Picture a group of wolves or with one male sitting at the top of the order: he eats first, the others move aside, and his mood sets the mood of the group. Alpha is the first letter of the Greek alphabet, so the phrase names the male who comes first. Every later use is that same picture carried over to people.
可以想成一群狼或黑猩猩裡，有一隻公的排在最上面：牠先吃、別的成員讓路、牠的脾氣決定全場的氣氛。alpha 是希臘字母的第一個，所以這個說法畫的就是「排第一的那隻公的」。之後的用法，都是把這張圖原封不動搬到人身上。
From the word’s history
The phrase came out of scientific work on rank in social animals, and was later applied to men with dominating characteristics; it was already around in the 1990s and spread further after Neil Strauss's The Game (2005).
這個詞原本出現在研究動物社會階級的科學文獻裡，後來才被拿去形容具有支配性格的男人。1990 年代已經有人在用，2005 年 Neil Strauss 的 The Game 讓它流行起來。所以「動物首領」不只是比喻的說法，它真的是這個詞的出發點。
Wiktionary, "alpha male"
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