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为什么有些鸟不会飞?

Why can't some birds fly? - Gillian Gibb

鸟儿漫步时 人们才发现它们有翅膀——安东尼·马丁·勒米埃尔
在澳大利亚的茂密雨林中
In the lush rainforests of Australia,
鸟类栖息在低矮的枝头 在森林的地面漫步
birds roost in the low branches and amble across the forest floor,
享受着树荫和热带水果
enjoying the shade and tropical fruits.
但雨林不仅仅属于它们
But the jungle isn’t theirs alone.
一只澳洲野狗正在树荫下觅食
A dingo is prowling in the shadows,
水果并不合它的胃口
and fruit won’t satisfy his appetite.
除了食火鸡 鸟儿们都逃到了安全的地方
The birds flee to safety all but the cassowary,
食火鸡无法依靠它脆弱的翅膀飞翔
who can’t clear the ground on her puny wings.
相反 它会攻击 用锋利的趾爪猛击野狗
instead,she attacks, sending the dingo running for cover
把野狗赶走
with one swipe of her razor-sharp toe claws.
食火鸡是现存近60种不会飞的鸟类中的一种
The cassowary is one of approximately 60 living species of flightless birds.
世界各地都有这种只在地面活动的鸟类
These earthbound avians live all over the world,
从澳大利亚内陆地区到非洲热带草原再到南极海岸
from the Australian outback to the African savanna to Antarctic shores.
它们包括一些鸭类物种 所有的企鹅物种
They include some species of duck and all species of penguin,
隐秘的沼泽居民 快速的鸵鸟 巨大的鸸鹋 以及娇小的几维鸟
secretive swamp dwellers and speedy ostriches, giant emus, and tiny kiwis.
尽管所有的现代鸟类其祖先都可以飞翔
Though the common ancestor of all modern birds could fly,
但如今很多不同的鸟类物种皆丧失了飞行能力
many different bird species have independently lost their flight.
会飞有极好的益处
Flight can have incredible benefits,
特别是对逃避捕食者 狩猎 长途迁徙来说
especially for escaping predators, hunting, and traveling long distances.
但飞行也有很大的代价:
But it also has high costs:
会消耗大量的能量 限制体型的大小和重量
it consumes huge amounts of energy and limits body size and weight.
一只不飞的鸟能保存能量
A bird that doesn’t fly conserves energy,
那么在食物来源少或匮乏时
so it may be able to survive on a scarcer
它可能比会飞的鸟更容易幸存下来
or less nutrient-rich food source than one that flies.
例如 新西兰的南秧鸟
The Takahe of New Zealand, for example,
几乎全都生活在高山草地的草甸上
lives almost entirely on the soft base of alpine grasses.
对那些在地面筑巢或生活的鸟类
For birds that nest or feed on the ground,
更倾向于不会飞
this predisposition to flightlessness can be even stronger.
当一个鸟类物种不再面临特殊飞行压力时
When a bird species doesn’t face specific pressures to fly,
最快在几代之内就会停止飞行
it can stop flying in as quickly as a few generations.
然后 千万年后
Then,over thousands or millions of years,
鸟的躯体开始改变来配合新的习性
the birds’ bodies change to match this new behavior.
它们的骨骼 一旦变得中空以使重量最小化 就会变得质密
Their bones, once hollow to minimize weight, become dense.
丰满的羽毛变得稀松
Their sturdy feathers turn to fluff.
翅膀萎缩 有些情况下 会完全消失
Their wings shrink, and in some cases disappear entirely.
胸骨上类似龙骨的突出
And the keel-like protrusion on their sternums,
即飞行肌肉依附的地方 会萎缩或消失
where the flight muscles attach, shrinks or disappears,
除了企鹅 会改变飞行肌肉和龙骨的用途来游水
except in penguins, who repurpose their flight muscles and keels for swimming.
通常 飞行能力都是在鸟类飞到岛上后退化的
Most often, flightlessness evolves after
因为小岛上没有捕猎者
a bird species flies to an island where there are no predators.
只要这种没有捕食者的情况持续下去
As long as these predator-free circumstances last,
鸟儿就会大量繁殖 但它们易受其生存环境的危害而发生改变
the birds thrive, but they are vulnerable to changes in their environment.
例如 人类定居者会把狗 猫
For instance, human settlers bring dogs, cats,
以及偷渡来的啮齿类动物带到岛上
and stowaway rodents to islands.
这些动物经常捕食不能飞的鸟类
These animals often prey on flightless birds
并能把它们吃到灭绝
and can drive them to extinction.
在新西兰 欧洲移民带来的白鼬
In New Zealand, stoats introduced by European settlers
威胁着很多当地不能飞的鸟类物种
have threatened many native species of flightless bird.
有些已经灭绝了 而其他的也濒临灭绝
Some have gone extinct while others are endangered.
尽管不会飞有省力的优势
So in spite of the energy-saving advantages of flightlessness,
在走上渡渡鸟的老路之前
many flightless bird species have
很多不能飞的鸟类只能逃一小会儿
only a short run before going the way of the dodo.
但一些不能飞的鸟类已经在
But a few flightless birds have survived on
陆地上与大量的捕食者共存了
mainlands alongside predators aplenty.
不像大多数小的不能飞的来无踪去无影的物种
Unlike most small flightless species that come and go quickly,
这些巨鸟已经数千万年都不飞了
these giants have been flightless for tens of millions of years.
它们的祖先与最初的小型哺乳动物同时期出现
Their ancestors appeared around the same time as the first small mammals,
它们之所以能幸存下来 是因为它们一直在进化和成长
and they were probably able to survive because they were evolving—and growing
与它们的哺乳类捕食者共同成长
—at the same time as their mammalian predators.
大多数鸟类 比如鸸鹋和鸵鸟
Most of these birds, like emus and ostriches,
尺寸剧增 超出数百磅以至于翅膀举不起来
ballooned in size, weighing hundreds of pounds more than wings can lift.
腿变得厚实 脚变得强壮
Their legs grew thick, their feet sturdy,
新出现的大腿肌肉 使它们变成了强大的奔跑者
and newly developed thigh muscles turned them into formidable runners.
尽管它们不再使用翅膀飞翔
Though they no longer use them to fly,
但很多鸟类改变了它们原有的用途
many of these birds repurpose their wings for other means.
它们能把自己的头裹在翅膀下取暖
They can be spotted tucking their heads beneath them for warmth,
向喜欢的配偶扇动翅膀 用翅膀来保护鸟蛋
flashing them at prospective mates, sheltering eggs with them,
甚至在穿过平原时 用翅膀来转向
or even using them to steer as they charge across the plains.
它们可能不能飞 但它们仍会扇动翅膀
They may be flightless, but they’re still winging it.

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视频概述

鸟的进化历程和有些鸟不会飞的关系

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收集自网络

翻译译者

呵呵呵呵

审核员

审核员 V

视频来源

https://www.youtube.com/watch?v=hRzRjHzvOts

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