Women with women Main article: Lesbianism in erotica: Lesbianism in contemporary pornography Lesbian or girl-girl pornography features videos gays women hentai porn engaged in sex together, but is typically aimed at a male audience. The introduction of the home video and Internet saw a boom in the worldwide porn industry that generates billions of dollars annually. Nevertheless, it is only an attempt at phe toon porn nomenological differentiation. Group sex also occurs in populations of non-human animals such as bonobo apes and chimpanzees. Some people masturbate by using machines that simulate intercourse. Mammary intercourse is known as titty-fucking, titfuck, or a titjob in the United States, as well as tit wank or French fuck in the United Kingdom - the latter term dating back to the 1930s; while a more jocular equivalent is a trip down mammary lane. A study of 164 males in Finland from two SM clubs found that 18.2% had engaged in coprophilia; 3% as a sadist, 6.1% as a masochist, and 9.1% as both. 18% of heterosexuals and 17% of homosexuals in the study pool had tried coprophilia, showing no statistically significant difference between heterosexuals and homosexuals. Works in this genre may explore philosophical, political or social issues. In POV porn, the shooting style is generally similar to gonzo pornography, with the person receiving sexual gratification holding the camera him or herself—aiming it down at the actress or actor who is performing the sex act. Often other friends and community members may witness the signing of such a document in a ceremony, and so parties violating their agreement can result in loss of face, respect or status with their friends in the community. Cartoon pornography includes, but is not limited to, hentai and parody renditions of famous cartoons and comics. Pegging films include Strap It On , Babes Balling Boys , Strap Attack, Strap-on Sirenz, Strap-on momporn Chicks, Boss mobile porn sites Bitches , various movies produced by LEDA Studios, and San Francisco-based Men In Pain. They signal the presence of estrogen and the presence of sufficient fat stores for pregnancy and lactation.