being sexually active does not necessarily correlate with their ovulation cycles. Sexual activity occurs between almost all ages and sexes of bonobo societies. Primatologist Frans de Waal believes that bonobos use sexual activity to resolve conflict between individuals. Immature bonobos, contrariwise, perform genital contact when relaxed. Macaque Similar same-sex sexual behaviours occur in both male and female macaques. It is thought to be done for pleasure as an erect male mounts and thrusts upon or into another male. Sexual receptivity can also be indicated by red faces and shrieking. Mutual ejaculation after a combination of anal intercourse and masturbation has also been witnessed, although it may be rare. In comparison to socio-sexual behaviours such as dominance displays, homosexual mounts last longer, happen in series, and usually involve pelvic thrusting. Females are also thought to participate for pleasure as vulvar, perineal, and anal stimulation is part of these interactions. The stimulation can come from their own tails, mounting their partner, thrusting or a combination of these. Dolphin Male bottlenose dolphins have been observed working in pairs to follow or restrict the movement of a female for weeks at a time, waiting for her to become sexually receptive. The same pairs have also been observed engaging in intense sexual play with each other. Janet Mann, a professor of biology and psychology at Georgetown University, argues that the common same-sex behaviour among male dolphin calves is about bond formation and benefits the species evolutionarily. They cite studies that have shown the dolphins later in life are bisexual and the male bonds forged from homosexuality work for protection as well as locating females with which to reproduce. In 1991, an English man was prosecuted for allegedly having sexual contact with a dolphin. The man was found not guilty after it was revealed at trial that the dolphin was known to tow bathers through the water by hooking his penis around them. Hyena The female spotted hyena has a unique urinary-genital system, closely resembling the penis of the male, called a pseudo-penis. Dominance relationships with strong sexual elements are routinely observed between related females. They are notable for using visible sexual arousal as a sign of submission but not dominance in males as well as females (females have a sizeable erectile clitoris). It is speculated that to facilitate this, their sympathetic and parasympathetic nervous systems may be partially reversed in respect to their reproductive organs.