INTRODUCTION

Mate choice, i.e., preference when choosing the partner to copulate with, is present throughout the animal kingdom. For example, in the case of cocks and hens, the adult cock prefers mating with successful mother hens. However, this observation is not usually seen since the mating behaviour of domestic animal is controlled by humans. However, much they are controlled by humans, there can be a time where there are no mates from the opposite sex present, and an animal can even escape their home to look for a partner. For example, cats would leave their house in the search of a mate, since they prefer not to mate with their siblings. In pigs, same situation can happen. In any case, these preferences are better studied in the wild, where an animal´s mate choice is not controlled. From their genetic closeness to humans, a very interesting species where to study mate choice are chimpanzees.

In humans, we have sex for pleasure, but for chimpanzees, copulation has a clear reason: to reproduce. Therefore, in chimpanzees, mate choice only happens when a female is receptive. But how do male chimpanzees know females are ready to ovulate, and thus, potentially fertile? Males use female’s oestrous to know in which part of their ovulatory cycle they are in. Like menstruation in humans, female oestrous in chimpanzees is a physical change that shows sexual maturity, and it can last for an average of 28 days. However, as an interesting fact, sometimes, the oestrus will be impacted by seasonality, reappearing much faster in dry season than wet season. Thus, more females are seen in oestrus in dry season. The oestrous is the swelling of a female reproductive organ that shows in which part of their ovulatory cycle they are in. There are different grades that show the evolution of the oestrous in chimpanzees, ranging from grades 0 to 4. Grade 0 is a flat oestrous, showing that that individual is not in oestrous at all. Grade 1 is a swelling of a 25% of a full oestrous; grade 2 a swelling of 50% (half-way); grade 3 a swelling of 75% and the oestrous looks wobbly; while grade 4 is a swelling of a 100% and the oestrous looks turgid. Real copulations start at grade 3 and grade 4, when ovulation takes place. Males check females’ oestrous to know in which phase of their cycle they are in by direct sniffing, touching, or by inserting their fingers in the oestrous and then sniffing them. When females are in grade 1, they are sometimes taken for consortship for a week or two. We talk about consortship when a male and a female will associate together away from the rest of the group. The pair normally comes back when the female is around grade 3 of her oestrous. During that time, the sexual drive of the female is high. Thus, all males, regardless of their rank, can mate with her, but mainly low- and middle-ranking males. However, the male that took her for consortship has an upper hand regarding the mating opportunities. Even if present, consortship doesn’t always happen: it is a matter of preference. When females are in grade 4, high-ranking males dominate the access to mate with the idea of conceiving. That is the time where the alpha male will take an upper hand, restricting his colleagues to have access to the female. During that time there is a lot of commission, aggression, changes in feeding rate, etc. For example, feeding rates decrease at the end of the oestrous, when the female will decide towards where to move, and males will follow. As a matter of fact, males will be so interested in the female’s movements that they will feed less, since their priorities changed. This can also affect males’ diet: they will eat what the female prefers, leading to sometimes eat the same throughout the day. At the end of the day, the alpha male will nest closer to the female to be the first to have access to her in the morning, or even during the night.

Like in humans, most initiations of male-female mating are done by males. Male and female chimpanzees can solicit copulation with the opposite sex in several different ways:

1. For instance, an individual can show its interest with leaf-clips, in two different ways. They can tear a leaf continuously by using their fingers, or by biting the leaf, while looking at the other individual, or the receptor of the solicitation. This is most common from males to females, but we can also find instances from female to male.

2. Another sex solicitation is twig or sapling shaking, where a male would shake a twig or sapling to attract the attention of the female.

3. Stamping can also be a sexual solicitation: when the male is fully erected, he would face the female while sitting to present his sexual organ and stamping with his feet. This can happen either on the ground or up in the tree – then, it would be called branch stamping. They sometimes also knuckle the branch.

4. Body scratching is another kind of sexual solicitation, where a male will scratch loudly their chest looking at the female.

5. For females, they can also present their oestrous to males to solicit for sex.

Even though males solicit sex to females, it is not a guarantee that the female will give in. Perhaps, this has something to do with the female’s decision. The female is free to choose whether to copulate with a male or not. A case when solicitation might not work is when there is more than one female in oestrous present. If there are two females in oestrous around a male, and the male solicits one of them, normally the female that was not solicitated would present first. For example, in the Sonso community, the female Edinburgh (ED) has been observed to interfere with males interacting with other females by presenting to them. It could also be the case that males solicit a female, and the female instead of copulating with them, copulates with another male in the party. Aggression could ensue this interaction if the male soliciting is higher-ranking than the male with whom the female copulated.

