洋·居然：動物也變性！雌性動物長出「小丁丁」
這，再也不能好好撩妹了...
動物也會變性？來來來，聽聽科學的解釋吧！
眾多物種中，雄性表現出明顯的性特徵吸引雌性。他們通過「比美」擊敗對手，從而獲得雌性的青睞。如雄性孔雀和天堂鳥如錦似緞般的羽毛，雄性馬鹿特有的兇猛鹿角，無一不印證著這一點。
那麼，問題來了。自然界中兩性的角色是如何確定的？為什麼通常是雄性主動追求雌性，而不是雌性追求雄性呢？
研究人員認為原因在於「貝特曼原理」。原理指出：在後代身上投資越少的性別，性選擇的作用更加強烈。
美國進化生物學家Robert Trivers在1972年提出了「親代投資」的概念。他認為親代投資在決定何種性別處於更高性選擇壓力的過程中起著至關重要的作用。
根據特里弗斯和貝特曼原則，分配較少資源給親代投資的性別，在性別選擇上承受的壓力更大。產生精細胞的成本比產生卵細胞的成本低。
這表明，雄性比雌性產生更多數量的配子（在性交過程中出現）。反過來說，兩性因此受到的影響也非常不同。
通常，雌性通過親代照顧對後代投資更多，如孵卵、哺育和保護。因此雌性是親代投資的主體，而雄性則需要競爭才能主動接近雌性而繁殖後代。
雄性四紋豆象甲蟲在交配時，生殖器上的刺毛會毀掉雌性的生殖器
性別角色轉換
然而自然界中，存在著性別轉換的例子，即雌性表現出比雄性更強度更大的行為尋求配偶，這種現象並不罕見。某種程度上，這種性別轉換的進化這有驚人的適應性。
雄性成為「家庭主男」，雌性外出求偶，自然界中有很多這樣的物種。
如水雉，紐西蘭幾維鳥，中美洲和南美洲本土的䳍形目鳥和一些岸禽類鳥的物種。
值得一提的是海馬。在這一物種中，雄性海馬受孕並照顧後代。類似的物種還有屬箭毒蛙屬的蛙類，哺乳動物中有非洲的轉角牛羚等。
在性別轉換中，最令人吃驚的生理變化出現在魚類身上。如雌雄同體的金頭鯛。當魚卵孵化時，所有個體都為雄性。但當他們到了一定年紀，受體重、荷爾蒙和一些社會因素的影響，又可以選擇是否變為雌性。
自然情理之外
在有性生殖中，生殖器形態是動物進化過程中變化次數最多、變化速度最快的一個特徵。
在保持傳統性別角色的物種中，由於強烈競爭引起的強烈選擇導致雄性比雌性發展出更複雜的生殖器官。
雄性在生殖器或射精（包括精液中的蛋白質）方面所表現出的適應性，提高了精子傳送速度，刺激了雌性更快的接受精子，或將對手競爭下去。某些種類的豆娘（一種昆蟲），雄性甚至能清除由上一個雄性留在雌性體內的精子。
大千世界，除常態之外，進化中也出現了特例。
脊椎動物中，雌性發育出一種「假陰莖」結構。這是由於幼崽發育的最後階段荷爾蒙快速分泌的結果，導致陰蒂生長延長，從而導致這種結構的形成。
這種結構從身體向外突出幾厘米並且十分狹窄，使雄性很難與之交配，在分娩時也十分困難。儘管他們的性器官起著雌性的作用，但卻保持著雄性的形態。
另一個不可思議的例子是最近在洞穴中發現的一種昆蟲——新穴蟲（Neotrogla）。這種昆蟲可以加深人們對性選擇壓力的理解。
在這種物種中，雄性沒有類似陰莖類、插入式的器官，反而是雌性擁有類似陰莖的結構（稱為gynosoma）。這一結構用於深入雄性體內收集所謂的精囊。
許多無脊椎動物和脊椎動物，比如蠑螈或火蜥蜴，都會用精囊保存精子。在新穴蟲的例子中，雌性用他們充滿創意且獨特的類似陰莖的器官將精子從雄性的體內拖出來。這一器官還有棘刺，能使雌性在雄性體內部固定在一起。
是什麼導致了這一器官的形成？答案很簡單：對精液的性競爭和性衝突。
新穴蟲棲於洞穴之中，水和食物十分稀缺。居於這種條件，對精液的競爭就變得十分激烈。雌性也努力奮鬥尋求配偶。一旦一隻雌性找到一隻雄性，她就會瞄準目標，其gynosoma結構能使其錨定在雄性身上進行長時間的交配。
破除誤區
自然界中存在著許多普遍的交配方式，也存在某些特殊情況。就是這些特例，加深了我們對自然的理解。
性別誤區並不像傳統思想。相反，不同性別的表現行為取決於不同的影響因素，比如親代投資的不對稱，性別比例或配偶的獲取難易度。
因此，下一次，如果你看到一隻擁有突出器官的動物，不要理所當然的認為他們是雄性啦！
英文原文：
When sex roles get reversed, some females develop a 'penis'
In many species, the males develop elaborated sexual traits to attract females and dissuade potential rival males through competition. Some iconic examples are the extraordinary feathers of the peacock or paradise birds, or the menacing antlers of dominant red deer males.
