Strategies for
Survival
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. An often
quoted 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. Recent research
however has shown that both of these species are unpalatable, thus
they are Mullerian mimics , not Batesian mimics.
There are nevertheless numerous
other examples of true Batesian mimicry 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 �
Adrian Hoskins
Perrhybris pamela, female, Satipo, Peru �
Adrian Hoskins
Mechanitis polymnia dorissides,
Rio Pindayo, Peru �
Adrian Hoskins
M�llerian mimicry
In
1879, M�ller 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
M�llerian mimicry, and is a very common phenomenon amongst the
Ithomiinae, Danainae and Pieridae.
M�ller 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
M�llerian
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 ) �
Adrian Hoskins
Eueides
isabella dissoluta ( Heliconiinae ) �
Adrian Hoskins
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.
Cosmosoma
species (
Arctiidae : Ctenuchini ), Manu cloudforest, Peru �
Adrian Hoskins
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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