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你可能喜欢试译——以假乱真的动物伪装术
The Canadian tiger swallowtail caterpillar is a plump green
creature that spends all its time munching leaves. It ought to be
an easy meal for a bird, yet many birds pass it by.
The caterpillar is protected by a remarkable defense, researchers
have found: It tricks birds into thinking it’s a snake.
The caterpillar grows a pair of concentric yellow and black rings
that look like a giant pair of eyes. When the caterpillar senses a
bird nearby, it quickly inflates the front part of its body, making
it resemble a snake’s head.
Astonishing as this deception may be, the tiger swallowtail is
hardly unique. Many species have evolved ways to fool their
would-be predators. Some insects look like twigs, even mimicking
the way that twigs sway in the breeze.
Harmless snakes scare off predators by mimicking the look of
venomous ones. Some species of hoverflies have the yellow and black
stripes of stinging wasps. They even pretend to sting their
enemies, despite having no stinger at all.
But mimicry has revealed a puzzle at its heart: Time and again,
scientists find examples of overkill. Animals don’t seem to get an
extra benefit from making their disguises more elaborate.
The Canadian tiger swallowtail is an excellent example. Over the
past few years, Thomas N. Sherratt, a behavioral ecologist at
Carleton University in Ontario, and his graduate student Thomas J.
Hossie have studied the caterpillar’s strategy by creating “pastry
caterpillars” of flour, water, shortening and green food
They are apparently tasty. When the scientists placed pastry
caterpillars on tree branches, many of them quickly disappeared.
“Birds are really motivated to eat them,” said Dr. Sherratt.
Dr. Hossie, who is now a post-doctoral researcher at Trent
University in Ontario, created a batch of pastry caterpillars with
eye spots and snake-like heads. A substantial number of these more
elaborate models remained on the branches, presumably because they
scared away many birds.
Surprisingly, though, pastry caterpillars don’t require a
complicated disguise to deter birds. The scientists also created
faux caterpillars with eyespots but no snakelike head. The birds
avoided these simpler mimics about as often as they did the more
elaborate ones.
If real caterpillars don’t gain extra protection from extra
deception, the scientists wondered, then how could their disguise
have evolved?
The researchers suspected that they had missed something in their
experiments in the woods. For one thing, they had not actually
watched birds as they ate or avoided the pastry caterpillars. They
had simply counted up the pastries that remained on branches.
To get a closer look at the birds, the Canadian scientists teamed
up with Dr. Skelhorn, who studies mimicry in his Newcastle
laboratory.
The scientists baked more batches of pastry caterpillars, but this
time they didn’t put them on tree branches. Instead, they presented
the models to day-old chicks.
They picked such young animals in order to judge whether the birds
are born with a fear of snakes and snakelike objects. “These chicks
haven’t seen anything like a snake before,” said Dr. Sherratt.
When the chicks were presented with simple green cylinders, they
happily pecked at them. But if the scientists added eyespots, the
chicks became far more wary. Consistent with earlier research, the
scientists found that adding a snakelike head didn’t make the
chicks any more fearful.
But the experiment didn’t end there. Over the next two days, the
scientists presented the chicks with pastry caterpillars five more
times. By the end, the chicks had learned that cylinders with eye
spots were in fact tasty snacks.
That was not the case when the chicks were presented with pastry
caterpillars with both eye spots and snakelike heads. Even at the
end of the study, the chicks were still fairly wary of the more
realistic mimics.
The researchers&&their
results on Aug. 14 in the journal Behavioral Ecology.
Dr. Skelhorn and his colleagues suspect that birds have evolved a
healthy fear of snakes, present from the moment they hatch from
their eggs. They don’t need to see a snake in full detail before
deciding to flee. Just a few distinctive features — like snake eyes
— may be enough. The caterpillars exploit that quick response to
deter the birds.
But birds, like other animals, can learn to tell objects apart.
This ability may have driven caterpillars to evolve more elaborate
disguises, which are harder for birds to learn to distinguish from
“It takes more information for the birds to be convinced they’re
not a snake,” said Dr. Skelhorn. The caterpillars “can fool
predators for longer and survive.”
Olof Leimar, an evolutionary biologist at Stockholm University who
was not involved in the study, said that it demonstrated how
deceptive animals can exploit the wiring of their predators’
brains. “Without understanding the psychology, you can’t understand
the phenomenon,” he said.
Dr. Sherratt hopes that their experiment will inspire other
researchers to run similar studies on other mimics. He suspects
that other “over-evolved” deceivers may actually be exploiting how
their enemies learn.
“Of course, you need to do it a few more times to see if it’s
real,” he said.
