Evolution. Handsome finches win a boost for their offspring.
Vogel
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D000818:Animals; D013196:Dihydrotestosterone / Q000378:metabolism; D004530:Egg Yolk / Q000378:metabolism; D005260:Female; D005796:Genes; D008297:Male; D010063:Ovum / Q000378:metabolism*; D012726:Sexual Behavior, Animal; D020308:Songbirds / Q000235:genetics / Q000502:physiology*; D013739:Testosterone / Q000378:metabolism*
D013196:Dihydrotestosterone; D013739:Testosterone