Although it is impossible to quantitatively measure a pleasure response in bonobos, Frances de Waal has proposed that an internal motivator must be at play. Working off of facial expression, de Waal notes that females often bare through teeth during sex, particularly at its end. This wide baring of the teeth dubbed “the pleasure grin” has also been observed during solitary masturbation in addition to when the bonobo was excited over food or novel objects to play with (de Waal 1995). Furthermore, Woods’ (2011) observations about the use of socio-sexual contact in infant orphaned bonobos during feeding corroborates the case for a strong internal motivator. The mechanisms governing primate arousal have not been studied. Testosterone governs sexual activity in many mammals, however, bonobo females were not found to have higher levels of testosterone than chimpanzee counterparts, suggesting the involvement of other hormones.


Incidence of sexual behavior rises sharply during feeding times, suggesting that there is a link between food and sexual behavior in bonobos. However, except among infants, nonreproductive sexual behavior has also been observed by many researchers outside the context of feeding. In fact, almost anything can trigger a sexual response in bonobos. As Frans de Waal recounts, even something as simple as two bonobos reaching for the same cardboard box can be an occasion for sexual behavior. Thus, socio-sexual behavior in bonobos must serve some must be triggered by something more general. De Waal suggests that this sexual behavior is a response to tension created by anticipated competition over resources. Hence, not just food, but just about anything that can capture the attention of multiple bonobos can be the stimulus needed to trigger sexual behavior in bonobos.