When all these solicitations fail, aggression can be the last resort. Males can put aggression pressure on females in order to copulate. This is observed in several individuals, mostly in middle- and high-ranking males. Indeed, we have seen cases of male chimpanzees coercing females to have sex with them. Two examples would be Musa (MS) from the Sonso community and Daudi (DAU) from the Waibira community. They have been seen soliciting several females and whenever they would not respond, they aggressed them and coerced them to copulate.

In some cases, when females are in oestrous, they walk from party to party in search for who to mate with. If in a party no one is interested on her any longer, she will not stay with them and move to the next party. ED and Janie (JN) are examples from the Sonso community that behave in this way.

RESULTS

The rank of the individual plays an important role in their mating access. Looking at paternity analysis, middle- to high-ranking males have a greater paternity within the community. They can guard females in oestrous during their ovulation period not letting them the chance to copulate with lower- ranking males. Also, low-ranking males have more possibilities to be in proximity of and copulate with oestrous females if they are friends with high-ranking males. However, towards the end of the cycle, when the females are suspected to be at their peak of fertility, higher-ranking males will become more aggressive and jealous of who is copulating with said female. The same happens with females: if a high-ranking and a low-ranking female are in full oestrous at the same time, and the lower ranking is solicited by or copulating with a male, the high-ranking female usually interferes with the copulation. This can also happen when both females are from the same rank category.

There are females that, when they are in oestrous, are so popular that each male wants to copulate with them. This is related to the age and how many offsprings she had (we talk about null parous or multi parous female according to the number of offspring they had), males usually preferring successful mothers. Indeed, male chimps prefer to go to older females because they prefer that they know how to take care of the babies, whereas young females don’t. However, human men prefer young ladies than old ones.

An important factor is the individual preference of the female, it does not always lie with the rank of the male and indeed is the female that likely engineers the whole process that exist amongst males. Sometimes there is more than one female in oestrous and the males prefer one of the females to the point of hurting and wounding each other and not caring about the other females in oestrous.

When a female is in the peak of its oestrous, males can hurt and cause many injuries to each other due to the tension over this female, and regular fighting. Because of this competition and power political struggle, the Sonso community lost an adult male, Zesta (ZT), in 1998. In this case, ZT was competing with Duane (DN), the alpha male of the community, over one of the preferred females, JN. ZT initiated a copulation when JN was in oestrous grade 4. In this case, the alpha male should take charge of the copulations with the females. When ZT solicited, JN approached and presented, when the copulation started, DN charged them to end the copulation. From there, ZT puffed up to start an aggression. Then, other males joined DN and killed ZT.

Female chimpanzees like going for social males. On top of that, anecdotally, they like the ones that have longer penis. For example, females in Sonso prefer Pascal (PS) over other males. They also have preferred partners, for example in the Waibira community, if Arua (ARU) is in oestrous, if Ben (BEN), the alpha, is not there, she will look for him throughout the parties. We can also find these preferences in the Sonso community, where Rachel (RE) had a clear preference for Kato (KT) and would present to him, but he would ignore her and go away. Some females can at times interfere the copulation between a male and their daughter. For example, Nambi (NB) used to aggress some males that wanted to copulate with Nora (NR). The same happened with Shy (SHY) and her daughter Sula (SUL) in Waibira community. When both of them were in oestrous, they wouldn’t like to share the same tree in order not to interfere with the other copulating with another male.

Another example of choice is the fact that adult males hardly ever copulate with their mothers and siblings (with an exception in the Sonso community, whereby Nick (NK), the alpha male at this time, copulated with his mother, Ruhara (RH)). However, infants and juveniles do copulate with their mothers as a way of teaching, for example, a juvenile male called Hadue (HD) has been observed copulating with Harriet (HT), his mother. This happened during a travel initiation, where HD wanted to go in one direction, but HT wanted to go in another. They both went in different directions, and then HD started whimpering and screaming. Then, when he joined the mother, HT presented to him, and he copulated with her. We know this was a copulation since HD had an erection and penetrated HT. After the copulation HD calmed down and they started traveling. This behaviour has not been observed in Waibira.

Even if each individual has their preferences, there are not set in stone. Chimpanzees can groom and build social bonds with individuals of the opposite sex with the idea to become more attractive in regards of the other individual and win popularity. As an example, when BEN, the alpha male of the Waibira community, is not around, the second in rank, the beta male, is taking charge of the group. This male is friend to juveniles, and they get access to mate with high-ranking females. The juveniles would groom the high-ranking male and then at the end the juveniles get the chance of copulating.

