Mating

Basic Mating Facts

Bonobos are a highly sexual species; females usually mature around ten years old, while males usually mature a little sooner (The Columbus Zoo & Aquarium 2004). Infants of 4 months have been observed trying to engage in sexual contact with their mothers which supports the idea that sex is used for other purposes than just procreation (Gibbons 1992). Bonobos engage in many different sexual positions with many different sexual partners (de Waal 2001). They have been known to french kiss as well as give a "nasalized scream" vocalization while copulating (de Waal 1989 and Dixson 1998). Bonobos participate in ventro-ventral copulation (front to front), which is made possible because the placement of the female bonobo's vaginal opening and clitoris (de Waal 1989). Not only do bonobos engage in male-female copulations, they also have female-female genital rubbing (see female-female contact below) and male-male penis fencing (de Waal 1989). Gibbons (1992) found that less than one-third of the sexual contact is between adults of the opposite sex. Bonobos use sexual contact for other purposes besides procreationg (de Waal 1989). Bonobos use sexual contact for calming infants, resolving conflicts among adults of the same sex, food distribution, and social bonding/community organization. Bonobos use sex to release tension, form social bonds, and smooth over agonistic encounters (Sagan and Druyan 1992).

Hierarchical Promiscuity

The bonobos are an extremely promiscuous species meaning they engage in sexual contact with many different partners (de Waal 1989). Promiscuity serves as a "mechanism for socialization" (Dixson 1998). It allows for affection, common goals, identification with others, and reconciliation (Dixson 1998). There are a couple of reasons that bonobos have a hierarchically promiscuous mating system. The first reason is that female bonobos prefer to mate with dominant males with good genes in order to insure the survival of their offspring (de Waal 1997). In order for the female bonobos to determine which male has the best genes there must be a male bonobo hierarchy (see Social Organization). Secondly, female bonobos are found in large groups which are too big for one male to control. Therefore, there are no harem groups in bonobo society. Thirdly, since groups are composed of both sexes, multiple males associate with multiple females everyday. This makes the females readily accessible to the males, which leads to an increase in competition between the males. This competition aids in the establishment of the male bonobo hierarchy. Bonobos probably developed into a promiscuous mating system in order to guard against infanticide (de Waal, 2001). Females have sexual swellings almost all of the time, so males do not know when they are in estrus. Therefore the males do not know which copulations result in offspring. Females also have multiple partners, therefore males do not know which offspring are theirs. Since the males do not know which offspring are theirs, they are not going to risk killing their own kin, if they kill the infants.

Allogrooming

There is no evidence that bonobos allogroom, either. Although male-female grooming occurs the most often in bonobos, this is most likely between the mother and her sons (Wrangham, McGrew, de Waal, and Heltne 1994). Adult females are often found grooming one another in order to establish strong social bonds, but there have been no observations of mothers allowing other females to groom their offspring.

Sexual Swellings

Female bonobos retain their sexual swellings for a lot longer than female chimpanzees, which allows for breeding to be non-seasonal (Dixson 1998). Females are in a sexually active state almost 75% of the time and copulations can occur at almost any stage in the female's menstrual cycle, which explains the frequency of the copulations (Dixson 1998). Nunn (1999) studied the evolution of exaggerated sexual swellings in primates and related it to the graded-signal hypothesis. He stated that some female primates demonstrate their sexual receptivity with exaggerated sexual swellings. Although the exact function of this trait has not been determined, other researchers have hypothesized that exaggerated sexual swellings can be correlated with some features that confuse paternity certainty in males, while other features bias paternity towards dominant males. In the case of the bonobos, the sexual swellings confuse paternity which helps reduce infanticide. If the male bonobos do not know which offspring is theirs, they are less likely to commit infanticide towards any individual in the group. Nunn proposes that the graded-signal hypothesis (swellings signaling the probability of ovulation) is the most valid explanation for the exaggerated swellings. Nunn also proposes that the exaggerated sexual swellings allow females to manipulate male behavior. Males are more likely to guard the female only at peak swelling; this allows the females to mate with multiple males at any other time, confusing paternity. According to Nunn, the evolution of exaggerated sexual swellings has been attributed to a number of hypotheses. The best-male hypothesis states that exaggerated swellings create male-male competition, which benefits the female because she can determine which male has the best genes. This partially explains the bonobos because the females are looking for the males with the best genes. Therefore the males who win the aggressive encounters are more likely to have good genes that will produce strong offspring. However, this hypothesis cannot account for the long duration of sexual activity in the bonobos which probably developed in order to confuse paternity. The other hypothesis that might apply to bonobos is the many-male hypothesis. This states that females use their sexual swellings to attract many males and therefore can confuse paternity. This increases the chance that the female will have multiple males taking care of her because they all think that she has their offspring.

Photo courtesy of Dave Liggett (www.daveliggett.com)

Female-Female Contact

Female - female contact is the most frequent sexual contact, mainly due to the close social bonds that female bonobos share (de Waal 1997). Hohmann and Fruth (2000) studied the use and function of genital contacts among female bonobos. The discovered that female bonobos show mounting behavior that is different from other primates. They embrace each other and engage in GG rubbing. Hohmann and Fruth hypothesized that the function of these genital rubbings is reconciliation, mate attraction, tension regulation, expression of social status and social bonding. They found that post conflict genital contacts occurred more often than pre-conflict contacts, therefore suggesting that genital contact serves as reconciliation for agonistic behavior. The frequency of this reconciliation was positively related to kinship and close social ties. Genital contact was found to be more frequent when food was present, which was attributed to the competition for food. This competition leads to aggression, which results in immediate GG rubbing. Hohmann and Fruth attributed the high rate of sexual contact to the ambiguity of social status when a large amount of food is present. Therefore, low ranking females would show submission to the higher ranking females and benefit from this behavior. Low ranking females initiated genital contact more than high ranking females, thus denoting a significant social order that is established with GG rubbing. Although GG rubbing occurred more in the presence of males it did not affect the rate of copulation between males and females. Therefore Hohmann and Fruth concluded that sexual contact between females was most likely not used to attract mates. They did not find a positive correlation between party size and the rate of genital contacts in trees. Hohmann and Fruth determined that social tension is not only a function of party size but also a combination of quality of food, accessibility to food and other types of competition that could lead to tension in the group. They concluded that high ranking individuals accept "solicitation" by the lower ranking females because it solidifies their position of dominance in the group. Accepting the solicitation reduces possible aggression from the lower ranking female, as well. However, when a high ranking female ignores the solicitation of a lower ranking female, it demonstrates that the benefit the high ranking individual would receive is not worth the cost. It has been suggested in past studies that this sexual contact has resulted in an effort to reduce physiological stress. Since stress could turn into negative long-term effects, such as a reduction in reproductive success, friendly social behavior would benefit both the initiator and the target. Low ranking females benefit from initiating sexual contact because they move closer to a good food source, they reduce their social stress, and they create closer social bonds. In the current study, Hohmann and Fruth could not determine which purpose was the most significant. They therefore concluded that GG rubbing was multifunctional and achieved all of the above results. Another use for female-female sexual contact has been demonstrated by mother bonobos helping their sons find mates. The mother will engage in sexual contact with another female in order to determine if she would be an appropriate mate for her son (Jolly 1999). The high amount of sexual contact within the female group aids in group cohesiveness and helps strengthen the "artificial sisterhood" (see Social Organization).