Sexual Selection
By: Patricia L. R. Brennan
Post-copulatory male-male competition is known as sperm competition a term coined by Parker (1970) who recognized that when females mate with multiple males, their ejaculates compete inside the female reproductive tract for access to eggs. Sperm competition has resulted in the evolution of morphologically modified sperm that increase the likelihood of fertilization in many taxa (Birkhead & Moller 1998). Post-copulatory female choice refers to the ability of females to affect the likelihood that sperm from a particular male fertilizes their eggs, and their decision to invest in offspring based on the identity of the male with whom they mate. Females exert this choice via morphological, chemical and behavioral adaptations.
https://www.nature.com/scitable/knowledge/library/sexual-selection-13255240/
Why does the peacock have such an elaborate tail? This cumbersome trait significantly decreases the male’s chances of survival. It only exists because it confers an advantage to its bearer in the form of increased reproductive success.