Ernest Schwarz, a German anatomist in a Belgian colonial museum first documented Pan paniscus in 1929. The species name reflects that the P. paniscus skull was first thought to belong to a juvenile chimpanzee. In 1933, Hal Coolidge classified P. paniscus as an independent species following a detailed morphological study. In fact, “pygmy chimpanzees” are just as large as some subspecies of the common chimpanzee (De Waal 1995). It is estimated that the two species within the Pan genus diverged 1,000,000 years ago, which reflects similarities in both the anatomy and behavior (such as male philopatry) between the species (Przeworski 2007). 

There are, however, profound differences between the two species (Cf. comments on Stanford). The rate of sexual interactions is higher among bonobos than chimpanzees, yet both maintain the same rate of reproduction. Furthermore, sexual interactions among bonobos occur in every possible partner combination regardless of age or gender with the notable exception incest among siblings. These interactions reflect a much broader sexual lexicon  in bonobos than in chimpanzee; in addition to vaginal, bonobos utilize oral and manual stimulation in a variety of copulation positions, preferring ventro-ventral position as in humans. Together, these differences in behavior suggests that sex in bonobo society serves a fundamentally different purpose than in chimpanzees that is not limited to reproduction. The differences in the behavior of these two species, however, is not limited to sex. Male chimpanzees express dominance over their female counterparts; by contrast, dominance is not strictly ordered by gender in bonobo society despite the fact that female bonobos are smaller than males (de Waal 1995).

Contrary to the proposition that  speciation in the Pan linage derived the common chimpanzee from the bonobo when the chimpanzee entered the open savannah  (der Waal 1989), genetic testing gives credence to the notion that a genetic mutation derived the bonobo from the chimpanzee  when the bonobo entered the protected forests (Furuichi 2011). Comparison of the banded chromosomes in humans, chimpanzees, and bonobos demonstrated that Pan paniscus is the most chromosomally specialized species (Stayon et al. 1986).

The question left for modern ethologists is the nature of the mutation that caused speciation. Furuichi (2011) postulates that small genetic changes altered one or a few crucial features during the bottleneck period, resulting in development of the bonobo social system, including its sexual behavior; “if genetic changes occurred in the physiology of females, causing them to show estrus during nonconceptive periods, this whole social system may have developed in an environment with abundant and dense food resources”.  Blount (1990) offers a theory for the mechanism by which speciation might have occurred. He suggests that comparisons between bonobo and chimpanzee anatomy are suggestive of paedomorphism, or the retention of juvenile characteristics in the adult form. This theory implies that at least part of the mutation responsible for the speciation of bonobos suppresses the onotological process necessary to derive adult chimpanzees from children. Perhaps the first observers of the bonobos were not so far off when they mistook a bonobo skull for a juvenile chimp.

It’s not immediately obvious why an environment would select for paedomorphism, but examining the bonobo’s habitat can provide some clues to reconstructing its phylogentic history. Bonobos inhabit the lowland forests south of the Congo River while chimpanzees inhabit a variety of environments north of the Congo River  including woodland-savannah and dry forest as well as moist evergreen forests typical of bonobos. Both bonobos and chimpanzees are primarily frugivorous, but chimpanzees display more variety in their diets. In chimpanzee habitats, trees grow farther apart, fruit is available only seasonally, and there is a higher level of anti-feedants (such as tannins which require detoxification, requiring greater energy intake) (Sommer 2010). Blounnt (1990) observed an increased in arboreal feeding, decreased daytime ranging, and less time budgeted for feeding in bonobos as compared to chimpzees. Therefore, the reduced cost of obtaining food in a resource rich environment, such as the one the bonobos occupy, may have contributed to selective pressures that caused bonobo speciation. An environment naturally abundant in resources will selectively favor those species with the ability to share. Sharing in this situation reduces the costs incurred by unnecessary competition for food. Bonobos resolve the tension caused by access to preferred foods using sexual interaction. The function of socio-sexual contact in facilitating food sharing will be discussed at greater length in the consequence section.  

Figure 1. Timetrees obtained from BEAST (2 + 1 model) from 4F degenerate sites. The time scale is in million years (Ma). Both trees used the gorilla/human + chimpanzee calibration (10.0–6.5 Ma). Additionally, the upper tree used the human/chimpanzee (H/C) calibration range of 6.5–4.2 Ma while the lower tree fixed


Bonobos form “fission-fusion” societies in which a large parent group splits into several smaller groups during the daytime for foraging and coalesces again during the night to sleep. The smaller parties that the bonobos form constantly change in their composition and remain in adjacent areas. Bonobos are male philopatric; while the male bonobos remain in their natal groups with their mothers, females between the ages of 7-9 leave their natal group to join another group (Kano 1992). Female bonobos may then join and leave several groups before settling on one which they remain in for the rest of their lives (Furuichi 2011). Upon entry to a group, adolescent females use socio-sexual contact and grooming to facilitate bonding with a senior, higher-ranking female. Once this bond is established, the female of higher rank introduces the adolescent to the rest of the group, where the adolescent female often engages in socio-sexual contact, such as gential-gential rubbing, with each member of the aggregation (Furuichi 1989). The new female’s position within the group is cemented once they have born offspring at approximately 14-15 years of age. The formation of bonds between non-related females crucially violates the assumption that the sex that stays within the natal group forms the strongest bonds (de Waal 1995). The migration of female bonobos between groups necessitates a mechanism for integrating new females into an established, largely female-dominant group.  


Bonobo infants are nursed by their mothers for a period of 5 years, reaching adolescence at the age of 7 and adulthood at 15 (de Waal 1995). Use of socio-sexual behavior in bonobos has been observed in infants as young as age 1. These socio-sexual behavior occur among infant bonobos indiscriminate of gender and in a rich variety of positions, suggesting that the behavior is not “practice” for reproductive behavior. A recent study (Woods 2011) compared socio-sexual behavior in orphaned infant bonobos to that of chimpanzees; while no sexual behavior was observed in the chimpanzees, it was observed frequently in infant bonobos. Because the focal animals in question were the unfortunately the product of bushmeat trade, the infants were raised in the absence of any adult member of their species whom they could emulate. Therefore, it appears as though socio-sexual contact in bonobos most likely has a genetic basis (“species-typical”) that manifests itself early in the development of the individual (Woods 2011).  

Furthermore, bonobo females have evolved to give dishonest signals regarding their ovulation periods. Relative to chimpanzees, female bonobos reach sexual maturity two years earlier than their cousins, displaying perineal sexual swellings on their rumps. Periodically these sexual swellings deflate for the duration of a couple days during de-tumescence before becoming engorged once again. The period of tumescence in chimpanzees is comparatively short, with large periods of de-tumescence in between (Kano 1989) and a long postpartum amenorrhea (Furuichi 2011). Through a fecal analysis of progestin levels it has been concluded that bonobos have a longer period of ovulation relative to chimpanzees (Reichurt 2002). In addition to a longer period of fertility, bonobos maintain swelling during infertile periods, such as between ovulation and during pregnancy and postpartum amenorrhea in a phase termed “pseudo estrus”. The ability to maintain perineal swelling when infertile evolved as a dishonest signal of the female’s readiness to mate. The actual mechanisms governing this response remains poorly understood, however, as what typically induces similar sexual swellings in other primates is correlated to an increase in progestin.