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.TOEFL Listening Comprehension TranscriptNarrator
Listen to part of a university lecture on Animal Behavior by a professor of Biology.
We''re looking at animal behaviour this week, and let''s turn now, class, to one of its most dramatic manifestations- animal mimicry. Organisms that are good to eat, or that are attacked for other reasons, often develop devices- through evolution, of course- techniques and devices to protect themselves from their attackers, in order to survive, and in order to reproduce and pass their genes on to the next generation. And one of these techniques, one of these strategies, is to look like something else, to look like something that is not good to eat, or something that is otherwise of no interest to the predator. An organism that does this, that resembles something else, is called a ''mimic'', and the thing that it has evolved to resemble is called the ''model'', while the predator that it is trying to mislead is called the ''recipient''- the one that receives the misleading image.
Some mimics do this by adopting camoflage, which is a cryptic resemblance to something of no interest to its enemy, and by doing this, they become invisible, they are hidden. Many animals- insects, lizards, amphibians- mimic the abundant plant life in the habitat around them. I''m sure that you''ve seen green grasshoppers and brown moths that seem to be well-hidden on grass stems and tree trunks when they''re motionless. But the Leaf-tailed Gecko, a small lizard in Madagascar, is a master at this. It avoids its enemies by looking exactly like a cluster of old dead leaves. And there are various species of katydids, grasshopper-like insects, that have managed to duplicate the appearance of leaves with startling accuracy, in all stages of growth, some species looking like fresh green leaves and others looking like old decaying leaves- complete with leaf veins, weathered edges and mildew spots! These adaptations make these animals difficult or impossible for a predator to identify or even notice, and so these otherwise defenseless creatures are overlooked or passed by.
Other organisms defend themselves directly with stings or bites, or with poisons or other noxious chemicals, and such organisms often assume bold, characteristic colors and markings- called warning coloration- that warns a predator, reminds it, that this creature can inflict pain or discomfort, or that it tastes very bad. The bold orange-and-black pattern of the common Monarch Butterfly, or the black-and-yellow bands on a bumblebee, are such warning colorations.
And sometimes, this warning coloration is so effective that another species, a species that doesn''t have any of the protective devices of sting or poison or whatever, will adopt the same warning colors and pattern. This sort of mimicry is called ''Batesian mimicry''. The name comes from the early zoologist, HW Bates, who, back in 1862, first suggested an explanation for the origins of mimicry based on Charles Darwin''s new Theory of Natural Selection. This was one of the earliest applications of Darwin''s ideas to an unknown biological phenomenon.
Now, Viceroy Butterflies taste good to many birds, but because they mimic the Monarch Butterfly model''s color pattern, because Viceroy Butterflies look like Monarch Butterflies, they are avoided, just like the Monarch is. In the same way, many harmless fly species resemble the bumblebee model, and also in this way they avoid being eaten by the recipients, birds. So these are Batesian mimics. There are several conditions that must be fulfilled, though, for a Batesian mimic to be successful- the mimic must of course share the same general region and habitat as its model, but the mimic must also be less numerous than its model, which must be relatively abundant. That way, the odds are that the recipient predator will sample an unpalatable model first, which is very important for keeping the trick effective.
A similar kind of mimicry is ''Müllerian mimicry''- named after another early biologist- and in this sort of mimicry, both the model and the mimic are dangerous or taste bad. A very obvious example is the way that so many unrelated species of bees, wasps, and ants have assumed similar, bold, black-and-yellow or black-and-orange banded patterns. By doing this, Müllerian mimics present a united image that predators soon learn to be wary of.
There''s also another aspect of mimicry that I''d like to mention, too, and that''s the mimicry used by predators. This is called ''aggressive mimicry'', and it is used to conceal or misrepresent a predator until its prey comes near enough to capture. Many mantids, for example, are green or brown, so that they blend in with their plant surroundings, but some tropical mantids are fantastically shaped and colored, like the beautiful Orchid Mantis, which resembles a petal of one of those tropical flowers, and it hides motionless next to one of these orchids until an insect comes within its reach. There''re also several green-colored vine and grass snakes of various families, which lie invisible among the tangled vines and branches of the jungle until they suddenly lash out to grab their prey.
Actually, there are an endless number of ingenious mimics in the natural world, and I recommend that you all try a Google Images search tonight for some more interesting examples of this fascinating behaviour.
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