The process by which spermatozoa from a male are deposited in the body of a female during sexual reproduction. In mammals the penis of the male becomes erect and stiff as its tissues become filled with blood, enabling it to be inserted into the vagina of the female. Thrusting movements of the penis result in ejaculation, in which semen, containing spermatozoa, is deposited in the vagina. The males of many invertebrates deposit their sperm in packets called spermatophores, which the female must then locate to transfer the sperm into her reproductive tract. In some cases the male assists in this process.