Strategies for
Survival
Page 4
1 - Mate recognition
2 - Camouflage & Disguise
3 - Aposematic
& Diematic
colouration
4
- Mimicry
5
- Sexual dimorphism
6 - Roosting behaviour
7 - Seasonal dimorphism
8 - Chemical warfare
Mimicry
As discussed
previously, unpalatable 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 species which were
palatable to birds had
uncannily similar patterns to unrelated toxic species.
A well known example illustrated below is the palatable North American species
Limenitis archippus which bears a quite
remarkable
resemblance to the highly toxic Monarch butterfly
Danaus plexippus.
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Danaus
plexippus (
image
© Ingo Arndt ) |
Limenitis
archippus (
image
©
Benny Mazur ) |
There are many 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.
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.
This form of defence is widely known as Batesian mimicry. It is
normally only effective because the toxic species far outnumber the
non-toxic species. If the situation was reversed so that most of
the butterflies attacked were palatable, the mimicry bluff would
fail.
There are however circumstances
where the mimic can outnumber the model, and the bluff will still
work. An example is Eresia pelonia
which produces several differently coloured forms or morphs, each
mimicking a different toxic model e.g. the nominate subspecies
E. pelonia pelonia is a mimic of the
Ithomiine
Callithomia alexirrhoe thornax;
while subspecies
Eresia pelonia callonia is
a very close "copy" of Hypothyris mansuetus meterus.
For any form of mimicry to work, both the mimic and the model must
fly together in the same area, i.e. they must be sympatric.
Another example is the Mocker Swallowtail ( aka the Flying
Handkerchief ) Papilio dardanus,
which produces several mimetic forms. Such species are known as
mimetic polymorphs. In the case of dardanus
all males look identical, but the female produces several morphs.
These correspond to a variety of different unpalatable models
including Amauris niavius,
Amauris echeria,
Acraea poggei and
Danaus chrysippus. The reasons why
only females produce polymorphs is poorly understood. Some
biologists postulate that if males also produced polymorphs that
certain morphs would be disadvantaged during male / male
territorial conflicts. If this was the case the "losing" morph
would end up holding lower quality territories, reducing their
chances of intercepting females.
A further example is Perrhybris pamela
- on
the upperside of the wings the males are white with a black
apex, but females are entirely different, patterned with bands
of orange, yellow and black.
The females are generally regarded as Batesian mimics of Ithomiines in the genus
Mechanitis.
Perrhybris
pamela, male, Rio Madre de Dios, Peru
Perrhybris
pamela, female, Satipo, Peru
Mechanitis polymnia dorissides,
Rio Pindayo, Peru
Müllerian mimicry
In
1879, Müller realised that there were also many cases where both
the mimic and the model were unpalatable.
When a bird catches any one of these butterflies, either model or
mimic, and realises it is unpalatable or toxic, it quickly learns
to keep away from all similarly patterned species.
This
type of evolutionary "cooperation"
is referred to as Müllerian mimicry, and is a very common
phenomenon amongst the Ithomiinae, Danainae and Pieridae.
Müller demonstrated
mathematically that this form of mimicry is biased in favour of
the scarcer species by a factor of the square of the ratio of
species abundance. It is advantageous for there to be a large
number of species involved in a
Müllerian
mimicry complex as it increases the power of the warning signal.
It
is important to note that mimicry cannot
be simply categorised into groups of distasteful models and Batesian
or Mullerian mimics. There is actually a 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 of butterfly to be unpalatable,
but a jacamar might find it quite acceptable.
The Tiger complex
A famous example of butterfly mimicry is the "tiger complex" - a
group of about 200 neotropical species which all share a similar
pattern of orange and yellow stripes on a black ground colour.
