The researcher stated "I was watching opportunistically, so I can't give an exact figure of how common it really is." It has been reported that "bartering of meat for sex ... forms part of the social fabric of a troop of wild chimps living in the Tai National Park in the Côte d'Ivoire." Pavlovian conditioning: The sexualisation of objects or locations is recognised in the animal breeding world. For example, male animals may become sexually aroused upon visiting a location where they have been allowed to have sex before, or upon seeing a stimulus previously associated with sexual activity such as an artificial vagina. Sexual preferences for certain cues can be artificially induced in rats by pairing scents or objects with their early sexual experiences. The primary motivation of this behaviour is Pavlovian conditioning, and the association is due to a conditioned response (or association) formed with a distinctive "reward". Viewing images: A study using four adult male rhesus macaques (Macaca mulatta) showed that male rhesus macaques will give up a highly valued item, juice, to see images of the faces or perineum of high-status females. Encouraging captive pandas to mate is problematic. Showing young male pandas "panda pornography" is credited with a recent population boom among pandas in captivity in China. One researcher attributed the success to the sounds on the recordings. Copulatory wounding and Traumatic Insemination: Injury to a partner's genital tract during mating occurs in at least 40 taxa, ranging from fruit flies to humans. However, it often goes unnoticed due to its cryptic nature and because of internal wounds not visible outside. Non-reproductive sexual behaviour There is a range of behaviours that animals perform that appear to be sexually motivated but which can not result in reproduction. These include: Masturbation: Some species, both male and female, masturbate, both when partners are available and otherwise. Oral sex: Several species engage in both autofellatio and oral sex. This has been documented in brown bears, Tibetan macaques, wolves, goats, primates, bats, cape ground squirrels and sheep. In the greater short-nosed fruit bat, copulation by males is dorsoventral and the females lick the shaft or the base of the male's penis, but not the glans which has already penetrated the vagina. While the females do this, the penis is not withdrawn and research has shown a positive relationship between length of the time that the penis is licked and the