英語聽力文摘500 鳥兒如何睡覺

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How do birds sleep?

How do birds hang onto trees when they’re asleep? Why don’t they fall off?

That's easy. It's because of the hallux toe. Hallux toe? Is that the toe on some birds that points backwards instead of forwards?

That's the one. We already mentioned the hallux toe once on the show, when talking about how well-adapted some bird feet are for perching on branches.

But you say it holds the bird in place even when it’s sleeping. Absolutely! It’s because of the way the tendons work in a perching bird’s leg. When the bird hunkers down for sleep, it bends its leg at the knee and the heel. Two thin wires, called flexor tendons, run down the leg around the knee and heel, and attach to the toes.

The bending of the bird’s legs when it perches tightens up these tendons, and, like a puppet’s strings, curls the toes. The neat thing is the bird doesn’t even have to think about it to make it happen.

鳥兒如何睡覺?

鳥兒睡覺時(shí)是如何掛在樹上的呢?它們?yōu)槭裁吹舨幌聛?

很簡單!那是因?yàn)樗鼈冇写笾?。大?就是某些鳥兒身上指向后方而不是前方的趾頭嗎?對，就是那個(gè)。我們曾在節(jié)目中談?wù)撨m應(yīng)性強(qiáng)的鳥兒的腳是如何幫助其棲息于樹枝上時(shí)，提到過“大趾”。

但是你剛剛說大趾能在鳥兒睡覺時(shí)助其保持平衡。正是!這與棲息的鳥兒腿里鳥腱運(yùn)作的方式有關(guān)。當(dāng)鳥兒蹲下睡覺時(shí)，鳥腱使鳥腿在膝蓋和腳踝彎曲。兩根纖細(xì)的屈肌腱使腿在膝蓋和腳踝處彎曲，然后就與腳趾連接在一起。

鳥兒棲息時(shí)，鳥腿的彎曲拉緊了這些屈肌腱，就像是拉動(dòng)木偶的繩子一樣，使得腳趾卷曲起來。有趣的是，鳥兒根本不用去想怎么讓其發(fā)生。