But how is the role of each sex determined in nature? Why do males generally compete for access to females?
Researchers believe the answer lies in what is known as the Bateman principle, which suggests that sexual selection acts with more intensity in the sex that invest less in the offspring.
Parental investment wasproposed in 1972 by the American evolutionary biologist Robert Trivers as a key factor determining which sex is under higher sexual selection pressure.
According to Trivers and the Bateman principle, sexual selectionis stronger in the sex that allocates fewer resources to parental investment. The costs associated with the production of (minuscule) sperm cells are lower than those associated to the production of (large) eggs.
This implies males can produce a much larger number of gametes – the cells that merge during sex – than females do, which, in turn, entails important consequences for differences between the sexes.
In general, females keep investing more in the offspring through parental care such as incubation, nourishment and protection. So parental investment is usually far greater in females, and males compete to get access to them.
Sex-role reversal
But examples of sex-role reversal – when females compete more intensely than males to obtain mates – are not rare in nature. In some cases, the evolution of this so-called role reversal comes with stunning adaptations.
There are a variety of examples in nature in which males are the caregivers or females compete for access to mates.
Examples include the jacana water birds, New Zealand's kiwi birds, the tinamou birds native to Central and South America and some shorebird species.
Then, there are sea horses, among whom males get pregnant and carry the offspring during development; amphibians such as some species of frogs of the genus Dendrobates and; mammals such as the African topi antelope (Damaliscus lunatus).
A most amazing process in regards to physiological changes associated to sex-role reversal is found in fishes, such as the hermaphrodite gilt-head bream (Sparus aurata). All the individuals are males when they hatch but, when they reach a certain age, they can become females, depending on weight, hormones and social factors.
Nature pushing borders
Genital morphology is one of the most diverse and rapidly evolving features of animals with sexual reproduction.
In species with traditional sex roles, strong selection due to intense sexual competition has generally shaped more complex genitalia in males than in females.
Males frequently exhibit adaptations in the genitalia or the ejaculates (for instance, regarding seminal fluid proteins) to improve sperm transfer, stimulate a female's uptake of sperm, or to outcompete rivals. In some species, such as in some damselflies, males even remove the sperm transferred to the female by previous males.
But, as with almost anything in nature, there are exceptions that commonly prove the evolutionary processes underlying general patterns.
Among vertebrates, the females of the spotted hyena (Crocuta crocuta) have developed a pseudo-penis structure. This is the result of an elongation of the clitoris due to a hormonal boost during the final stage of cub development.
This structure, which protrudes several inches from the female's body and is very narrow, makes it more difficult to achieve successful copulation by males as well as giving birth for females. Although their genitals are female in function, they are male in form.
But another remarkable example, which constitutes a huge step towards understanding the selective pressures acting on the sexes, has been recently observed in cave insects of the genus Neotrogla.
In this species, males lack an intromittent, or penis-like, organ and females have developed a penis-like structure (called gynosoma), which is used to penetrate the body of the males to collect so-called spermatophores.
Commonly used by many invertebrate species but also by some vertebrates such as newts and salamanders, the spermatophores are sperm-containing capsules. In the extraordinary case of Neotrogla, females are pulling the sperm out of the male body using their innovative and exclusive penis-like organ. This organ has spines that allow females to grasp and hold mates from the inside.
What leads to the development of this structure? The answer is simple: sexual competition and sexual conflict over seminal fluid.
Neotrogla inhabit caves where water and food resources are very scarce. In these conditions, competition to obtain seminal fluid is fierce; and females strive to mate. Once a female finds a male, the gynosoma enables prolonged copulation by anchoring her to the male.
Don't stereotype
Nature provides us with a deluge of common mating patterns, but also with exceptions. These exceptions enhance our knowledge of how nature and evolution work.
Sexual stereotypes are not as universal as traditionally thought. Instead, the way each sex behaves depends on several factors such as asymmetries in parental investment, sex-ratio or the availability of mates.
So the next time you see an animal with what seems to be an intromittent sexual organ, don't take for granted it's a male.
鏈接尋找： Icy
譯者：Icy
2016.3.4.
校對：Candy
2016.3.5
原文鏈接：
http://phys.org/news/2016-02-sex-roles-reversed-females-penis.html
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