Inbreeding avoidance in primates

A Cul-de-sac of Sexual Evolution. Takeshi Furuichi. Chapter 2 of Bonobo and Chimpanzee: The Lessons of Social Coexistence; pp 37-62. November 23 2019. https://link.springer.com/chapter/10.1007/978-981-13-8059-4_2

Abstract: Incest. It is a word that has an extremely unpleasant echo. Incest taboo (or inbreeding avoidance) between close kin in relation to the societies of numerous ethnic cultures has occupied a significant place in anthropological study. Many people might think that incest taboo is a uniquely human quality, but inbreeding avoidance, in general, is found in almost all animal species. Interestingly, the word “inbreeding avoidance” resonates healthier to our ears than “incest avoidance.”

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2.1.5 Male-philopatric Society in which Females Leave

Not only the Japanese monkey and other primate species [53] but also many groupliving mammals form matrilineal societies where males transfer between groups. Elephants, lions, and the Japanese Sika deers have similar systems [54]. For this, we consider why females do not transfer.

Animals forage for grasses, fruits and leaves of trees, and other foods in the habitats where they are born. Carnivores must hunt prey animals. Prey animals survive by protecting themselves from predators. To do so, they need to have a thorough knowledge of when and where to go to find which kinds of food, understand where prey animals are located and are easy to hunt, learn where predators roam, and where to escape to defend themselves. There are close relatives in their group who have supported them since they were born, but when they move to another group, they have to be accepted by members of that group and build relationships all over again. While moving, they may have to, temporarily, spend a dangerous period alone. Given all of that, it is better to stay in their natal groups throughout their lives.

However, males and females differ in terms of survival strategies. Females can live a normal life and bear a fixed number of offspring during their lifetimes as long as they respond to males that want to mate with them. Therefore, what is important for females is not the number of offspring they produce, but rather how they obtain a stable food supply and effectively protect themselves from predators to rear offspring with certainties. This is why it is more beneficial for them to stay in their natal group than transferring out. In contrast, the number of offspring males can sire during their lifetime varies from zero to almost infinity depending on their efforts and degrees of success. For them, it is more important to obtain as many mating opportunities as possible and impregnate as many females as they can. To do so, it might be better to find another group that offers possibilities of mating with more females rather than staying in a group with a compromised low-rank position in which they are born. Moreover, females are sexually attracted to novel males coming from outside, rather than closely related males or males that are familiar from their childhoods. Consequently, males that actively move between groups, rather than those males that attempt to stay in natal groups, will have a greater chance of obtaining breeding opportunities.

To live in a group and avoid inbreeding, either a male or a female must transfer between groups. If both sexes transfer, this could incur a possibility that brothers and sisters would encounter in a newly transferred group unbeknownst to them. Therefore, if one sex transfers, individuals of the opposite sex do not normally need to transfer. If males are more likely to greatly benefit from transferring, it seems natural for many species to live in female-philopatric societies where males transfer.

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However, immigrant females are desperate. They somehow have to build relationships with males in the group, and the only thing they can depend on is sex.

One day I witnessed an interesting scene. Until 1996, to carefully observe the behavior of the E1 group, we sometimes brought their favorite sugarcanes, cut them into about 30 cm in length, and scattered them on the forest floor beneath the trees where they spent the night. The scene happened when I was observing the bonobos that were coming down to take the sugarcanes.

In the center of the feeding area, the male bonobo named Ten that had quite recently climbed up to the top rank position was eating a sugarcane. He had some in his hands and was eating slowly. A young female named Nao that had recently immigrated into the E1 group came closer to him. As Nao approached Ten, so closely as if their faces would stick together, she peered at Ten’s mouth and she reached for his mouth with her hand. This is a behavior displayed when a bonobo begs for food. However, Ten pretended not to notice Nao and continued to eat slowly.

Then Nao changed her tactic. She changed the direction of her body to protrude her buttocks to Ten and began inviting him to copulate. Even though males are usually not interested in young females, there was no reason for Ten to refuse copulation if the buttocks are pressed to his nose. Ten stood up with a few sugarcanes in his hand and copulated with Nao as he clasped her from behind.

Then the power relationship between them completely changed. Nao reached out for the sugarcanes that Ten was holding in his hands and took the longest piece from him as if to say that reciprocating with the sugarcanes was an appropriate thing to do. She finally got the piece and started eating it alongside him.

Readers may think that this is an episode in which food was exchanged for sex, like prostitution. However, if we look at this situation more closely, there was no need for Nao to get sugarcane from Ten, because there were a lot of sugarcane stalks still lying on the forest floor. If she really wanted a sugarcane, she could have just picked any one up.

Nao took the trouble to approach Ten and beg for a sugarcane, but when he ignored her, she made a series of negotiations to get a sugarcane to eat together simply because she wanted to begin a relationship with him. It is analogous to being in the university library and asking to borrow stationery that you do not really need from a person of the opposite sex whom you are interested in getting a chance to talk to.