加拿大虎斑燕尾蝶幼虫是一种体型丰满的绿色生物，它们整天都在安静地啃食树叶。对鸟类来说，这应该是一顿轻而易举的美餐，然而它们却视而不见。
研究发现，虎斑燕尾蝶幼虫有一层奇异的保护罩，让鸟类误以为看到的是一条蛇。
幼虫身上长了一对大眼似的黄黑同心圆。感应到附近有鸟类时，幼虫会迅速胀大前半部分身体使之看起来像是蛇头。
伪装术也许匪夷所思，但这并非虎斑燕尾蝶幼虫的独门绝技。经过长时间进化，许多物种已形成一套骗过捕食者的方法。比如一些昆虫形似细枝，甚至模仿细枝在风中摇摆的姿态。
伪装成毒蛇的模样，即使无害也足以吓走捕食者。有几种花蝇有黄蜂的黄黑条纹，没有黄蜂的毒刺，却可以假装黄蜂叮人来吓走天敌。
但是科学家们发现了一个拟态伪装关键问题：时间，还有过度伪装，有许多例子可以说明。动物们似乎并没有因伪装的精致而受到更多保护。
虎斑燕尾蝶就是个极好的例子。过去几年时间，安大略卡尔顿大学的行为生态学家汤玛士·N·谢拉特和当时的研究生汤玛士·J·候斯利用点心毛毛虫对虎斑燕尾蝶幼虫的伪装策略进行了研究，该点心毛毛虫由面粉，水，起酥油，绿色食用色素制成。
很显然，这些点心很美味，放上树枝，不久就被吃掉了。谢拉特博士说：为了吃掉这些点心，鸟儿们不遗余力。
候斯博士——如今是安大略特伦特大学的博士后——制作了一批有“眼睛”，看起来像蛇头的点心毛毛虫。同样放在树枝上，却有很大一部分剩余，想必是形状吓走了一些鸟类。
事实上，令人意外的是，要震慑住鸟类，不需要太复杂的伪装。科学家们又制作了一些有“眼睛”但不像蛇头的人工毛毛虫进行试验。就像避开点心毛毛虫一样，鸟类避开了这些简单的模仿物。
这不禁让科学家们感到疑惑，自然情况下，如果多一层的伪装并不能使幼虫获得多一层的保护，那么他们的伪装是如何进化的？
研究人员怀疑是不是试验中错过了什么。首先一个就是他们并没有实际观察鸟类吃掉或避开点心毛毛虫的全过程，只是对仍留在树枝上的点心数进行清点。
为了更近地观察鸟类行为，这两位加拿大科学家与纽卡斯尔实验室的伪装研究专家斯凯霍恩博士展开了合作。
他们制作了更多的点心毛毛虫，不过这次，没有放到树枝上，而是放到了一些一天大的小鸡面前。
之所以选择年幼动物，是想判断鸟类是否天生惧怕蛇类或类蛇形物体。谢拉特博士说：“这些小鸡此前从未见过蛇或像蛇的东西。”
面对简单绿色的圆柱形物体，小鸡们啄得很欢快。然而，面对加了“眼睛”圆柱形物体，小鸡们变得非常谨慎。与之前的研究结果一致，面对再加上蛇头的圆柱形物体，小鸡没有表现出更多的恐惧。
实验到此并没有结束。接下去两天，科学家们又进行了5次实验，都是把点心毛毛虫摆在小鸡面前。最终，小鸡们会明白那些带“眼睛”的东西其实是好吃的点心。
但是面对同时有眼睛和蛇头的点心毛毛虫时，小鸡们又是另外一番表现。直到实验最后，小鸡都表现得相当警惕。
这一实验结果于8月14号刊登在了《行为生态学》杂志。
斯凯霍恩博士和同事们怀疑鸟类已经形成了对蛇类的健康恐惧，从他们破壳而出的那一刻起就存在。鸟类飞走之前不需要看到蛇的全貌，只需要一些特征——如蛇眼——也许就足够了。虎斑燕尾蝶幼虫就是运用这一点，吓走了捕食者。
但是，同其它动物一样，鸟类能够学会判断。也许正因如此，幼虫为自保进化出更复杂的伪装形态，加大鸟类辨别的难度。
斯凯霍恩博士说：“鸟类需要更多的信息来确信这些是毛毛虫而不是蛇。”虎斑燕尾蝶幼虫能够继续迷惑捕食者而存活下来。
未参与这项研究的斯德哥尔摩大学进化生物学家奥洛夫·雷马尔说，这项实验证明了利用捕食者的脑回路，动物们可以做到相当地具有欺骗性。他说“不了解这些心理的话，你无法理解这种现象”。
谢拉特博士希望他们的实验可以激励其他研究者展开类似的拟态伪装研究。他猜测其它“过度进化”的伪装者甚至是参透了敌人的学习方式。
“当然，你得做更多的实验才能知道”他说。
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