Many of the earliest artifacts of human culture (such as cave drawings and sculptures) indicate that humans have been preoccupied with sex for as about as long as there have been humans. For thousands of years, human cultures have also been concerned with identifying what each culture felt were appropriate expressions of sexuality and which expressions were inappropriate. Often, the justification for the appropriateness of sexual acts centered on whether or not it was "natural". As our understanding of animal sexuality has increased, the variations of sexual behavior which occur in nature (and thus are "natural" in the original sense of the word) has grown.

Open mouth kissing (including same sex kissing) occurs in Bonobo chimps, common chimps, and squirrel monkeys. Open mouth kissing can be intense in Bonobos (Bagemihl, 1999; Fouts, 1997).

Apes have displayed a diverse array of sexual practices, some of which were previously thought to be unique to humans. These include the "missionary" position (Orangs and Bonobos; common chimps do rarely while this is the position of 1/3 of Bonobo matings), a variety of sexual positions (Bonobos); and sex used for purposes other than procreation (to reconcile, make friends, and calm tension in Bonobos; in exchange for food in common chimps).
In gibbons, Bonobos, and Old World monkeys, sexual activity can occur when females are not able to conceive. Bonobo females are continually sexually receptive (Savage-Rumbaugh, 1998). Face to face intercourse occurs in gorillas, Bonobos, gibbons, and dolphins. Anal stimulation frequently occurs in heterosexual encounters between orangutans Bagemihl, 1999).
The former name for chimpanzees was Pan satyrus for their sexual activity. In a colony with four males, a female may mate six times a day during estrus (although the mating typically lasts for less than 15 seconds). Dominant males mate more frequently than males of lower rank. Females experience climax (De Waal, 1982). Female common chimpanzees may mate several hundred times per conception. (Bagemihl, 1999). Bonobos engage in sexual activity more frequently than in common chimpanzees and this activity plays a factor in female relationships and group cohesiveness (Wrangham, 1994).

Female gorillas average 7 births per female while chimpanzees average 4 (Wrangham, 1994).

Clitoral rubbing in bonobo chimps, rhesus monkeys, and gorillas. Female penetration can involve fingers, tails (dolphins), an erect clitoris (Bonobo chimps), and foreign objects . Mutal masturbation occurs in macaques. Masturbation is known in in gorillas, rhesus monkeys, macaques, vampire bats, and proboscis monkeys (Bagemihl, 1999). Lucy (a chimp) would masturbate with a vacuum cleaner (after plugging it in) and would "read" National Geographic regularly until estrus at which time she preferred Playgirl (Fouts, 1997).

Oral sex which involves sucking occurs in Bonobos, orangutans, siamangs, stumptail macaques; licking more common in animals as diverse as hyenas, cheetahs, sheep, and vampire bats (Bagemihl, 1999).

Same-sex sexual activity is known in more than 450 kinds of animals including representatives of every major group of animals and every continent of the world. Same-sex contact includes lip contact and open-mouth kissing (Bagemihl, 1999). Male anal intercourse occurs in orangutans, bonobo chimps, rhesus monkeys, bison, and bighorn sheep. Same sex pairs may raise young, especially in birds.

Same-sex mounting and genital manipulation has been observed in a number of species of New World monkeys. Same-sex mounting has been observed in some prosimians. (Bagemihl, 1999).

All female Hanuman langurs have same-sex relationships (Bagemihl, 1999). Amoung macaques, most females are bisexual while many are heterosexual and some are exclusively homosexual. In an number of Old World monkeys and apes, the frequency of homosexual encounters varies from population to population (Bagemihl, 1999).

Homosexual activity in orangutans includes male anal intercourse, oral genital contact, kissing, and manual masturbation. Most males probably bisexual, at least when young and most females are bisexual (Bagemihl, 1999).

Gorilla females and males frequently engage in same-sex affectionate relationships, often with a particular individual of the group. In both males and females, same-sex interactions last longer than heterosexual ones and more frequently use face to face interactions as opposed to mounting from the rear. Most males are bisexual (at least when young and in all-male groups) and some are exclusively heterosexual or homosexual. There is variation among females as to whether they are bisexual, predominantly heterosexual, or predominantly homosexual (Bagemihl, 1999).

About 1/3 of the mounting between common chimps occurs between males. Manual stimulation and same-sex kissing can also be common in males and females. In some populations, almost all males may participate in homosexual activity. Some individuals seem to have a homosexual orientation (Bagemihl, 1999).

Bonobo chimps utilize G-G rubbing, orgasm, female group sex, anal stimulation, oral sex, same sex kissing; 40 to 50% of sexual activity is homosexual and almost all bonobos are bisexual. (Bagemihl, 1999).

Bonobos can have sexual interactions with redtail monkeys in the wild. Male orangutans may have homosexual encounters with male crab-eating macaques (Bagemihl, 1999).