DISCUSSION

Oestrus is a swelling of a female organ as an indicator of time of maturity. Real copulation starts at grade 3 about 75%, but overall, adult males pick more interest of the females at grade 4. Ranks greatly influence mating in a chimpanzee party. Indeed, at grade 3, females will mainly attract all males, but only low- and middle-ranking males will be interested to copulate. However, at grade 4 the high- ranking males will start to get interest and will compete to access the female. Usually, the alpha will be the one dominating, but other males also take chances of mating the female. It is believed that at this time females have higher chances of conceiving once mating is done. In this line the females decides which male will be successful by ejecting the sperm plug to aid fertilization. All other males may mate so many times, but the sperm plug won’t allow their semen to succeed. The multiple mating is a strategy to prevent future infanticide. Infanticide is the grabbing of an infant from the mother and killing it. This can either be by biting of swinging or hitting it against the tree. This can happen mainly if the father is suspected to be from another community or not a popular male. Also, when paternity is not clear, some individuals might commit infanticide. These cases are not related with mate preference, any male could kill any infant. For example, Frank (FK) from the Sonso community has killed a lot of infants for which he suspected he was not the father or that didn’t belong to the community. Moreover, sometimes it is thought males commit infanticide for the females to come into cycle again. In fact, a female with an infant will take about 4 years to come back to oestrus. Other females of the group also sometimes participate in infanticide.

Sometimes, some low-ranking males can adopt strategies to have access to a female and pregnant her. These strategies are the following ones: 1) they can mate when in poor visibility; 2) some males can pretend some conflicts a few meters away to attract the attention of the alpha male and then, give the opportunity to folks to copulate with the female; 3) they can solicitate the female with specific behaviours and the female will quietly and quickly join the said male to copulate. Copulation is a strategy game where males are waiting for the good opportunity and for a mistake of the alpha to have access to the female. Then, most of the time, you can observe the alpha male guarding the female in oestrus, but all other males will surround them and resting, waiting for an opportunity. Feeling harassed, the alpha will often chase some of the other male, which will give an opportunity to other to reach the female and mate. As a matter of fact, in the Sonso community there has been a chance of low-ranking male siring an offspring with support of DNA analysis.

Therefore, mating is universal, and all adult males are given chance to access the female, but juvenile too as they are given the opportunity to practice. Indeed, mothers present to their young sons and copulate with them as a way of initiating them into reproduction. However, once males are mature, they don’t mate with their mothers as a way of avoiding incest. In addition, age is also playing a role in partner selection as males have proved to select older females because they have already given birth and taken care of their offsprings. However, around 50-year-old, older females approaching menopause are gradually ignored by males due to their low chances of conceiving. In the same way, older males become less interested in matting perhaps because it becomes too costly to avoid aggression from young energetic male, or too costly to simply mate.

Mate selection seems to be a trade-off as female seems to prefer males with whom they have better social interaction. For example, males will sometimes provide more and longer grooming bouts to some females or share meat with them to later obtain their sexual favours. Once, in Waibira community, Kipepeo (KIP), a multi-parous female, was in oestrus and got meat. While usually males are dominating meat-sharing and monopolize the prey, they were observed begging to her for pieces. Then, still holding the prey, she was presenting her swelling to the same males and copulated.

Sometimes, during a mating, a female will take off from a male just before ejaculation, as to avoid being pregnant. It seems confirmed by the fact that females have been observed to stay until the end of the mating with other males. As an anecdote, as it need to be tested, it seems that penis size and time of erection may affect female’s choice. For example, according to these patterns, PS, an adult male in Sonso community, seems to be one of the preferred mating partners for females despite being a low-ranking male. On the other hand, personality may also play a role in female preference. For example, Zefa (ZF) in the Sonso community was able to sire many offsprings because of personal traits such as calmness or other strategies such as consortship, sneaking, inter-individual relationships.

CONCLUSIONS

In chimpanzees, mating is driven by the oestrus status of a female. Several pattern impact reproductive successes such as age, rank, competition, preference, experience, parous-stage, season, etc ... When a female is sexually receptive, mating is a universal behaviour within a community, with every male individual having chance of reproducing with a female. However, high ranking males and preferred individuals stand high chance of siring offspring. Mating is then highly competitive among males, but at the end, this is the female who will choose her preferred one. However, female preference for a specific male is still unclear. Therefore, further observations focusing on females’ behaviours are needed to support our statements and better understanding competition and mating choice from the female chimpanzees’ perspective.

CITE AS

Perspectives Collective. (2024). Mate Selection in Wild Chimpanzees (Vol. 2, Number 1). Zenodo. https://doi.org/10.5281/zenodo.11115877