MIMIC YOUR NEIGHBOR
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A
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There’s no
animal that symbolizes rainforest diversity quite as spectacularly as the
tropical butterfly. Anyone lucky enough to see these creatures flitting
between patches of sunlight cannot fail to be impressed by the variety of
their patterns. But why do they display such colourful exuberance? Until
recently, this was almost as pertinent a question as it had been when the
19th-century naturalists, armed only with butterfly nets and insatiable
curiosity, battled through the rainforests. These early explorers soon
realized that although some of the butterflies’ bright colours are there to
attract a mate, others are warning signals. They send out a message to any
predators: “Keep off, we’re predicting poisonous.” And because wearing certain
patterns affords protection, other species copy them. Biologists use the
term ‘mimicry rings’ for these clusters of impostors and their evolutionary
idol.
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B
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But here’s the conundrum.
“Classical mimicry theory says that only a single ring should be found in
any one area,” explains George Beccaloni of the Natural History Museum,
London. The idea is that in each locality there should be just the one
pattern that best protects its wearers. Predators would quickly learn to
avoid it and eventually, all mimetic species in a region should converge
upon it. “The fact that this is patently not the case has been one of the
major problems in mimicry research,” says Beccaloni. In pursuit of a
solution to the mystery of mimetic exuberance, Beccaloni set off for one of
the mega centres for butterfly diversity, the point where the western edge
of the Amazon basin meets the foothills of the Andes in Ecuador. “It’s
exceptionally rich, but comparatively well collected, so I pretty much knew
what was there, says Beccaloni. “The trick was to work out how all the
butterflies were organized and how this related to mimicry.”
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C
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Working at
the Jatun Sacha Biological Research Station on the banks of the Rio Napo,
Beccaloni focused his attention on a group of butterflies called
ithomiines. These distant relatives of Britain’s Camberwell Beauty are
abundant throughout Central and South America and the Caribbean. They are
famous for their bright colours, toxic bodies, and complex mimetic
relationships. “They can comprise up to 85 per cent of the individuals in a
mimicry ring and their patterns are mimicked not just by butterflies, but
by other insects as diverse as damselflies and true bugs,” says Philip
DeVries of the Milwaukee Public Museum’s Center for Biodiversity Studies.
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D
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Even though all ithomiines are
poisonous, it is in their interest to evolve to look like one another
because predators that learn to avoid one species will also avoid others
that resemble it. This is known as Mullerian mimicry. Mimicry rings may
also contain insects that are not toxic but gain protection by looking like
a model species that is: an adaptation called Batesian mimicry. So strong
is an experienced predator’s avoidance response that even quite inept resemblance
gives some protection. “Often there will be a whole series of species that
mimic, with varying degrees of verisimilitude, a focal or model species,”
says John Turner from the University of Leeds. “The results of these
deceptions are some of the most exquisite examples of evolution known to
science.” In addition to colour, many mimic copy behaviours and even the
flight pattern of their model species.
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E
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But why are
there so many different mimicry rings? One idea is that species flying at
the same height in the forest canopy evolve to look like one another. “It
had been suggested since the 1970s that mimicry complexes were stratified
by flight height,” says DeVries. The idea is that wing colour patterns are
camouflaged against the different patterns of light and shadow at each
level in the canopy, providing the first line of defence, against
predators.” But the light patterns and wing patterns don’t match very
well,” he says. And observations show that the insects do not shift in
height as the day progresses and the light patterns change. Worse still,
according to DeVries, this theory doesn’t explain why the model species is
flying at that particular height in the first place.
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F
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“When I first went out to
Ecuador, I didn’t believe the flight height hypothesis and set out to test
it,” says Beccaloni. “A few weeks with the collecting net convinced me
otherwise. They really flew that way.” What he didn’t accept, however, was
the explanation about light patterns. “I thought, if this idea really is true,
and I can work out why it could help explain why there are so many
different warning patterns in any one place. Then we might finally
understand how they could evolve in such a complex way.” The job was
complicated by the sheer diversity of species involved at Jatun Sacha. Not
only were there 56 ithomiine butterfly species divided among eight mimicry
rings, but there were also 69 other insect species, including 34 day-flying
moths and a damselfly, all in a 200-hectare study area. Like many entomologists
before him, Beccaloni used a large bag-like net to capture his prey. This
allowed him to sample the 2.5 meters immediately above the forest floor.
Unlike many previous workers, he kept very precise notes on exactly where
he caught his specimens.
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G
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The attention
to detail paid off. Beccaloni found that the mimicry rings were flying at
two quite separate altitudes. “Their use of the forest was quite
distinctive,” he recalls. “For example, most members of the clear-winged
mimicry ring would fly close to the forest floor, while the majority of the
12 species in the tiger-winged ring fly high up.” Each mimicry ring had its
own characteristic flight height.
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H
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However, this being practice
rather than theory, things were a bit fuzzy. “They’d spend the majority of
their time flying at a certain height. But they’d also spend a smaller
proportion of their time flying at other heights,” Beccaloni admits.
Species weren’t stacked rigidly like passenger jets waiting to land, but
they did appear to have preferred airspace in the forest. So far, so good,
but he still hadn’t explained what causes the various groups of ithomiines
and their chromatic consorts to fly in formations at these particular
heights.
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I
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Then
Beccaloni had a bright idea. “I started looking at the distribution of
ithomiine larval food plants within the canopy,” he says. “For each one,
I’d record the height to which the host plant grew and the height above the
ground at which the eggs or larvae were found. Once I got them back to the
field station’s lab, it was just a matter of keeping them alive until they
pupated and then hatched into adults which I could identify.”
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Questions 28 – 32
The
reading Passage 3 has seven paragraphs A-I.
Which
paragraph contains the following information?
Write
the correct letter A-I, in boxes 28-32 on your answer sheet.
NB
You may use any letter more than once.
1. Criticism against flight height theory of butterfly
2. Explained why Beccaloni researched in Ecuador.
3. Different mimicry ring flies at different height
4. The method of catching butterfly by Beccaloni
5. Not all Mimicry patterns are toxic information sent out from
insects.
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Questions 33-38
Do the following statements agree with
the information given in Reading Passage 3
In boxes 33-38 on your answer sheet,
write
TRUE
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if the statement is true
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FALSE
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if the
statement is false
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NOT GIVEN
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if the information is not
given in the passage
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33. All butterflies’ colours of the wing reflect the sense of warning
to other predators.
34. Insects may imitate butterflies’ wing patterns as well.
35. Flying the Altitude of a butterfly is determined by their food.
36. Beccaloni agreed with the flight height hypothesis and decide to
reassure its validity.
37. Jatun Sacha has the riches diversity of breeds in the world.
38. Beecaloni has more detailed records on the location of butterfly
collection than others.
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Questions
39-40
Choose
the correct letter, A, B, C or D
Write
your answers in boxes 39-40 on your answer sheet.
39
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Which is correct about butterflies’ flight altitude?
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A
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Flight height
theory already established
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B
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Butterfly always flies at a
certain height
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C
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It is like
the aeroplane’s flying phenomenon
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D
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Each butterfly has its own
favourable height
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40
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Which is
correct about Beccaloni’s next investigation after flight height?
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A
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Some certain statistics have
already been collected
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B
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Try to find
connections between larval height and adult ones
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C
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It’s very difficult to raise
butterfly larval
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D
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Different
larval favours different kinds of trees
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