Sexual intercourse
Two animals coming together for the purposes of sexual reproduction is called mating. Most mammals only mate when the female is at the point of estrus, which is the most fertile period of time in its reproductive cycle. In certain animals, sexual intercourse is not only used for reproduction, but has taken other functions as well. These animals include bonobos, dolphins, and chimpanzees  which also have sexual intercourse even when the female is not in estrus, and to engage in sex acts with same-sex partners. In most instances, humans have sex primarily for pleasure. This behavior in the above mentioned animals is also presumed to be for pleasure, which in turn strengthens social bonds.