soared over cliffs like a gull and
over wave crests like an albatross or shearwater.
Among mammals we have various parachuters and gliders, but true flight is
achieved only in the bats, in which a patagium is extended by greatly elongated
radius and second to fourth digits, and embraces most of the hind legs and often the
tail. This type of wing structure precludes
the aerodynamic refinements that are found
in birds. It is thus only to be expected that
bats should be slow fliers and generally
quite small.
In some respects the gliding mammals
are more interesting than the bats. It is
clear that parachuting has developed repeatedly in arboreal mammals. Active animals that jump from branch to branch
must be partly preadapted to parachuting
through an improved sense of balance and
short reaction time. If a red squirrel (Tamiasciurus hudsonicus) is shaken out of a high
tree it spreads its legs and comes down in
GLIDING AND FLIGHT
the familiar attitude of the flying squirrels.
At first it falls almost vertically but later it
swerves away at an angle of perhaps 60°
and lands quite lightly. The stretched skin
between limbs and flank provides enough
surface to check and deflect the animal appreciably. Thus any small mammal that
can control its attitude is the potential ancestor of a glider. Parachuting must be distinctly adaptive to many active arboreal
mammals; and the next step, the development of a true patagium, is a simple adaptation that has occurred several times. Thus
we have the cobegos (miscalled "flying lemurs"), the "flying" squirrels, and, in particular, numerous "flying" phalangers.
Some of the latter, such as the flying opossum, look astonishingly like a flying squirrel, one of the many marked instances of
convergence between marsupials and placentals. In some of the smaller phalangers
there is no appreciable patagium, but only
a flattened body and flattened tail—these
seem to be adequate in a very small, and
consequently light, animal. Most of these
gliding mammals can evidently land on a
selected tree trunk far from the point of
take-off, indicating ability both to steer and
to pull up into a stall just before striking
the tree.
The flight of birds is interesting both because it is almost universal in a large class
and because of its extreme perfection. The
perfection is due mainly to the development of feathers, which allow a mechanical
and aerodynamic refinement never achieved
by other means. Mechanically, feathers
have great resilience, graded flexibility, and
high strength to weight ratio. Aerodynamically, they provide smooth contours and
streamlined sections; permit the development of slotted wings, which produce high
lift at low speed; and allow birds to achieve
laminar flow.
To appreciate these aerodynamic points
we must sketchily review the properties of
an airfoil, which is a thickened, generally
curved plate of streamline section. Fig. 1
shows the section of a low-speed high-lift
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airfoil such as a bird wing. When such a
foil strikes an airstream at a slight positive
angle some lift is produced by positive pressure beneath the foil, but much more is
caused by negative pressure above it. The
airfoil is, in effect, a venturi (or half venturi), like the throat of a carburetor, filter
pump, or atomizer. Air passing through a
throat must increase in velocity; and, according to Bernoulli's theorem, this increased velocity must be accompanied by
decreased pressure.1 If the angle of attack
of the airfoil is slightly increased we get
added lift but also added drag. Too great
an increase in the angle of attack causes the
airstream to break away from the wing in
violent eddies. This leads to stall, whereby the wing abruptly loses almost all lift.
The stall can be delayed by forcing air over
the wing at increased velocity in a flat jet
(Fig. 2). This is achieved by means of a slot
—the venturi once more. Two types of slot
are found in birds. The alula forms a midwing slot; and emarginated primaries form
wing-tip slots. Fig. 3 shows the plan of the
wing of a catbird (Dumelella carolinensis),
which is strongly slotted. The slots of large
birds such as vultures, buteos, pelicans and
crows are open in level flight; but in small
birds they are used mainly in take-off and
landing.
If we have negative pressure above the
wing and positive pressure below it, air will
inevitably spill up over the wing tips in
vortices, causing an unavoidable induced
drag. This is essentially a loss at the wing
tip. The upper aircraft in Fig. 4 has an aspect ratio (that is, ratio of span to chord)
of 5. Let us arbitrarily suppose the shaded
portion to be ineffective due to this tip effect. If we double the span and halve the
chord as in the lower plane, giving an aspect ratio of 20, we clearly increase the effective proportion of the wing; thus induced drag varies inversely as aspect ratio.
1
Bernoulli's Theorem: At any two points along
the same .streamline in a nonviscous, incompressible
fluid in steady flow, the sum of the pressure, the
kinetic energy per unit volume, and the potential
energy per unit volume has the same value.
164
D. B. O. SAVILE
Turbulent
Laminar
Flow
Flow
A.R. = 20
FIGS. 1 to 5. FIG. 1. Low-speed airfoil at shallow
angle of attack, showing pressure distribution. FIG.
2. Aerodynamic slot in operation. FIG. 3. Highly
slotted wing of catbird. FIG. 4. Effect of increased
aspect ratio in increasing lift; shaded portion of
wing is ineffective. FIG. 5. Distinction between
turbulent and laminar flow.
If the habitat permits it, a high aspect ratio
is generally adopted, the ultimate example
being the oceanic soaring birds. In land
birds of closed habitat extensive slotting is
used as a substitute for high aspect ratio;
but the aspect ratio is always increased
when conditions permit.
It is now becoming clear that the feathered wing gives birds a further aerodynamic
advantage almost certainly denied to all
other flying animals, and one that has only
recently been achieved in aircraft.
