Why do the largest animals have the tiniest sperm? A brief investigation. 为什么全球最大动物的精子却是最小的？一份简要调查报告。 作者：Brian Resnick @ 2015-11-20

翻译：Drunkplane(@Drunkplane)

校对：慕白（@李凤阳他说）

来源：Vox.com， http://www.vox.com/science-and-health/2015/11/20/9768864/largest-animals-have-the-tiniest-sperm Stefan Lüpold is a sperm guy. The Swiss evolutionary biologist did his masters work on sexual selection and bat genitalia, his PhD on the evolution of bird sperm, and a postdoctorate on how fruit fly sperm compete. "I’m fascinated by the almost unlimited diversity in both size and shape of sperm," he writes me in an email from Zurich, describing his chosen sub-sub-discipline.

Stefan Lüpold 是个精子达人。这位瑞士进化生物学家硕士时研究的是性选择和蝙蝠的生殖器，博士时研究的是禽类精子的进化，博士后时则研究果蝇的精子如何竞争。“精子大小和形状近乎无穷的多样性让我着迷。”他从苏黎世给我发来电邮，讲述了他所选择的这一学科分支。

I've emailed him because he's recently found evidence to answer a perplexing question: Why are sperm so weird?

我给他发邮件是因为他最近的发现能回答一个令人费解的问题：精子为什么这么诡异？

Mice, for instance, have sperm that's twice as long as elephants'. The world's longest sperm belongs to a fruit fly. And across the animal kingdom, sperm take on extremely odd and varied shapes and sizes. The "tadpole" shape we most associate with sperm is not at all common outside of mammals. Rat and mice sperm can have hook-like attachments on their heads. "In some species they seem to allow sperm to connect by their heads and form so-called sperm trains," Lüpold says. "These groups of sperm seem to swim faster than individual sperm." Fascinating!

举例来讲，老鼠精子的长度是大象精子的两倍。世界上最长的精子属于一种果蝇。纵观整个动物王国，精子的形状和大小极其古怪和多变。“蝌蚪”状这一我们最为熟悉的形状在哺乳动物以外根本不常见。老鼠精子的头部会有钩子形状的附属物。“一些物种的精子似乎可以通过头部连接在一起，形成所谓的精子列车。” Lüpold说，“这种精子群似乎比单个的精子游得更快。”多么神奇！

From an evolutionary perspective this raises an intriguing question: Why are sperm so varied among different species when they all have the exact same purpose — fertilizing eggs?

从进化的角度看，这带来一个引人入胜的问题：所有的精子都有一个相同的目的——让卵子受精，但为什么不同物种的精子相差如此巨大？ [Four wildly different shapes of sperm, as illustrated in the text Sperm Competition and the Evolution of Animal Mating Systems (only 700 pages long!) . A) Silverfish sperm, B) sponge sperm. C) molluscan sperm.,D) gyrinid beetle sperm.]

【四种截然不同的精子形状，来自《精子竞争和动物交配系统的进化》一书（也就700页而已啦！）A) 蠹虫的精子 B) 海绵的精子 C)软体动物的精子 D) 一种豉甲的精子】 Lüpold has a theory. Analyzing the sperm of 100 species of mammals, he and a co-author found a pattern amid the chaos: the larger the species, the smaller the sperm. The results were recently published in Proceedings B , a journal of the Royal Society of London.

Lüpold有个理论。他和一位合著者分析了近百种哺乳动物的精子后，从这千头万绪中发现了一个规律：物种个体越大，精子越小。最近其成果已发表在伦敦皇家学会的《Proceedings B》杂志上。

Why would evolution favor such a pattern? Lüpold explains that longer sperm has some advantages — they are better at "elbowing" aside the competition. But it also takes a lot of energy to make long sperm, which larger animals can't afford. So it's a trade-off:

为什么进化倾向于这样一种规律？Lüpold 解释说，更长的精子具有某些优势——它们更能将竞争者“排挤”开；但是制造长精子会消耗大量能量，这是大型动物无法负担的，所以这是一种权衡取舍。 If there were no constraints on sperm production and assuming that longer sperm are advantageous, each male would probably produce lots of impressively big sperm. But in nature there are always constraints because resources and energy are not unlimited. For a testis of a given size, producing bigger sperm thus means it cannot produce as many of them (producing big sperm takes more resources, energy and time).

如果在精子生产方面没有限制，并且假设更长的精子确实更有优势，那么每个雄性也许会造出大量个头大得吓人的精子。但是自然界中总是存在种种限制，因为资源和能量不是无限的。对给定大小的睾丸来说，生产更大的精子意味着它生产的精子数量会减少（生产大精子会消耗更多资源、能量和时间）。

So, whether investing more in sperm size or in sperm number to maximize sperm competitiveness really depends on the circumstances, for example the size of the female reproductive tract. In large species, the female reproductive tract is massive compared to the tiny sperm, so sperm can easily be lost or diluted in it. Males have to compensate by transferring more sperm. Simply making longer sperm really wouldn’t make a difference in an elephant. They would have to be incredibly large. So males are better off making lots of tiny sperm.

所以，为了将精子的竞争力最大化，是在精子尺寸还是精子数量上“投资”取决于环境条件，比如雌性的生殖道。对大体型的物种来说，雌性的生殖道相较微小的精子来说太大了，所以精子很容易在其中迷失或被稀释掉。雄性只能通过投送更多的精子来弥补损失。简单通过制造更长的精子对大象来说于事无补，精子得大到离谱才行【 编注：是指大到离谱才能产生阻挡其他精子的效果 】。所以对雄性来讲，更好的策略是制造大量的小精子。 This inverse correlation between animal size and sperm size might be a consistent pattern across the animal kingdom. Almost all animals with sperm longer than a 10th of a millimeter, he explains, weigh less than one or two pounds. "Our results certainly suggest a unifying pattern that is likely to explain much of the diversity in mammalian sperm size and possibly beyond," he says, while noting more research is still needed.

动物的体型尺寸和精子尺寸之间这种负相关关系也许是动物王国里的普遍规律。几乎所有精子尺寸长于十分之一毫米的动物，体重都不超过一两磅。“我们的研究成果明确揭示了一个统一的模式，基本可以解释哺乳动物精子的多样性，也许还不止于此。”Lüpold说，然而他同时也表示还需要做更多的研究。

The mammal with the longest sperm? It's not the human. That distinction belongs to the honey possum, a very small (they grow to 3.5 inches long ) marsupial that lives in western Australia. They are adorable. Their sperm is 350 micrometers (.014 inches) long.

拥有最长精子的哺乳动物？不是人类。这一殊荣属于长吻袋貂，一种生活在澳大利亚西部的非常小（它们能长到八九厘米长）的有袋动物。它们很是可爱。它们的精子有0.356毫米长。

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