Two theories have been put forward to explain the
evolution of flight (flight refers to powered, flapping
flight, as distinct from gliding): the arboreal and the
cursorial. The arboreal theory for the evolution of
bat flight was probably put forward first by Darwin
(1859), and has been elaborated many times since.
Put simply, tree or cliff dwelling ancestors evolved
flight through an intermediate gliding stage. The
cursorial theory is more recent, and has not had a
wide following, but Caple et al. (1983) formalized it,
and raised numerous objections to the arboreal theory.
These objections have been effectively countered by a
number of authors (for example
Norberg 1985, Rayner 1992), and the arboreal or
gliding theory was widely favoured, but recent
bird fossils have revived interest in the cursorial
theory. Hedenstrom et al. (2009) provide an up-todate
review comparing flight in bats and birds,
including evolutionary aspects. In birds, there are
reasons for considering the cursorial theory (Rayner
1986, Hedenstrom 2002), but it can be discounted
when considering bats. The cursorial theory demands
that the animal runs and leaps, gaining sufficient speed
to get lift from its outstretched wings to
glide, and then fly. There is little doubt that our
protobat was unable to run at any speed! In the
absence of an abundance of evidence from the fossil
record, work on the evolution of flight in bats (and
birds) has concentrated on a study of living animals,
often from a biomechanical viewpoint. Let’s take a
look at the idea that bats evolved from a gliding
ancestor, and see why it is the preferred option. A
very concise and readable account has been given by
Rayner (1992).
First of all, if the ancestral bat is going to glide, it
has to be able to climb, to gain height for its jump.
When an animal climbs a tree its muscles do work
and use energy. Some of this muscular energy is
converted into potential energy (potential energy =
the mass of the animal X its height above the ground
X acceleration due to gravity). The higher it climbs,
the more energy it expends, but the more potential
energy it gains. This potential energy is used by the
animal to glide. When the protobat jumps (Fig. 2.8),
gravity pulls it to the ground, and it loses potential
energy.
Gliding is powered by potential energy: when the
protobat opens its wings as it falls, they generate a
force which counteracts the force of gravity acting
on its body, holding the protobat up. In other words
the wings act as an aerofoil. However, as the aerofoil
moves through the air, there is a backwardly
directed force (drag) as well as the upward force
(Fig. 2.8 and see previous section). Drag resists the
forward motion of the animal, and slows it down.
A gliding animal maintains speed because it is falling.
The energy needed to overcome drag and maintain gliding
speed comes from the loss of potential
energy as the animal approaches the ground. Good
gliders have good aerofoils, with little drag. They
can glide a long way because only a little potential
energy (height loss) is needed to overcome drag, so
less height is lost over a given horizontal distance:
their glide angle is small (Fig. 2.8).
So, we have established that a glider and future
flyer needs to be able to climb. We do not need to
look very far to find evidence which suggests that
our protobat, a small mammal, could climb: the
world is full of very agile mammalian climbers of
all sizes. What does it need to be able to glide?
A gliding surface or aerofoil, and the strength to
hold the aerofoil open. Rayner (1992) has calculated
that the muscular strength required to hold the aerofoil
open is much less than that needed for climbing,
so a climber can glide if it has an aerofoil. Aerofoils
and gliding have evolved at least once in every
vertebrate group, and several times in most. There
are gliding fish, frogs, lizards, snakes, and mammals,
and some of their aerofoils (pectoral fins,
webbed feet, extended ribs, flattened bodies, flaps
of skin between the fore and hindlimbs) differ little
from the standard anatomical plan of their nongliding
cousins. The protobat would have little difficulty in
becoming a bad glider as the skin flap
between fore and hindlimbs developed, and from
there, a better one.
How was the aerofoil’s performance improved?
First of all, the amount of lift generated increases as
the area of the aerofoil increases. More lift means
slower, safer speeds, and longer glides, so an
evolutionary selective pressure for increased wing area is
understandable. Another factor is wing shape. Modern
mammalian gliders such as flying squirrels and
marsupial sugar gliders have aerofoils of low aspect
ratio (Fig. 2.9).
Aspect ratio is a measure of wing shape, calculated
as the length from tip to tip (span), divided by
the width, front to back (chord). For the oddly
shaped wings of animals, a better measurement is
wing span squared divided by area. Low aspect
ratio wings generate lots of drag, the lift to drag
ratio is high, the glide angle is steep, and the glide
therefore short (Norberg 1985). The length of the
glide can be increased by lengthening the wing,
thus increasing aspect ratio, which increases the lift
to drag ratio. Although the area of the gliding
membrane can be increased, there is little scope for
increasing aspect ratio without significant modification
of the limb skeleton. If the gliding membrane
extended to between the fingers, then simple elongation
of the fingers could increase aspect ratio, and
this was probably a very important step in the evolution
of powered flight. Not only would it allow an
increase in aspect ratio, but adjustments to the flight
path could also be made by movement of the fingers
to change the shape of the aerofoil. Modern-day
flying lemurs (dermopterans) have webbed hands
with short fingers—a step away from a longfingered flying
bat. Sears et al. (2006) have shown
that the lengths (relative to body size) of the third,
fourth, and fifth fingers in bats have remained similar in
length for the last 50 million years. They show
that the critical step, occurring at an early stage in
the evolution of bats, was probably an increased
local expression of a single protein, Bmp2, which
causes the proliferation of bone-forming chondrocytes and
an elongation of the finger bones.
So, there’s no reason why a gliding protobat
should not have evolved, and there are several
good reasons why it should. A single tree would
have been unlikely to provide our protobat with all
of its food, or cater for other needs, such as a safe
shelter and mates, and like many modern mammals
it would have moved from tree to tree. It had two
choices, jump from one tree to the next, or climb all
of the way down, cross the ground, and climb back
up again. The latter is an expensive option: climbing
down can use as much energy as climbing up. Gliding on
the other hand is cheap. A good glider, like a
colugo, can travel over a 100 m, lose only 10 m
height, and expend almost no energy. Gliding is
also fast: five or more times faster than running on
the ground (Rayner 1986, Scholey 1986). Although
the cost of climbing is high, the high gliding speeds
mean that the overall cost of transport (cost per unit
distance travelled, or unit time travelling) is low. A
small animal on the ground could pay the ultimate
cost—its life—if caught by a ground predator.
A jump, or better still a glide from tree to tree, is a
good way to avoid a more agile predator. Lots of
animals have learnt to move through trees by jumping,
and many spread their limbs for stability and
for landing. It is a small step from there to gliding.
The next step is to active, flapping flight. Rayner
(1986) has calculated that flapping the wings over
an amplitude of 25º or more, with flexing on the
upstroke, is sufficient to generate useful thrust to
assist gliding, and that a climbing animal could
supply the appropriate energy: the way to flapping
flight is therefore open. A detailed aerodynamic
model for this transition has been developed by
Norberg (1985, 1990).