Why is the mating between the closely genetically related Chimpanzees (Pan troglodytes) and Bonobos (Pan paniscus) so different?

Taken from an assignment in which I was set the task to combat a controversial question and then create a conference paper on the topic for a Primate Behavioural Ecology unit. This is a comparative approach of factors that could attribute to the differences, and not a definitive answer – just my analysis of theories/my own conclusion.

Abstract:

Primates, Pan troglodytes and Pan paniscus, both speciated from a shared last common ancestor, yet they have very different mating behaviours. Pan troglodytes are known for their aggressive behaviour, even when it comes to mating. Pan paniscus on the other hand are known to perform sexual activities for purposes other than reproduction. There are many social, physiological and environmental factors which could have caused this diverse approach to reproduction. Within this paper I will be utilising past research, conducted on these contributing factors, to theorise why these primates have created two different behaviours types when they are so close genetically.

Introduction

Chimpanzees (Pan troglodytes) and Bonobos (Pan paniscus) speciated from their last common ancestor and share similar genetic information (Ruvolo et al, 1994). Both Pan paniscus and Pan troglodytes habit Africa, where they are separated by the River Congo. Pan troglodytes commonly habit the west and central parts of Africa whereas the Pan paniscus habit the Democratic Republic of the Congo. Due to this separation it would seem that genetics have no vital role in the different mating styles, but the environment and social interactions do. To understand the behaviours of adult Pan troglodytes and Pan paniscus we need to understand their anatomy when it comes to mating.

Within the lifespan of Pan paniscus and Pan troglodytes they encounter many physiological changes, some of which are adaptive to their environments. Male Pan troglodytes reach maturity between the ages of nine and sixteen, and male Pan paniscus reach maturity around the age of nine. Female Pan troglodytes have their first estrus around the age of ten, which is characterised by the sex skin swelling. Pan paniscus have their first estrus before their menarche. Menarche, their first ovulatory cycle, occurs between six and eleven years old whereas for P. troglodytes it occurs after the first estrus (Cawthon-Lang, 2010).  As we can see there are certain similarities, such as the male maturity age, within both primates biological development, but as with menarche there are differences. So does this influence their mating behaviours?

Mating Behaviours

Both Pan troglodytes and Pan paniscus have been observed to practice opportunistic (multiple) mating (Cawthon-Lang, 2010). Multiple mating, also known as promiscuous mating is where a female will mate with numerous males within the population. Throughout the animal kingdom there are many benefits of practicing opportunistic mating;  increase genetic diversity, avoid infanticide, exchange resources, increase possibility of fertilisation and avoiding male harassment, compared to their costs: male retaliation, male aggression, and energy/time. There are numerous benefits for practicing multiple mating within a group of primates compared to the few costs. It must be noted that from multiple mating, female Pan troglodytes do not receive resources but receive decreased levels of male aggression and the choice of better males (Matsumoto-Oda, 1999). A key aspect of participating in multiple mating is ‘female choice’. The female can time which males she mates with in relation to her current ovulation state. When she feels she is at most fertile, she could choose to mate with more successful males within the group and when she isn’t at her most fertile mate with the lower ranking males (Matsumoto-Oda, 1999).

Pan troglodytes have also been observed practicing possessive and consort-ship mating (Cawthon-Lang, 2010). Possessive (restrictive) mating is when a male forms a short-term exclusive relationship with a female within a group, and consort-ship is when a female and male create a mutually exclusive relationship isolated from the group. It is thought that males make the choice to have a restrictive mating relationship while for consort-ship it is a mutual choice, as female co-operation is very important for this relationship to be established (Matsumoto-Oda, 1999).

There are many factors to consider when comparing the mating behaviours of these two primates. The main factor which has heavy influence on the sexual behaviour of are sociological. Sociological factors include gender dominance, philopatry, community type, and infanticide. Environmental factors such as predation and food abundance, play a minor part.

Influencing Factors

Pan troglodytes and Pan paniscus have some shared sociological factors such as both communities are of a fission-fusion social structure and that both are male-philopatric (Cawthon-Lang, 2010). A fission-fusion social group is where a large community is made up of temporary smaller sub groups and the population of the sub groups change due to movement between the subgroups. Smaller groups of Pan troglodytes can be made up of eight to ten members whereas larger groups could reach forty. An influencing factor on group size is food abundance; if there are a lot of resources a group will become larger, due to migration from smaller groups, to exploit the high supply of food (Boesch, 1996).

This fission-fusion social group system plays a vital role in the male-philopatric nature of these species. Male-philopatry is where the females leave their natal subgroups and migrate to another subgroup once reaching sexual maturity (Cawthon-Lang, 2010). This practice helps create better gene diversity within a population as there is less chance of inbreeding due to the males within the new subgroup usually not being related to the emigrated female (Inoue et al, 2008). During this migratory process the females have adolescent infertility which lasts from one to two years for P. paniscus and then two to four years for P. troglodytes. This is hypothesised to be an adaption to protect the female. For example, if a female arrived at a new subgroup with an infant or pregnant there could be violence and aggression towards them (Cawthon-Lang, 2010).

A big difference between Pan troglodytes and Pan paniscus is the dominate gender, with P. paniscus it is the females who are dominant, with P. troglodytes it is the males. Female Pan paniscus obtain rank as they age and have offspring, especially males. Their sons often have corroborating rank as they mature and as the adult female becomes more central to the group (Surbeck et al, 2010).

