It sounds a little trampy to humans but in nature, it's not unusual for a female to copulate with several males in quick succession. Chimpanzees are a well-known example.
When that happens, sperm war breaks out.
"The sperm of the different males then compete within the female to fertilize the eggs," says evolutionary biologist Steven Ramm from Bielefeld University. "Generally speaking, the best sperm wins. This may involve its speed or also be due to the amount of sperm transferred. It can also be useful for the seminal fluid to be viscous, meaning it sticks inside the female reproductive tract to try to keep other sperm at bay."