How and why do bats hang upside down all day?
[ 2006-08-04 10:19 ]
蝙蝠為什么總是倒掛著休息？它們那么吊著不累嗎？它們這個(gè)姿勢起飛容易嗎？這種會飛的哺乳動(dòng)物總是有很多讓人捉摸不透的地方。下面這篇小文章將試著為你解答這些問題。
 During
the day, bats hardly move at all. Instead, they pass the time hanging upside
down from a secluded
spot, such as the roof of a cave,
the underside of a bridge or the inside of a hollowed-out tree.
There are a couple
different reasons why bats roost this way. First of all, it puts them in an
ideal position for takeoff. Unlike birds, bats can't launch themselves into the
air from the ground. Their wings don't produce enough lift to take off from a
dead stop, and their hind legs are so small and underdeveloped that they can't
run to build up the necessary takeoff speed. Instead, they use their front claws
to climb to a high spot, and then fall into flight. By sleeping upside down in a
high location, they are all set to launch if they need to escape
the roost
.
Hanging upside down is also a
great way to hide from danger. During the hours when most predators
are active, bats
congregate where few animals would think to look and most can't reach. This
allows them to disappear from the world until night comes again.
 Bats
have a unique physiological adaptation that lets them hang around this way
without exerting any energy. If you want to clench your fist around an object,
you must contract several muscles in your arm, which are connected to your
fingers by tendons. As one muscle contracts, it pulls a tendon, which pulls one
of your fingers closed. A bat's talons
close in the same way, except that their tendons
are connected only to the upper body, not to a
muscle. To hang upside down, a bat flies into position, pulls its claws open
with other muscles and finds a surface to grip. The weight of the upper body
pulls down on the tendons connected to the talons, causing them to clench. The
talon joints lock into position and the bat's weight keeps them closed.
Consequently, the bat doesn't have to do anything to hang upside down.
It only has to exert energy to release its grip, flexing muscles that pull its
talons open. Since the talons remain closed when the bat is relaxed, a bat that
dies while roosting will continue to hang upside down.
seclude:
隔離
roost：歇息
predator：食肉動(dòng)物
talon：爪
tendon: 肌腱
（英語點(diǎn)津 Annabel 編輯）
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