The evolutionary tango of animal genitalia. Can you guess what you're looking at? If you answered "duck vagina," you'd be right. Although the bird's outward appearance may not strike you as especially odd, it uses this strange, intricate, cork-screw shaped contraption to reproduce. We see similarly unbelievable genitalia in insects, mammals, reptiles, fish, spiders, and even snails. Apparently, no organs evolve faster and into more variable shapes than those involved in procreation. Superficially, it makes sense because evolution works via reproduction. When an animal leaves more offspring, its genes will spread. And since genitalia are an animal's tools for reproduction, any improvement there will have immediate effect. And yet, what's the point of having such decorative nether regions? After all, the function of genitalia seems simple. A penis deposits a bit of sperm and a vagina receives it and delivers it to the egg. A pipette-like thingy on the male and a funnel-like gizmo on the female should do just fine for any animal. And yet, that's not what we see. The penis of a chicken flea, for example, looks nothing like a pipette, more like an exploded grandfather clock. And the vagina of a featherwing beetle resembles something you'd find in a Dr. Seuss book. Throughout the animal kingdom, genitalia are very complex things, much more complicated than seems necessary for what they're meant to do. That's because genitalia do more than just deposit and receive sperm. Many male animals also use the penis as courtship device, like crane flies. In some South American species, males have a tiny washboard and scraper on their penis, which produces a song that reverberates throughout the female's body when they mate. It's thought that if female crane flies enjoy this unusual serenade, they'll allow the male to father their offspring. This way, the genes of the most musical penises spread, leading to rapid evolution of insects' phalluses. Similarly, some beetles have two little drumsticks on either side of the penis. During mating, they'll rub, slap, or tap the female with these. And some hoofed mammals, like rams and bulls, use a whip-like extension on the penis's left side to create a sensation during mating. But how can females really choose between males if she can only assess them after mating? This is where the power of female adaptation comes into play. In fact, insemination is different to conception, and the female genitalia exploit this distinction. For instance, in some dung flies, the vagina contains pockets for separating sperm from different males depending on how appealing they were. Males using their penises for courtship and females controlling their own sperm management are two reasons why genitalia evolve into such complex shapes. But there are others because genitalia are also where a sexual conflict is played out. A female's interests are best served if she fertilizes her eggs with the sperm of the best fathers and creates genetic variability amongst her offspring. For a male, on the other hand, this is bad news. For him, it would be best if a female used his sperm to fertilize all of her eggs. So we see cycles of adaptation in an evolutionary arms race to retain control. Black widow spiders have a disposable penis tip that breaks off inside the vagina blocking the attempts of his rivals, and bed bug males bypass a female's genitalia altogether using a syringe-like penis to inject sperm cells directly into her belly. Not to be outdone, females have evolved their own countermeasures. In some bed bug species, the females have evolved an entirely new set of genitalia on their right hand flanks where the males usually pierce them. That allows them to maintain the power to filter out unwanted sperm with their genitalia. And duck vaginas are shaped like a clockwise spiral so that when the male inflates his long, counterclockwise coiled penis into her, and she disapproves, all she needs to do is flex her vaginal muscles and the penis just flubs out. So, genitalia differs so much, not just to fascinate us, but because in every species, they're the result of a furious evolutionary tango of sex that has been going on for millions of years and will continue for millions of years to come.
動物生殖器官的演化探戈 你猜得到你正在看什麼嗎? 如果你回答「鴨子陰道」 那就答對了! 雖然鳥類的外型或許並不會 讓你覺得有什麼特別奇怪 牠們卻是運用像這樣 奇怪、複雜的 螺旋形器官來繁衍後代 這般不可思議的生殖器 也能在昆蟲身上找到 或是哺乳類 爬蟲類 魚類 蜘蛛 甚至是蝸牛 顯然,其他器官的演化 都不如生殖器官這般迅速 這般多樣化 表面上,這很合理 因為演化即由繁衍而來 當一隻動物產下愈多後代 牠的基因就愈能遠播 由於生殖器官是 動物繁衍後代的工具 每當生殖器官有任何演進 便會隨即產生影響 但是,何必要有 這麼裝飾性的下體呢? 畢竟,生殖器官的功能 看起來終究簡單 陰莖產生一些精子 陰道接收精子,並將精子傳送至卵子 雄性提供滴管狀的東西 雌性擁有漏斗狀的構造 在任何動物身上應皆能發揮作用 然而,事情並不全然那麼簡單 舉例來說,雞跳蚤的陰莖 看起來一點都不像滴管 反而像是一個炸開的老爺鐘 至於羽翼甲蟲的陰道 就好似某個在蘇斯博士的 繪本裡會看到的插圖 在動物王國裡 生殖器官其實是很複雜的東西 遠比其看似簡單的功能更為複雜 因為生殖器官的作用 事實上不僅止於產生與接收精子 對很多雄性動物來說 陰莖也是求愛工具,例如大蚊 某些南美洲品種的大蚊 陰莖上具有小型洗衣板和刮具 交配時,能夠演奏出曲子 在雌性的體內迴盪不絕 科學家相信,如果雌蚊 喜歡這首特別的小夜曲 牠就會願意與該雄蚊產下後代 如此一來,最具音樂性的 陰莖基因就會傳承下來 促使昆蟲的陰莖快速演化 有些甲蟲的陰莖兩側 各有一枝小鼓棒,原理相似 交配時,雄性會用它們 摩擦、拍擊或敲打雌性 部分有蹄類,包括公羊、公牛 其陰莖左側具有鞭子狀的延伸物 能在交配時產生快感 但假如雌性只能在交配後 才能夠評判雄性優劣 那麼,牠們到底該如何選擇伴侶呢? 這時候,雌性的調適能力 就要發揮其重要性了 事實上,受精並不等同於受孕 而雌性生殖器官 充分運用了此一差異 舉糞蠅為例 其陰道具有多個口袋 能夠分裝不同雄蠅的精子 雄蠅的個別魅力即為決勝關鍵 雄性使用陰莖求愛 而雌性擁有自己的精子管理系統 以上兩個理由促使生殖器官 演化成如此複雜的形狀 不只如此 由於生殖器官 也是性別衝突展現之處 雌性的利益唯有在以下情況 才能獲得最佳保障: 卵子受精自雄性伴侶最佳人選 以及,為其後代創造遺傳變異性 然而,這對雄性來說卻是壞消息 雌性最好只採用某單一雄性的精子 來為所有的卵受精 因此,我們能看到一連串的 調適循環在演化過程中較勁 以爭取主導權 黑寡婦擁有拋棄式龜頭 會斷裂遺留在陰道中 阻擋其他對手精子的去路 而雄床蝨則乾脆略過雌床蝨的陰道 用針筒狀的陰莖 直接將精子細胞注射進雌性的腹部 雌床蝨也沒那麼好欺負 牠們演化出與之抗衡的對策 在某些床蝨品種中 雌性演化出一整套全新的生殖器官 位於腹部右側 也就是雄性經常刺穿的部位 如此一來,牠們就能夠 繼續運用生殖器官 篩選掉不要的精子 至於鴨子的陰道 則呈現順時鐘螺旋狀 這樣一來,當公鴨將其 逆時針盤繞狀陰莖送進牠的陰道 但牠不同意的時候 牠只需要收縮陰道的肌肉 陰莖就會被擠出來 所以說,生殖器官如此多樣化 並非只是要讓我們驚豔 更重要的是 生殖器官是各物種性別間 激烈的演化探戈的成果 這場探戈已經持續幾百萬年了