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Strategies for
Survival
PAGE 4
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
Mimicry
As discussed
previously, unpalatable or toxic butterflies often have prominent
patterns. Experiments have shown that some avian predators can
memorise these patterns and learn to avoid eating similarly
patterned species in the future.
Batesian mimicry
The 19th century
naturalist Henry Walter Bates realised that many edible species had
uncannily similar patterns to unrelated toxic species. An example is
the male of the Amazonian species Patia
orise ( Pieridae ), which bears a quite extraordinary
resemblance to the toxic Glasswing Methona
confusa ( Ithomiinae ). The female of
Patia orise is totally different in
appearance, and bears a strong resemblance to another toxic species
Ituna ilione ( Danainae ).
Bates published a
scientific paper in 1862, in which he theorised that
palatable species occasionally produced mutant forms with visual
characteristics similar to toxic species. He believed that they
would therefore be less likely to be killed by birds, and would pass
on their characteristics to their offspring. He proposed that as a
result of further mutations over the millennia, that palatable
species had evolved to become almost identical to the toxic species.
Perhaps the most
commonly quoted example of
Batesian mimicry is the Viceroy
Limenitis archippus
which is said to mimic the Monarch
Danaus plexippus.
There are a myriad of other examples such as the palatable
Dismorphia
and
Heliconius
species which mimic toxic Ithomiines; and the palatable Spicebush
Swallowtail
Papilio troilus
which mimics the toxic Pipevine Swallowtail
Battus philenor.
Müllerian mimicry
In
1879, Müller realised that many unpalatable species shared very
similar patterns. He proposed that this was a form of evolutionary
"cooperation". It spreads the chances of
any
one butterfly being eaten over a larger number of species, and
over a larger number individuals within a species. When a bird
catches any one of these individuals, it quickly learns to keep
away from all the species within the group.
This is referred to as Müllerian mimicry, and is a very common
phenomenon amongst the Ithomiinae, Danainae and Pieridae.
In cases of Müllerian mimicry, all the
species involved are unpalatable or toxic to birds and other
small vertebrates, and
the commonest species in the group serves as the model for the
others.
A well known example is Perrhybris pamela
- on
the upper surface of the wings the males are white with a black
apex, but the females are entirely different, patterned with bands
of orange, yellow and black.
They are generally regarded as Müllerian mimics of the
"tiger-complex" Ithomiines in the genus
Mechanitis.
It
is important to realise that while the Batesian, Müllerian and
more recent theories of mimicry are widely accepted as being
sound, very little experimental study has been carried out to
determine whether the supposed "models" are all unpalatable or
toxic. Equally little work has been carried out to test the
supposed palatability of Batesian mimics. Much is based on
untested assumption.
What
is increasingly being understood is that mimicry groups cannot be
simply split into distasteful "models" and Batesian or Mullerian
"mimics". There is actually a very broad spectrum of palatability
from species to species, and within any given species. Many butterflies only become unpalatable after
sequestering toxins from plants, so when they first emerge they
are palatable, but after a few days feeding they become
unpalatable. Birds and other predators must also vary considerably
in what they find palatable - a tanager for example might be find
a particular species or individual of butterfly to be unpalatable,
but a jacamar might find it quite acceptable.
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Methona
confusa is considered to be an unpalatable
species. It is a Nymphalid from the subfamily
Ithomiinae. The butterfly contains toxins derived
from it's larval foodplants, and from adult food
sources such as
Heliotropium flowers.
A bird that has tasted this species is unlikely to
attack anything that looks similar.
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Patia
orise is a remarkably similar
Pierid species from the subfamily Dismorphiinae. Clearly
there is some sort of mimicry going on, but little or no
work has been carried out to determine the degrees of
unpalatability of either species, so it is uncertain
whether Patia
is a Batesian or Müllerian mimic.
