While many insects are fair game for bats, not all
of them are easy to catch; some moths have evolved remarkable
strategies to evade bats. . .
While many insects are fair game for bats, not all of them
are easy to catch; some moths have evolved remarkable strategies
to evade bats. . .
Before the appearance of bats, more than sixty million years
ago, the night was a relatively safe time for insects to move
about, free from most hungry birds and mammals. The eyes of most
nocturnal vertebrate predators were poorly designed for the dark,
and as a result, many insects like moths, caddisflies, and
beetles evolved a nocturnal lifestyle.
Staying alive during the day is a complicated affair for
insects. Many use visual defenses, such as camouflaging
themselves or packing their bodies with noxious chemicals and
then advertising their distastefulness with bright colors. One of
the best ways of remaining uneaten is simply to be very still.
This tactic, however, presents problems when it comes to finding
food, mates, or places to rear young. Becoming nocturnal
initially solved these problems, but once bats evolved their
sonar into the sophisticated tool it is, the night was no longer
Different insects adopted different strategies to cope with
such efficient nocturnal predators. Some insects moved back into
the day, and others remained nocturnal, but evolved a new sense:
a way of hearing the echolocation calls of hunting bats. The
anti-bat ear was born. Today, ears that serve as bat detectors
are found in certain moths, lacewings, and preying mantises and
are suspected in some beetles. The ears of moths were the first
to be discovered and still amaze us with their exquisite design.
Depending upon what family they belong to, moths have ears on
their waists, their abdomens, or their faces, and they equip them
with only one to four nerve cells. Our ears, by contrast, have
over 15,000 sound receptor cells each, but we use our sense of
hearing for more complicated tasks. When we listen, our ears pick
out thousands of subtle changes in the pitch of the voices we use
When it comes to social communication, moths, for the most
part, have little use for sophisticated ears. They usually
communicate to members of the opposite sex, or to others, using
chemicals called pheromones. Some moths do use sounds in their
mating behavior, but the relative rarity of this suggests that
bat-detection is the most common purpose for their ears. The
nonsocial use for hearing has resulted in moth ears being
tone-deaf, though this limitation apparently has not hindered the
moths much. Moths that can hear the approach of a bat and respond
to its attack stand a 40% less chance of being consumed.
It was the work of Kenneth Roeder and his students and
colleagues (notably Asher Treat) in the 1950s and 1960s that
provided the initial insights into the hidden lives of moths and
bats. Since then, researchers around the world have continued to
fill in the picture.
This is what we think is happening. When a flying moth first
hears a bat's echolocation calls, the sounds are faint and affect
only one of the moth's ears. This lets the moth know from which
side the bat is approaching and how far away it is. The moth acts
on this information by adjusting its wingbeat to fly away from
the bat. This early-detection system works well for the moth
because its ears are so sensitive. North American moths can hear
the sonar calls of a big brown bat (Eptesicus fuscus), one
of their most common predators, when it is almost 100 feet away.
Even the most optimistic echolocation researcher will admit this
is much further than the distance at which the bat hears its own
echoes from the moth. The moth can out-hear the sonar system of
the bat in much the same way as a radar detector in your car
warns you of a police car in wait.
Why then, do any moths ever get eaten? This dilemma always
confronts people working on evolutionary questions: If a defense
is so good, why does it sometimes not work? The answer is in the
unpredictability of the real world. The military hardware that
works superbly on the drawing board often fails when it has to
perform in the uncertain conditions of a battlefield; similarly,
the defenses of the moth are also imperfect. For instance, when a
bat feeds around trees, the trees become obstacles to the moth's
ability to hear the bat's echolocation signals. The moth,
therefore, may not have early warning of the bat's approach and
must use short-range defenses. For this purpose, some moths have
a back-up system in the form of a single nerve cell in each ear
that alerts the moth only to very close bats.
One of the ways that this cell alerts the moth that a bat is
close is based on the kind of sonar the bat uses. When a bat
begins its final approach, it speeds up its rate of echolocation
pulsing (often called a "feeding buzz"). This sonar
gunfire appears to activate a special part of the moth's flight
circuitry, which instructs the moth to begin a series of wild,
looping aerial maneuvers designed to out-fly the heavier and less
agile bat. If these aerial acrobatics fling the moth outside of
the bat's narrow echolocation beam, the bat seems to give up its
pursuit to search for less troublesome prey. If, however, the bat
locks onto the echoes off its intended prey, real problems begin
for the moth since the bat's greater speed works in the bat's
The final tactic for the moth is to fold its wings and dive
toward the ground. Some moths have more elaborate defenses.
