Studying sperm competition in fruit flies, scientist Mollie Manier color-coded their junk and made videos of sperm "racing around the female's genital tract at high speed, like miniature formula-one cars," according to one awed science writer.
Female fruit flies' ornate vaginas contain a "special pouch" for storing male fruit flies' jiz, which means the males' sperm compete inside her vagina. The reproductive advantage goes to the last guy who bangs her, because his young, feisty sperm will knock the previous guys' old, sluggish goo out of the way. Here, two male's sperm—color-coded red and green—compete in a Christmas-themed mating race.
Update: Night editor Adrian and commenter AvJ simultaneously and independently improved this video, in a video tableau that shall henceforth be known as "Yakety Sperm."