In Larry Niven's Ringworld novels, rishathra is "sexual practice outside one's own species but within the intelligent hominoids". It is not generally considered a taboo and is often used by the myriad hominids of the Ringworld as a way of sealing agreements, such as trade contracts and peace treaties. Humans, though not native to the Ringworld, share a common descent with the hominids of the Ringworld and may participate freely in rishathra.
Rishathra is also used as a form of birth control. Because it is impossible to impregnate a member of another species, sexual intercourse with such an individual provides sexual release without the risk of pregnancy. The issue of sexually transmitted diseases is not explored in the Ringworld novels until the last book, Ringworld's Children in which it is briefly mentioned that Ringworld STDs cannot jump the species barrier.
The (non-sentient) "Vampire" species of hominid on Ringworld uses rishathra to obtain prey. A powerful pheromone is released that brings any compatible hominid into an extreme state of arousal. The victim immediately attempts to mate with the nearest compatible hominid, usually the vampire. During this mating, the vampire gains sustenance by draining the victim's blood.
The "Ghoul" species of hominid on Ringworld are generally not permitted to "rish" with members of another species. This is due to their place in the Ringworld ecosystem as carrion eaters, specifically the dead bodies of other hominid races, a practice that while accepted by other hominids prohibits such close relations.
The "Water People" species of hominid on Ringworld are not able to rish with most other hominids due to the requirement that such activities take place underwater. Air breathers are typically not able to hold their breath for sufficient time to allow the act of sexual intercourse to take place. (This does not seem to disappoint them, however.)
Popular culture and science
The discovery of populations of Homo denisova cohabiting with Neanderthal and Homo sapiens in a cave in Siberia in 2011 raised the interesting possibility that Niven may have accurately "hindcast" a previous state of hominid behaviour. Scientists now believe that Denisovans may have passed on useful immunity alleles to the Homo sapiens genome.