Polyandry, the practice of females mating with multiple males, affects sexual selection even after mating. Sperm competition occurs when males try to fertilize a female's eggs after mating. This competition led researchers to discover that females can influence which male's sperm fertilizes their eggs. This ability of females to choose the fathers of their offspring is called cryptic female choice. Male traits evolving due to sperm competition are selected based on how well they perform inside the female reproductive tract. Female behaviors, physical features, and biological functions act as filters for selecting the best sperm for fertilization, highlighting the importance of post-mating sexual selection processes.