Certain tiger moths have one last salvo they can launch at the
bat. At the last possible moment of the bat's final attack phase,
when it is very close and flying at its highest speed, the tiger
moth blasts back streams of high-pitched clicks. Some scientists
think these sounds are advertisements of the moth's bad taste,
while others feel the clicks are the moth's way of saying
"Boo!" to the bat and startling it. We will never know
exactly what goes on in the mind of the bat during these critical
last few milliseconds in pursuit of a meal. More often than not,
the harried tiger moth flies away, alive, to resume its search
for mates or places to lay its eggs.
The evasive flight of the moth, as effective as it is, may not
be the last word in this aerial warfare. Some tropical bats,
especially those of the Old World leaf-nosed family, emit
echolocation calls so high-pitched that even the sensitive ears
of moths cannot detect them. This kind of echolocation may
function mainly to allow the bats to avoid collision within dense
vegetation, but another advantage to this sonar is that it has
rendered them acoustically invisible to moths. What, if anything,
a moth does to defend itself from these chiropteran stealth
fighters is unknown.
In our lab we are particularly interested in how bat-detector
ears evolved. As effective as ears that can detect bats are, some
moths do not have them, and how they avoid bats is a mystery.
Luna and Cecropia silk-worm moths, for example, may simply rely
on their large size to protect them from most bats. One of my
students, Jayne Yack, found that earless moths in the upper
Northeast region of North America emerge as adults very early in
summer, before most bats have returned from their overwintering
hibernation sites. Moths have a very short adult life span (they
are essentially flying reproduction machines), and limiting their
flying life to times when bats are not active may be one way to
live without ears.
Another student, Scott Morrill, tested Ken Roeder's idea that
deaf moths co-exist with bats by flying low to the ground away
from where bats normally hunt. Scott found that deaf moths,
overall, fly less often and when they do, they fly in areas that
bats generally avoid, such as deep forests.
Other possible defenses exist for earless moths. Some moths
are almost as large as the bats that hunt them and could, in an
aerial dogfight, pose serious problems to a bat's ability to stay
aloft. Since it is very likely that the echolocation signals bats
emit can determine the size of their prey, they may simply ignore
large moths. These moths may not have been subjected to the
evolutionary pressure to evolve a way of detecting bats. Of
course, not all echolocating bats are small; some species in the
tropics have wingspans close to foot and a half. What deaf moths
do to avoid these giants is yet another tantalizing question.
One type of bat that poses real headaches for moths is the
kind that has evolved faint echolocation calls. Gleaning bats
snatch their prey either directly from vegetation or from the
ground instead of while in flight. Early investigators called
them "whispering bats," because their echolocation
calls were so faint. Gleaning bats hunt by using the sounds
insects make as they walk among foliage, fan their wings, or sing
for mates. We have found that moths in north temperate areas
likely never hear the ultra-quiet, high-pitched sonar emitted by
such bats as they fly in for the kill.
Studying the simple ears of moths and how they have been
modified to meet the needs of the moths that possess them can
tell us much about the mechanisms of evolution. The simple, yet
critically important, behaviors these sensory structures govern
in moths can give us insight into the ways in which our own
nervous systems work. Ultimately, though, the most fascination
comes from simply watching the life and death struggles of moths
and bats as they try to out-maneuver each other. It tells us that
mortal battles rage continuously in the natural world of
animals--a reminder, perhaps, that our own warfare may be only a
modern version of these evolutionarily ancient conflicts.
James H. Fullard is an Associate Professor of Zoology at the
Erindale College of the University of Toronto in Ontario, Canada.
He has studied the interactions between bats and moths in North
America, Central America, Africa, Australia, and the South
To avoid being eaten by bats, some moths have evolved
sensitive hearing that warns them of a bat's approach. Still,
even the best defenses are imperfect and many moths are eaten.
"Whispering bats" pose a real
problem for moths trying to avoid them. Even though the moth has
ears, it could not hear the faint echolocation calls of the bat
and was captured.
North American moths can hear the sonar calls of a big brown
bat (above) when it is as far away as 100 feet, much further than
the distance the bat hears its own echoes from the moth. In
contrast, some Old World trident-nosed bats (below), hold the
record for high-pitched echolocation calls at 212 kHz. Even the
sensitive ears of moths cannot hear them and, consequently, a
high proportion of moths are included in the diets of these bats.