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The Tiger Complex
A famous example of butterfly mimicry is the "tiger complex" - a
group of about 200 mostly toxic species which all share a very
similar pattern of orange and yellow stripes on a black ground
colour. Any bird which suffers the unpleasant experience of
tasting one member of the tiger-complex quickly learns to avoid
attacking any similar looking species.
The tiger complex includes the toxic Ithomiines
Tithorea harmonia,
Melinaea marsaeus &
Forbestra equicola;
toxic Danaines
such as
Lycorea pasinuntia, and
smaller numbers of species considered to be palatable, including
Perrhybris pamela ( Pierinae ),
Eresia eunice ( Nymphalinae ),
Heliconius ismenius
( Heliconiinae ),
Stalachtis calliope ( Riodinidae ),
Pterourus zagreus (
Papilioninae ), and
Consul fabius ( Charaxinae ).
Members of the tiger complex
habitually aggregate in large numbers in damp shady gullies in the
forest at the end of the dry season, and it is at this time, when
they are docile and easy prey for birds, that mimicry has its
greatest potential as a defence mechanism. A bird which samples a
single unpalatable species is likely to leave the rest of the
aggregation unmolested, and may even be capable of communicating
their distasteful nature to other birds.
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Heliconius numata
HELICONIINAE
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Tithorea harmonia
ITHOMIINAE
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Melinaea menophilus
ITHOMIINAE
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The
species in this group share a similar "tiger" pattern
of orange bands or stripes on a blackish ground colour.
All also have elongated wings,
creamy antennae, and a habit of fanning their wings to
draw attention to themselves - advertising their toxic qualities to predators.
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Wasp and bee mimicry
Many
day-flying moths in the family Sesiidae have small transparent
wings, and bodies banded in yellow and black. They bear a remarkable
resemblance to wasps, and
almost certainly escape predation as a result of their similarity
to these stinging insects. A
few, such as the Hornet moth Sesia apiformis
are such convincing mimics of wasps and hornets (
Vespa sp ), that most people would
avoid getting near them for fear of getting stung.
In
the neotropical region there are dozens, possibly hundreds of
Arctiid moth species which have transparent wings, and boldly
patterned red, orange, or yellow and black bodies. Most of these
species are nocturnal in behaviour, and hide away amongst foliage
during daylight hours, so unlike tiger-complex butterflies they do
not actively advertise their bold patterns to predators. It is
likely however that many are chanced upon by foraging birds, and
that at such times their mildly threatening appearance may save
them from being devoured.
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Cosmosoma species ( Arctiidae :
Ctenuchini ), Manu cloudforest, Peru |
Mimicry, or
coincidence ?
Although widely
accepted, mimicry theories are not totally convincing.
Clearly mimicry
can only evolve if the "mimics" and "models" spend their evolutionary history in the same
habitat and geographical region. There are however many examples of
strong similarities in colour and pattern amongst unrelated species from widely separated
regions. Such similarities cannot be attributed to mimicry, so how
did they come about ?
The biological mechanisms
& processes which generate wing patterns
are fundamentally the same for all butterfly and moth species. It is
therefore logical to assume that similarities will commonly occur
amongst unrelated species, particularly where they evolve in the same habitats
, where the mechanisms are
affected by the same climatic and environmental factors.
Equally it
is likely that the common mechanisms will cause similar visual
attributes to appear even in unrelated species in other parts of the
world. In temperate Europe for example there is a moth called the Buff-tip
Phalera bucephala, a member of the
family Notodontidae. In Brazil there is a group of virtually identical moths
in the genus
Cossula - members of the family
Cossidae. Phalera and
Cossula both have silvery grey forewings with a
yellowish patch at the apex. Both species rest with their wings
wrapped around their body, and look remarkably like fragments of broken
twigs. They have followed completely different evolutionary
pathways, and have almost certainly never co-existed in a shared
habitat, yet amazingly 2 almost identical
moths have evolved, one living in the temperate deciduous woodlands
of Europe, and the other in the
rainforests of the Amazon.
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