The complex includes
many unpalatable Ithomiines such as
Tithorea harmonia,
Tithorea tarricina, Melinaea marsaeus &
Forbestra equicola, unpalatable Danaines
such as
Lycorea pasinuntia, and
several highly toxic day-flying moths from the Arctiid subfamily
Pericopinae. It also
includes many unrelated species that are considered
to be palatable e.g. Heliconius
ismenius, Heliconius hecale,
Eueides isabella
( Heliconiinae ), Eresia eunice ( Nymphalinae ),
Stalachtis calliope ( Riodinidae ),
Consul fabius ( Charaxinae )
and
Pterourus zagreus (
Papilioninae ).
Members of the tiger complex
habitually aggregate in large numbers in damp gullies in the
forest at the end of the dry season. It is at this time when
they are very docile and easy prey for birds, that mimicry has its
greatest potential as a defence mechanism. Any bird that suffers
the unpleasant experience of
tasting a member of the tiger-complex quickly learns to avoid
attacking any similar looking species,
and may even be capable of communicating their distasteful nature
to other birds.
Eresia
eunice ( Nymphalinae )
Eueides
isabella dissoluta ( Heliconiinae )
Mimicry rings
Although the "tiger complex" is the most well known group of mimics
and models in South America, there are a number of other mimicry
rings. The "glasswing ring" is a group of largish transparent
species that includes toxic Mullerian models
in the Ithomiine genera Methona and
Thyridia, a toxic Danaine
Lycorea ilione, and a Batesian mimic -
the palatable Dismorphiine Patia orise.
The "orange ring" is comprised of a group of bright orange species
including Marpesia petreus,
Dryas iulia and
Eueides aliphera.
Additionally there are several species pairs, the most well known of
which is Heliconius erato and
Heliconius melpomene. These species
each produce 29 different geographically isolated subspecies. For
each subspecies of erato, there is an
equivalently patterned melpomene
subspecies.
Why so many mimicry rings? The answer in the case of Ithomiines
is vertical stratification of their forest habitats. The smaller
glasswings regardless of genus tend to fly and utilise foodplants in the
lower understorey. Tigers occupy the layer between about 2-4
metres above ground level. Other groups inhabit progressively higher
layers, right up to the sub-canopy. In the case of
genera such as Heliconius, which all tend to fly at roughly the same height,
Mullerian mimicry complexes are apparently segregated horizontally
by vegetation type.
Wasp 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 hornets, and
almost certainly escape predation as a result of their similarity
to these stinging insects.
In
the neotropical region there are dozens of
Arctiid moth species in the genera Cosmosoma, which have transparent wings and boldly
patterned red, orange, or yellow bodies. Most are nocturnal in behaviour and hide away amongst foliage
in the daytime, so unlike members of the tiger-complex they don't actively advertise their bold patterns to predators. It
seems
likely however that many are chanced upon by foraging birds, and
that at such times their threatening appearance may save
them from being devoured.
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Cosmosoma species ( Arctiidae :
Ctenuchini ), Manu cloudforest, Peru |
Transformational
mimicry
Mimicry is not
just confined to adult butterflies and moths. Many palatable
caterpillars also mimic unpalatable species. Caterpillars often
change their appearance periodically after moulting, and it can
happen that a caterpillar can mimic different models during
different instars. Furthermore the adult butterfly or moth that
ultimately results from that caterpillar could mimic yet another
species. This is known as transformational mimicry. There are many
other forms of mimicry known.
A note of
caution
It
is very easy to make assumptions about mimicry that may not prove
valid. Not all examples of
apparent
mimicry are genuine cases. There are numerous examples of almost
identical butterflies occurring on opposite sides of the world.
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 similar habitats, where the mechanisms are
affected by the same climatic and environmental factors.
It is
obvious for example that many moth species across the world will
have evolved to look like green leaves, or like bits of lichen or
patches of tree bark, because these occur in all habitats, and the
moths need a convincing camouflage when they settle on them. It is
highly unlikely however that these near identical moths share
common predators. Clearly these are simple cases of convergent
evolution, but nothing whatsoever to do with mimicry.
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