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Strategies for
Survival
PAGE 3
PAGE 1 - MATE RECOGNITION
PAGE 2 - CAMOUFLAGE and
DISGUISE
PAGE 3 - APOSEMATIC
COLOURATION
PAGE 4
- MIMICRY
PAGE 5
- SEXUAL DIMORPHISM
PAGE 6 - ROOSTING BEHAVIOUR
PAGE 7 - SEASONAL DIMORPHISM
Aposematic
coloration
bright colours and patterns to
warn, startle, distract and confuse
Easily remembered patterns
Altinote dicaeus callianira - it's
distinct pattern advertises it's unpalatability to avian predators
Some butterfly
species are distasteful or toxic to birds. Such species invariably
tend to "advertise" their toxic properties by the use of
eye-catching and easily remembered patterns, usually comprising of
bright orange markings on a contrasting black or white ground
colour.
Any bird that attempts to eat one of these
species will find the experience extremely unpleasant, and is likely
to suffer an immediate attack of vomiting.
Having tasted such a
butterfly, the bird will quickly learn to associate the colour and
pattern with the unpleasant experience, and will avoid eating
similarly coloured butterflies in the future.
Tithorea harmonia, a conspicuously
marked and highly unpalatable species from Peru.
Ocelli
Many
species, such as the Peacock Inachis
io are
marked with conspicuous ocelli
( false eyes ) which can scare off a predator, or at least deter it
long enough for the butterfly to make it's escape. The ocelli make
the butterfly appear larger and "scarier" to predators, and in
many cases have the effect of simulating the face of a small
mammal or reptile.
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In a study by Stockholm University, the ocelli of 20 Peacock
butterflies were blanked out with a marker pen. When exposed
to blue tits, 13 of them were attacked and eaten. A
control group of 34 Peacocks with intact ocelli fared
much better - only a single butterfly was attacked.
It can be concluded that in 97 percent of encounters with
blue tits, the ocelli are effective as a deterrent, and
the butterfly will escape unharmed.
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Inachis io
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very
beautiful to human eyes, but frightening to a
small bird.
Ocelli
can
also serve to divert and confuse predators :
If a bird
attacks a butterfly, it naturally focuses it's aim on an obvious "target".
The presence of a decoy target such as a "false eye", diverts the attack away from the
butterfly's body, and towards the wing borders. Usually this results
in nothing more than a small chunk being pecked out of the wing, and
the insect escapes virtually unharmed.
It is common to find ocelli-marked butterflies with chunks taken out
of their wings by birds. Butterflies are able to fly and go about their lives with quite
large chunks
missing from their wings, but an attack on their body would be
fatal.
Sometimes the ocelli are huge, as in the neotropical Owl
butterflies (
Caligo sp. ), but even the small ocelli on
species such as the Gatekeeper Pyronia tithonus are enough to divert a bird attack away
from the butterfly's body.
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Caligo
idomeneus,
Venezuela
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Chloreuptychia marica, Peru
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Decoys to distract
birds
The White-letter Hairstreak
Satyrium w-album and many other
Lycaenid butterflies have wings that are
marked with bright streaks that lead the eye away from their head, and
towards "tails" on their hindwings. The tails simulate antennae
and often there is a red or black spot near the tail,
which simulates an eye.
The effect is to create the illusion of a "false head", and to give the butterfly a
back-to-front appearance. This is further enhanced by the
butterfly's habit of immediately turning to face the other way as
soon as it lands on a flower or leaf. It is also likely to dip it's
real head, and raise the false head. Periodically, it oscillates the
hindwings, causing the "false antennae" tails to wriggle.
An attacking bird will try to anticipate the escape route of it's
prey, and aim it's attack at a point fractionally in front of the
head. The "false head" fools it into aiming behind the butterfly
instead. The butterfly then darts off in the opposite direction to
that which the bird expects, and makes it's escape.
Satyrium w-album
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false antennae and false eyes act as decoys.
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Patterns to confuse
Many butterflies have patterns which at first
glance seem to serve no purpose. What for example would be the point
of a striking and easily remembered pattern such as the checkerboard
appearance of the Swallowtail
Papilio machaon
? One explanation could be that the butterflies might be
toxic or unpalatable, and that the patterns act to warn off birds,
as in the case of the
Altinote
and
Tithorea
species illustrated at the top of this page.
Watching the behaviour of butterflies often
helps to reveal the answer to such riddles. The Swallowtail normally
rests with it's wings closed, but if it is disturbed it suddenly
flicks them open in exactly the same manner as adopted by the
Peacock and other ocelli-equipped species such as Bull's-eye
Silkmoths or Eyed Hawkmoths. Furthermore, when alarmed the butterfly
often moves the outspread wings in a jerky and almost threatening
motion, as if to deliberately draw attention to itself.
It seems likely therefore that the pattern
acts either to make the butterfly appear too large to eat, or that
it simply confuses the bird - causing it's eyes
to wander all over the pattern while the bird tries to fathom out
what it all means - Is it edible ? Is it dangerous ? Is it small
enough to eat ? Which bit of it should I aim my beak at ? The bird
may be so confused that it decides to abandon the attack, or the
attack may simply be delayed long enough to allow the butterfly to
escape.
The purpose of the crimson ocelli on the
hindwings remains a mystery. They are not big enough to be scary,
and their positioning close to the vulnerable body effectively rules
out any possibility that they act as decoys or targets. The
possibility that they serve some purpose in mate recognition is also
ruled out, because the patterns of male and female Swallowtails are
identical.
The Swallowtail Papilio machaon flicks
it's wings open in a threatening manner if alarmed
Signalling danger to fellow butterflies
Warning colouration is generally assumed to be
targeted directly at birds and other predators, but in the case of
Panacea prola from Peru the purpose is quite
different :
Large groups of males gather to feed on
mineralised moisture on riverbanks. They bask when feeding,
displaying their metallic blue uppersides, which help other passing
males to locate them and home in on the feeding grounds.
A group of butterflies on the ground could easily
be attacked by birds however, so the butterflies need to employ a
defence strategy. Any kind of minor disturbance causes one or two
butterflies to get nervous, at which point they start fanning their
wings so that the bright red undersides are visible to the other
butterflies.
Increased levels of nervousness cause them to fan
their wings faster, which attracts the attention of further basking
males, which join in the fanning activity. This "red
for danger" signalling acts as a very
effective alarm system, quickly alerting all
Panacea
in the vicinity to any
perceived threat, and allowing them time to take
evasive action before the threat becomes severe.
Panacea prola
basks
while feeding on mineralised water on a Peruvian riverbank.
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Panacea prola
group wing-fanning to warn each other of potential danger. |
Flash
colouration
South American
Morpho butterflies have
brilliant iridescent blue uppersides, making them highly visible to
predators as well as to potential mates. If alarmed, they
immediately land, snapping their wings shut so that only the dark
brown underside is visible. This alternating "now you see me, now
you don't" display of brilliant uppersides and sombre underside is
known as flash colouration.
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