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  • Marsupials - 高级

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    Is it really a bear?
    ::真的是熊吗?

    Its a koala. Or sometimes called the koala bear. The koala is a marsupial native to Australia. And it is very different from any bear.
    ::或有时叫卡拉熊 卡拉是澳大利亚土生土长的 与任何熊都非常不同

    Marsupials
    ::Marsupial 义体

    Marsupial mammals are viviparous mammals in which the embryo is born at a very early stage and completes its outside the mother’s body. Marsupials belong to the infraclass of therian mammals called Metatheria, which includes both living and extinct mammals. There are fewer than 300 hundred of marsupials living today. Examples of living marsupials include bandicoots, kangaroos, wombats, koalas, kolokolos, and opossums ( Figure  ).  About two-thirds of living marsupials are found in Australia. Most non-Australian marsupials are found in South America. They include the kolokolo and some species of opossum. Like , marsupials are endothermic, but they tend to have a slightly lower body temperature than placental mammals.
    ::马苏比哺乳动物是活生生的哺乳动物,胚胎在非常早的阶段就出生,在母体外完成。马苏比动物属于被称为Metatheria(Metatheria)的亚类动物,包括活的哺乳动物和灭绝的哺乳动物。今天,有不到300万只马苏比动物生活在其中。活的马苏比动物的例子包括带状动物、袋状动物、子宫动物、鳄鱼、科洛科洛斯(Figure ) , 大约三分之二活的马苏比动物在澳大利亚。大多数非澳洲的马苏比动物都在南美洲。它们包括Kolokolo和某些直观动物种类。像马苏比胎哺乳动物的体温要低一点。

     

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      A Bandicoot

      ::班迪库特
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      Kangaroo

      ::袋鼠
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      A Wombat

      ::乌蒙巴特
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      An opossum

      ::假阴

    Marsupial Reproduction
    ::二次复制

    The best-known trait of marsupials is their method of . Offspring are born while they are still in the embryonic stage, and they crawl to a pouch on the surface of their mother’s body. They remain in the pouch until they complete their development. Because of the short gestation period, the marsupial mother does not need to develop a placenta to nourish the embryo. Instead of a placenta, she develops a large yolk sac in her uterus . The yolk sac and secretions of the uterus are sufficient to nourish the embryo for the short period of time (typically three to seven weeks) that the embryo remains in the uterus before leaving the uterus and moving to the pouch.
    ::哺乳动物最著名的特征是它们的方法。在胚胎阶段出生后,它们会爬到母亲身体表面的袋子里。它们会留在袋子里,直到发育完成。由于妊娠期短暂,哺乳动物的母亲不需要开发胎盘来养育胚胎。她没有胎盘,而是在子宫里开发了一个大蛋白囊。子宫的乳房和分泌物足以在短期内(通常为三至七个星期)养活胚胎,因为胚胎在离开子宫和转移到袋里之前就留在子宫里。

    The short gestation period also means that the marsupial embryo is very small and immature when it enters the pouch. However, it has relatively strong forelimbs that allow it to climb to a nipple inside the pouch ( Figure  ). On the other hand, the embryo is too weak to suck, so the mother feeds it by contracting over her mammary glands . This causes milk to be released from the nipple. The developing marsupial offspring remains attached to the nipple in the pouch for several months (about nine months in the case of most kangaroos). Then it passes through a stage in which it temporarily leaves the pouch but returns periodically for warmth and nourishment. This stage may last for several more months. Eventually, the young marsupial is mature enough to remain outside of the pouch on its own.
    ::短暂的妊娠期还意味着哺乳胚胎在进入邮袋时非常小和不成熟,然而,它具有相对强大的前额,允许它爬到邮袋(图)内的乳头(图 ) 。 另一方面,胚胎太弱,不能吸,因此母体通过割乳腺喂养它。这导致乳头释放牛奶。发育中的哺乳后代在邮袋的乳头上待了几个月(大多数袋为九个月左右 ) 。随后,它经过一个阶段,暂时离开邮袋,但定期返回取暖和营养。这个阶段可能还要持续几个月。最后,年轻的哺乳动物已经成熟到可以自己留在邮袋外面。

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    An embryonic kangaroo suckles a nipple inside its mother’s pouch.

