This entry was written by Shaniah Elliott as part of a project done in BIAN 6133 ‘Human Reproductive
Strategies’ at The Australian National University in 2019 Semester 2.”
Introduction
Located in the Congo Basin of Central Africa is one of human’s closest relatives, bonobos (Pan paniscus). They are commonly mistaken for their genus partner, the chimpanzee (Pan troglodytes), with whom they share 99.6% of their DNA. However, the social and breeding structure of bonobos and chimpanzees differ greatly. While bonobos are known for their matriarchal and egalitarian polygamous social groups, chimpanzees are known for their patriarchal and aggressive polygynous social groups with a high rate of infanticide (Hare & Yaramoto 2017).
Infanticide is the premeditated killing of an infant, often by a non-relative male. Although many factors contribute to the nature of infanticide, this homicidal exploit is commonly associated with species’ social structure and the intensity of male intrasexual competition. Lukas & Huchard (2014) theorize that social groups which consist of few males and numerous females while still maintaining aggressive male domination, will experience high rates of infanticide largely due to jealous and tactical behaviour among males. This behaviour is linked to sexual conflict resulting in actions driven by jealousy, the goal of which is to either eliminate an unwanted gene pool entering the society or accelerate the copulation process to mate again (Opie et al. 2013). In the case of chimpanzees, infanticide is theorized to occur as an attempt by male chimpanzees to limit extra group males’ genes and terminate lactational amenorrhea in females, hence allowing them to mate again (Stanford 1998). This aggressive behaviour is puzzling when compared to bonobos, who have a drastically different social organization and experience little to no infanticide.
This entry will examine the lack of infanticide among bonobos via evolutionary selection against aggression and the increased sexual activities among bonobo parties. Studies surrounding the self-domestication hypothesis and female domination will be analysed.
Main Text
The Self-domestication Hypothesis
Several factors aid in the explanation for why bonobos experience a lack of infanticide. The overall ideology points to the lack of aggression found in bonobos. Although bonobos are very capable of expressing aggressive behaviour when necessary, it is not commonly observed in social situations (Standford 2013). This behaviour contributes to bonobos being a prime candidate for the self-domestication hypothesis. The self-domestication hypothesis in this case, is a theorized concept (Hare, Wobber & Wrangham 2012; Hare & Yaramoto 2017) explaining how bonobos came to be a non-aggressive matriarchal society compared to their sister species, even though they evolved from the same ancestor. The theory analyses the bonobo's location of origin, their variance in developmental pathways and their societal organization, with regards to the selection against male aggression. Consequently, this selection had theoretically led to the lack of infanticide observed in bonobos.
Tolerance via Location
The southern side of the Congo river marked the start of self-domestication of bonobos. The species evolved from the same ancestor of chimpanzees when the Congo River formed around 2 million years ago, splitting the ancestor’s land. Chimpanzees evolved from the ancestor on the northern side, while bonobos from the same ancestor on the southern side. This common ancestor is theorized to be both physiologically and socially similar to a chimpanzee (Hare & Yaramoto 2017). The southern side of the Congo River provided a different environment for the common ancestor with the critical environment difference being increased food availability due to the absence of gorillas. This absence of gorillas potentially led to the decreased aggression in bonobos because they had ample food and no rival species to defend their territory against; only potential for aggression in their own society remained. The decrease of aggression in the species allowed for an increase in tolerance and pro-social behaviour (Hare, Wobber & Wrangham 2012). This tolerant society led to behavioural differences in how one should react to another’s actions. In the case of infanticide, tolerance aids in the prevention of male’s overreaction to a female’s infant with another male, therefore decreasing the risk of the jealous, non-paternal male committing infant homicide.
Female Domination
Female dominance in primates is uncommon, as most primates either express male dominant polygamy or pair living. Bonobos are female-dominated, not due to the lack of an alpha male, but rather, because female coalitions in the group are a stronger leading force than males. This differs from normal male domination, which is often controlled through sexual dimorphism that allows males to be physically larger and more aggressive than females. Although bonobos experience a degree of sexual dimorphism with regards to size, the sexual dimorphism ratio of male to female size is not as dramatic in comparison to male-dominated societies. A study conducted by Furulchi (2011), demonstrated that females also maintain a higher social status due to the lower oestrus sex ratio among bonobos. A ratio of 1.9 (6 females for every 3.1 males), led to the suggestion that bonobos experience lower intermale sexual competition, therefore resulting in a greater level of female choice. This female dominance among the species allows for the creation of a hierarchical structure within the society. A strong female coalition creates stable societies amongst bonobos and allows for females to thwart any aggressive or unwanted behaviour, such as infanticide, by males.
