Mating - Coitus

In animals where internal fertilization occurs, mating refers to the act of pressing the genital opening together and sending the male's sperm into the female's body. During the course of evolution, animals that moved onto land could no longer use the water in their habitat as a means of transporting sperm, as fish and many aquatic invertebrates do. This meant that males needed to send sperm directly into the female's body. For this reason, many species have developed copulatory organs for sending and receiving sperm. Usually, the male's copulatory organ is called the penis, and the female's is called the vagina. The most typical example is mammals, where mating takes place by inserting a well-developed penis into the vagina. In most birds, the copulatory organ is not particularly developed, and mating takes place by pressing the cloaca together. In reptiles, mating takes place using a copulatory organ called the hemipenis, which is formed by developing the inner wall of the cloaca. In amphibian frogs, internal fertilization does not occur; females lay eggs near water and the male deposits sperm on them, but at this time the male holds the female and brings his genital opening close to her. This is not called mating, but rather concupiscence. Fish often fertilize externally, but some, such as the swordtail, have mating organs in their anal fins and perform internal fertilization. Many sharks also have developed mating organs in their anal fins. Invertebrates such as insects, earthworms, and snails mate, and some have developed unique mating organs. Males also engage in various courtship behaviors to get close enough to the female to mate.

[Masaru Wada]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

体内受精が行われる動物で、生殖口を密着させて雄の精子を雌の体内に送り込む行動をいい、交接ともいう。進化の過程で陸にあがった動物は、魚や多くの水生無脊椎(むせきつい)動物のように、生活している場にある水を精子の運搬手段として使えなくなった。そこで雄が直接、雌の体内に精子を送り込む必要が生じた。このため、多くの種では、精子を送りあるいは受けるための交尾器官が発達している。普通、雄の交尾器官を陰茎、雌のものを腟(ちつ)という。もっとも典型的な例は哺乳(ほにゅう)類で、よく発達した陰茎を腟に挿入して交尾が行われる。多くの鳥類では交尾器官はとくに発達せず、総排出口を密着させて交尾する。爬虫(はちゅう)類では総排出腔(こう)の内壁が発達してできた半陰茎とよぶ交尾器官によって交尾が行われる。両生類のカエルの仲間では体内受精は行われず、雌は水辺で産卵し雄が精子をかけるが、この際、雄は雌を抱きかかえ生殖口を近づける。これは交尾とはいわず抱接という。魚類は多くの場合体外受精であるが、ソードテールのように臀(しり)びれが変化した交尾器官をもち体内受精を行うものもある。サメの仲間にも、臀びれの変化した交尾器が発達したものが多い。無脊椎動物では昆虫やミミズ、カタツムリなどが交尾を行い、独特な交尾器官が発達しているものもある。また、雄は、交尾可能なところまで雌に近づくために、さまざまな求愛行動を行う。

［和田　勝］

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