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讲的好。 -- 淞岛 - (118 Byte) 2007-2-16 周五, 下午5:56 (127 reads) |
秃公
加入时间: 2006/09/12
文章: 1667
经验值: 1777
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作者：秃公 在 罕见奇谈 发贴, 来自 http://www.hjclub.org
听：
TEXT：(http://amos.indiana.edu/library/scripts/rivalry.html)
Today on A Moment of Science, another take on sibling rivalry. Sibling rivalry
among sharks, that is. There are nearly four hundred different species of
sharks out there and, depending on the species, shark embryos develop in
three distinct ways.
Some, like hammerhead and blue sharks, nurture developing embryos through
a placenta, much like mammals. Others, like horn sharks, release eggs into
the ocean. However, the most common kind of embryonic development in sharks
is when the female secretes a thin protective case around the embryo and
the embryo remains in one of her two uteri. These sacs have a yolk that
provides the embryos with nutrients, but there is no placental connection
to the mother. When the embryos are mature enough, they hatch within the
uterus, and the female gives birth to live pups.
So where does the sibling rivalry come in? Well, in some species of sharks,
the embryos may obtain additional nutrients by swallowing the unfertilized
eggs in the mother's uterus, or else--you got it--by eating other developing
embryos. This practice is known as intrauterine cannibalism, and was discovered
accidentally in 1948 when a scientist was bitten as he probed a very pregnant
sand tiger shark's uterus.
Because the world they're born into is hardly any more hospitable than the
womb, these shark pups are ready to feed and survive on their own from the
moment they come out of the womb. And what better preparation is there for
this dog-eat-dog world than surviving a shark-eat-shark womb?
在看一遍：
http://www.flicklife.com/62963ccde717b18f9759/Sand_tiger_shark_eats_siblings_in_womb.html
作者：秃公 在 罕见奇谈 发贴, 来自 http://www.hjclub.org |
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