    Pros and Cons of Marsupial Reproduction
    ::火星复制的Pros和 Cons Pros 和 Cons Pros 和 conss Pros

    The very short gestation period of marsupials may be an adaptive advantage. It reduces the risk of the mother’s immune system attacking the embryo as a “foreign invader.” This is a potential problem in placental mammals, which have longer gestation periods, and is one reason for the development of . A marsupial mother avoids this risk without using up the extra energy needed to form and maintain a placenta. The marsupial mother also does not have the risks associated with an extended and the birth of a large infant . In addition, she can easily expel the embryo from her pouch if she is pursued by a predator or if there is not enough food available to support both herself and the embryo.
    ::哺乳期的短暂妊娠期可能是一种适应优势,可以降低母亲的免疫系统作为“外国入侵者”攻击胚胎的风险。 这是胎内哺乳动物的潜在问题。 胎内哺乳动物的妊娠期较长,也是发育的原因之一。 哺乳期母亲避免了这一风险,而没有使用所需的额外能量来形成和保持胎盘。 哺乳期母亲也不具备与婴儿长寿和出生相关的风险。 此外,如果捕食者追逐胎儿,或者没有足够的食物来养活自己和胚胎,她可以轻易地将胚胎从邮袋中驱逐出来。

    On the other hand, the tiny newborn marsupial is at greater risk of death than a newborn placental mammal is. A marsupial mammal is born at a less mature stage of development than a newborn placental mammal. Therefore, a newborn marsupial is much smaller and more fragile than a newborn placental mammal.
    ::另一方面,与新生儿胎盘哺乳动物相比,幼小的新生儿哺乳动物的死亡风险更大。 哺乳哺乳动物的出生发育阶段不如新生胎盘哺乳动物成熟。 因此,新生儿哺乳动物比新生胎盘哺乳动物小得多,而且比新生胎盘哺乳动物脆弱得多。

    The North American Marsupial: The Opossum
    ::北美马苏比亚:

    Most people think of opossums as scary creatures. Is this because they look kind of funny, walk kind of funny, have beady and sharp teeth, and can emit a very foul odor? Maybe. But what is so different about opossums is that they are the only marsupial in North America.
    ::大部分人认为阴部瘤是可怕的生物。 这是因为它们看起来有点滑稽,行走方式有点滑稽,牙齿有珠子和尖锐的牙齿,能够发出非常肮脏的气味吗?也许吧。但是对于阴部瘤来说,有什么不同呢?它们是北美唯一的麻木动物。

    Opossums can be beneficial to humans. They use their sharp teeth to crush – which means that they are good at getting rid of unwanted rodents in your neighborhood. They have excellent immune systems, and they emit that terrible odor for protection. 
    ::食用人可以对人类有利。 他们用尖牙粉碎 — — 这意味着他们善于驱散邻居的不受欢迎的鼠类。 他们有很好的免疫系统,他们用这种可怕的气味来保护自己。

     

    Summary
    ::摘要

    • Marsupials are most well-known for their method of reproduction.
      ::Marsupials最著名的是其复制方法。
    • Offspring are born while they are still in the embryonic stage, and they crawl to a pouch on the surface of their mother’s body. They remain in the pouch until they complete their development.
      ::他们出生时还处于胚胎阶段,他们爬到母亲身体表面的邮袋里。 他们留在邮袋里,直到他们完成发育。
    • The short gestation period means that the marsupial embryo is very small and immature when it enters the pouch.
      ::短暂的妊娠期意味着婚姻胚胎在进入邮袋时非常小和不成熟。
    • The short gestation period of marsupials may be an adaptive advantage that reduces the risk of the mother’s immune system attacking the embryo.
      ::短暂的婚姻存续期可能是一种适应优势,可以减少母亲免疫系统攻击胚胎的风险。

    Review
    ::回顾

    1. Describe how marsupial offspring grow and develop.
      ::描述海产后代的生长和发育方式。
    2. How do marsupial offspring receive nourishment from their mothers?
      ::婚生后代如何从母亲那里获得营养?
    3. What are some advantages of marsupial development?
      ::婚后发展有什么好处?
    4. What are some disadvantages of marsupial development?
      ::婚后发展的一些不利之处是什么?