性选择如何影响物种生存能力？

Many scientists have tried to predict how sexual selection might influence a species’ fate. “Does it make organisms more susceptible to collapse because they are too big or too flashy?” says Patricia Brennan from the University of Massachusetts at Amherst. “Or does it make them better able to overcome challenges because they have better genes, or are somehow better adapted to their environments?”

许多科学家试图预测性选择是如何影响物种命运。“它是否使生物体更容易灭绝，因为它们个体太大或太过暴躁？”来自马萨诸塞大学阿莫斯特分校的帕特里夏布莱南说。“抑或是会让他们更好地克服挑战，因为他们有更好的基因，或者更好地适应他们的环境？”

There are several studies that support both of these conflicting possibilities, showing variously that sexual selection can protect against extinction, lead to it, or neither. But almost all of this research focused on animals that are still alive, and merely estimated their odds of going extinct by looking at population trends, local disappearances, or conservation status. “A lot of these results are ambiguous,” says Fernandes Martins. “When you work on living species, you’re not actually analyzing extinction.”

数项研究表明两者都有可能性，性选择可能会防止或者促进种族灭绝，也可能对灭绝没有影响。但是几乎所有的研究都集中在那些还活着的动物身上，仅仅通过观察种群趋势、当地的消失或保护状况来估计它们灭绝的可能性。“很多这样的结果都是模棱两可的，”费尔南德斯说。“因为研究活物种时，你实际上并不是在分析物种灭绝。”

That’s why she and her team decided to study ostracods. Their hard shells can withstand the ravages of time, so there are plenty of ostracod fossils to examine. Different species have distinctive shapes, so it’s easy to chart the rise and fall of particular lineages over time. Males are more elongated than females, so it’s also easy to tell the sexes apart—a rarity among prehistoric creatures. And among males, shell size reflects penis size, so that even when these soft organs have long since decayed, you can estimate their dimensions by looking at the entire animal.

这就是为什么她和她的团队决定研究介形虫的原因。它们坚硬的外壳能够经受住时间的破坏，所以有大量的介形虫化石可以检验。不同的物种有不同的形状，随着时间的推移，很容易绘制出特定血统的兴衰。由于雄性比雌性更为细长，所以很容易区分两性—这在史前生物中是罕见的。在雄性中，壳的大小反映了阴茎的大小，所以即使这些软器官已经腐烂很久，你也可以通过观察整个动物来估计它们的尺寸。

Fernandes Martins did this for 93 ostracod species, which lived between 66 and 84 million years ago in what is now Mississippi. She found that those where males were biggest relative to females went extinct 10 times faster than those whose males were much smaller. On average, species whose males invested most heavily in their sexual organs persisted for just 1.6 million years. By constrast, species whose males stepped away from the sexual rat race persisted for 15.5 million years.

研究团队观察的标本涵盖了93个生活在距今大约8500万至6500万年前生活在密西西比（白垩纪晚期）的介形虫物种研究发现性别差异最大，也就是性选择最强的物种平均幸存时间只有160万年，而那些雄性在交配战争中花费最少的物种，在地球上存活的平均时间有1550万年-近乎前者的10倍。

Perhaps that’s because it takes a lot of energy to build large sex organs and long sperm—energy that could go towards other adaptations. “In the short term, it’s beneficial for the individual, since he’ll have more babies,” says Fernandes Martins. “But in the long-term, it can be a problem. If you’re investing in reproduction, you’re not investing in coping with a changing environment.”

也许这是因为它需要大量的能量来建立大型的性器官和长精子-这些能量也可以适应其他环境条件。费尔南德斯说短期而言，性选择有利于个体，它们可以产下更多的后代。但从长远角度考虑，性选择会带来问题。如果你在繁殖上投入太多，你将无力应对其他类型的环境变化。

“It’s a great study,” says Brennan, “but it’s hard to predict how representative ostracods may be of larger [animals] that reproduce much more slowly and in smaller numbers.” Are elephant seals, whose males can outweigh females by more than four times, doomed to extinction? Is the peacock’s tail a death sentence? “We can’t answer that,” says Fernandes Martins. “It’s just one study.”

“这是一项伟大的研究，” 布莱南说。如果这一结论同样适用于其他体型更大、生殖周期更长的动物，这是否意味着雄性体型更大的象海豹、长着漂亮羽毛的孔雀被宣判了死刑？需要指出的是，这项研究的结论目前只适用于介形虫，性选择对其他动物的影响仍不清楚。“我们还不能回答这个问题，”费尔南德斯说，“这只是一项研究。”