Under most circumstances a series of eddies
forms between the wing and the airstream,
as in the left diagram in Fig. 5. This is
turbulent flow, in which skin friction is so
high that it forms a large proportion of the
total drag. Under some conditions it is possible to achieve laminar flow, in which a
layer of gas molecules is held against the
wing and successive layers slide smoothly
over them like the cards of a deck being
swept across a table (Fig. 5, right). Usually
the first requirement for laminar flow is a
very smooth surface—at least as smooth as
writing paper. Their relatively rough wing
surfaces seemed to preclude all possibility
of birds achieving it. However, laminar
flow may also be achieved through the process of boundary layer control. This mathematically complex process can be achieved
by drawing in air through the upper surface and releasing it from the lower surface.
Raspet (I960) has presented evidence, from
power availability studies, that this must
actually occur in bird wings, a contention
supported by the observation that the flight
feathers are about seven times as porous to
downward-moving as to upward-moving
air. These findings may explain the pronounced sweepback that has occurred repeatedly in the wings of fast birds. If laminar flow occurs over the wings and turbulent flow over the body, a reduction in drag
presumably results from shortening the
GLIDING AND FLIGHT
body and tail and sweeping back the wings
to stabilize against pitching.
I have elsewhere (1957) discussed wing
actions and adaptations; most other aspects
of flight are qualitatively explained in
terms of aircraft aerodynamics. The little
remaining space will therefore be given
over to discussing some unanswered quantitative problems that must engage the attention of those who believe that taxonomy
should reflect the whole biology of the organisms studied.
Hamilton (1961) has recently discussed
some of these problems, and I know, from
correspondence, that various workers in
Europe and North America are concerned
about them. Here is the sort of problem
that we may run into. Bergmann's rule
tells us that subspecies at high latitudes are
larger than those at low latitudes. This is
commonly interpreted as an adaptation for
reducing heat loss, the ratio of surface to
volume falling with increased body size.
How do we measure body size? Often we
rely on the length of the bent wing, which
may represent merely primary length and
not size and weight of body. Hamilton has
suggested that humerus length be used as
an index of body size. Even when the body
really is larger the wing may be disproportionately longer, keeping wing loading
nearly constant. Races breeding at high
latitudes often make long migrations, which
may make a longer wing adaptive. How
great an increase in wing area or length
will be adaptive in facilitating take-offs or
long migrations for races with a given increase in body size? If we managed to measure power consumption in take-off or level
flight accurately we might be able to come
close to answering such questions.
Another vexing set of questions concerns
adaptation to changes in altitude. Mountain races are subjected to low temperature,
which tends to encourage an increase in
body size. They also operate at reduced atmospheric pressure, which may make an increase of wing area adaptive (to keep lift
nearly constant) even if body size does not
165
increase. Can we be sure that reduced air
pressure does not sometimes impose respiratory limitations that would make a larger
wing inadaptive? There may also be
changes in habitat density, which will affect
the permissible wing changes. If the birds
inhabit subalpine parkland the wings are
likely to become relatively long; if they inhabit dense krummholtz the wings will
probably be short and added lift must then
come from increase in the wing chord or
from increased slotting; but only a change
in length will be revealed by the orthodox
bent wing measurement.
If we accept the standard atmosphere of
the International Commission on Air Navigation as representing pressure change with
altitude, we can say that at 5,000 ft the wing
area must increase by 16% over that at sea
level, and at 10,000 ft by 35%, to maintain
equal lift. If wing shape is unchanged the
spans will increase by 7.8 and 16.3%. As
most of this increase is likely to be in the
length of the primaries, the bent wing measurement could show a substantial altitude
effect of this sort; but we must obviously be
cautious about assuming that a change is
actually an air pressure effect. Even if none
of the other factors alter wing size, the altitude effect might produce a slightly smaller
increase in area than expected. The larger
wing may be slightly more efficient because
it has less edge in proportion to area than
the smaller one, and hence less turbulence
at the edges. Moreover, changes in wing
size will be mainly advantageous at takeoff; for drag, as well as lift, is reduced at
altitude. Thus there might be little or no
increase in wing area of sedentary ground
feeders, but a substantial increase in very
active insectivorous birds like the wood
warblers that make thousands of take-offs
each day. Obviously we dare not draw
sweeping conclusions without considering
the whole natural history of the species.
REFERENCES
Cott, H. B. 1926. Observations on the life-habits
of some batrachians and reptiles from the lower
166
D. B. 0. SAVILE
Amazon: and a note on some mammals from
Marajo Island. Proc. Zool. Soc. London, 2:11591178.
Gray, J. 1953. How Animals Move. Cambridge
Univ. Press.
Hamilton, T. H. 1961. The adaptive significance
o£ intraspecific trends of variation in wing length
and body size among bird species. Evolution 15:
180-195.
Heilmann, G. 1926. The origin of birds. H. F. and
G. Witherby, London.
Oliver, J. A. 1951. "Gliding" in amphibians and
reptiles, with a remark on arboreal adaptation in
the lizard Anolis carolinensis carolinensis Voigt.
Am. Nat. 35:171-176.
Raspet, A. 1960. Biophysics of bird flight. Science
132:191-200.
Savile, D. B. O. 1957. Adaptive evolution in the
avian wing. Evolution 11:212-224.
Smith, J. M. 1952. The importance of the nervous
system in the evolution of animal flight. Evolution 6:127-129.