A behavioural experiment (Gottfried et al. 2018) observed potential aggression in adult male bonobos that was aimed towards immature individuals. It was perceived that although adult males showed aggressive behaviour towards weaned infants, infanticide never occurred. Researchers theorized that the adult males used this aggression to instil dominance over the immature bonobos rather than resorting to committing infanticide. Females were still recognized as the dominant sex in the experiment. Though females are considered to be the dominant sex in bonobos, it should still be noted that the society is very egalitarian in terms of resources and sexual activity (Hare & Yaramoto 2017).
The Many Uses of Sexual Activity
While the potential for infanticide still exists, bonobos engage in several sexual strategies to avoid its occurrence. Bonobos are known to be very sexual primates, even more so than humans (Wrangham 1993). Like humans, they engage in both heterosexual and homosexual relations and not always engage in sexual activity with the intent of reproducing. They are a polygamous species that use sexual activity to diffuse tension in their communities (Hare & Yamamoto 2017). This has been observed (Wrangham 1993) in varying partnering of same and opposite sexes and generally involves genital rubbing to diffuse intracommunity tension. Genital-genital rubbing has also shown to increase female bonds among communities (Wrangham 1993 & Standford 1998).
Perhaps the most effective strategy used to avoid infanticide, is that in which individual females copulate with multiple males in a short period of time. This mating strategy obscures the paternal parentage of the infant, thus decreasing the likelihood of males committing infanticide, as the infant could possibly be theirs and thus impacting their fitness (Opie et al. 2013). This fitness is vital for males if they are to pass their genes on to the community. If a male kills his infant, he would eliminate this contribution to the gene pool. In a discussion by Gottfried et al. (2018), researchers analyse the female sexual components that prevent male infanticide and draw upon bonobos multiple copulations as well as the female bonobo’s reproductive cycle. The ambiguous fecundity of females and their changes in sexual swelling are thought to reduce individual male to female mating. Therefore, multi-male copulations are focused when fertility is highest, as it maximizes reproductive chance and obscures paternal parentage.
Conclusion
The behaviour of bonobos when compared to their sister species, chimpanzees, is a multifaceted area of interest in evolutionary anthropology. Behavioural studies depicted bonobos’ ability to avoid infanticide through their selection against aggression, which is explained by the self-domestication hypothesis, as well as their highly sexual and female-centric society. The rarity of infanticide in bonobos is a bewildering concept when compared to the various primates in today’s modern world. Therefore, many researchers regard the female’s involvement in obscuring paternity and their inexact fecundity as the core hypothesis explaining the rarity of infanticide in bonobos. This is due to the hypothesis being a broad fit across primates.
Literature Cited
Furuichi, T. (2011). Female contributions to the peaceful nature of bonobo society. Evolutionary Anthropology, 20, 131-142.
Gottfried, H., Vigilant, L., Mundry, R., Behringer, V., & Surbeck, M. (2019). Aggression by male bonobos against immature individuals does not fit with predictions of infanticide. Aggressive Behavior, 45, 300-309.
Hare, B., & Yamamoto, S. (2017). Bonobos: Unique in mind, brain, and behaviour. Oxford University Press: Oxford Scholarship Online.
Hare, B., Wobber, V., & Wrangham, R. (2012). The self-domestication hypothesis: evolution of bonobos psychology is due to selection against aggression. Animal Behaviour, 83(3), 573-585.
Lukas, D., & Huchard, E. (2014). Sexual conflict. The evolution of infanticide by males in mammalian societies. Science, 346, 841-844.
Opie, C., Atkinson, Q.D., Dunbar, R.I.M., & Shultz, S. (2013). Male infanticide leads to social monogamy in primates. PNAS, 110(33), 13328-13332.
Stanford, C.B. (1998). The social behaviour of chimpanzees and bonobos: Empirical evidence and shifting assumptions. Current Anthropology, 39(4), 399-420.
Wrangham, R.W. (1993). The evolution of sexuality in chimpanzees and bonobos. Human Nature, 4(1), 47-79.