There are a few unique behaviours and social practices to both species. There are two behavioural practices within P. paniscus, which stems from the female dominance, which are the use of sex for communication and sex between females. Same-sex behaviour between dyads occurs in both wild and captive bonobos. The use of these behaviours has been theorised for the use of practice, paternity confusion, exchange, communication and appeasement (Mason et al, 1996). It is thought that the effective female-female coalitions against males within a population of Pan paniscus is facilitated by the same sex behaviours.

Wrangham (1993) identified five functional categories of sexual behaviour within a Pan paniscus population; conception, practice, paternity confusion, exchange and communication. The function of practicing sex, paternity confusion and an exchange of behaviours is also used within populations of Pan troglodytes. Practice by juvenile Bonobos may be vital for developing effective courtship and copulatory techniques. The paternity confusion aspect applies to situations in which adult males could become a threat to infants which are not their own. Use of sexual behaviour for exchanging of resources or benefits such as food or grooming. From the communication function, sex between dyads can be used for appeasement without affecting their relationship and to communicate about their social relationship (Mason et al, 1996).

A unique practice to P. troglodytes is infanticide. Infanticide has been documented at Gombe, Mahla and Kibale within the chimpanzee study sites and this practice has been often attributed to sexual selection theory. Male P. troglodytes are more likely to exhibit infanticidal behaviour, mainly killing infants that are unlikely to be theirs. Also the act of infanticide shortens the inter-birth intervals as it induces cycling in the females that have lost their offspring (Cawthon-Lang, 2010). Therefore the use of infanticide can increase a male’s chance of producing offspring. It is hypothesised that this is due to the link between suckling and amenorrhea (Fedigan and Rose, 1995). There has been documentation of females committing infanticide, but there is some question whether these were isolated incidences or could be related to dominance rank in females (Pusey et al, 1997). There hasn’t been any documentation of infanticide within a population of Pan paniscus which could be either assessed as the use of non-conceptual sexual behaviour causing paternity confusion, or that there just hasn’t been any witnessed accounts.

 Conclusion

The fact that Pan paniscus and Pan troglodytes share many social factors, such as fission-fusion social group structure and are both male-philopatric, shows that these factors in no way affect the aggressiveness of mating within the species. The main factors to contribute to this stark difference are the unique practices, such as sexual contact as communication for Pan paniscus and male dominance for Pan troglodytes, they have adopted into their communities.

The use of sexual behaviour within a Pan paniscus population to diffuse situations and establish relationship hierarchy has meant that most conflicts are resolved somewhat peacefully through sexual activity. Within a population of Pan troglodytes most conflicts are resolved by a fight or aggressive display.

There is a hypothesis that the adaption of violence is all down to Pan troglodytes habitat during the earlier eras. Earlier chimpanzees occupied a small compact area below the river, now known as the Democratic Republic of the Congo, isolated with no predators and an abundance of food. Whereas Chimpanzees above the river had to habit this area with a population of Gorillas. This caused a high demand for food, as both Pan troglodytes and Gorillas share the same diet which caused competition for food sources (Wrangham and Parish, 2001). This is used to explain why chimpanzees have adapted to use violence to solve conflicts.  There isn’t much supporting evidence for this ‘evolved to utilise violence’ approach but gives an interesting look into how environmental factors could have played a much bigger part in the moulding of Pan troglodytes aggressive behaviour.

It is more creditable to acknowledge that the communication within the Pan paniscus is a lot clearer, with resolutions met instead of an escalation of violence, which has a higher influence on the peaceful aspect of their society. This peaceful aspect along with the fact females entice the males into sexual activity, instead of the chimpanzee males soliciting sex, creates a mutual beneficial mating style.

References:

Boesch C. 1996. Social grouping in Taï chimpanzees. Great ape societies. Cambridge, England: Cambridge Univ Pr; p 101-13.

Cawthon Lang,  K. 2010. Primate Factsheets: Bonobo (Pan paniscus) Behavior . National Primate Research Centre.

Fedigan, L. Rose, L. 1995. Interbirth interval variation in three sympatric species of neotropical monkey. American Journal of Primatology. Volume 37 (1), 9-24.

Inoue, E. Inoue-Murayama, M. Vigilant, L. Takenaka, O. Nishida, T. Relatedness in Wild Chimpanzees; Influence of Paternity, Male Philopatry and Demographic Factors. American Journal of Physical Anthropology. Volume 137 (1), 256-262.

Manson, J. Perry, S. Parish, A. 1996. Nonconceptive sexual behaviour in bonobos and capuchins. International Journal of Primatology. Volume 18 (5), 767-785.

Matsumoto-Oda,  A. 1999. Female choice in the opportunistic mating of wild Chimpanzees at Mahale. Behav Ecol Sociobiol. Volume 46 (1), 258-266.

Pusey, AE, Williams, J, Goodall, J. 1997. The influence of dominance rank on the reproductive success of female chimpanzees. Science. Volume  277(5327), 828-831.

Ruvolo, M. Pan, D. Zehr, S. Goldberg, T. Disotell, T. 1994. Gene trees and hominoid phylogeny. Proceedings of the National Academy of Sciences of the U.S.A. Volume 91 (1), 8900-8904.

Surbeck, M. Mundry, R. Hohmann, G. 2010. Mothers matter! Maternal support, dominance status and mating success in male bonobos ( Pan paniscus ). Proceedings of the Royal Society Biological Sciences. Volume 278 (1), 590-598.

Wrangham, R. 1993. The evolution of sexuality in chimpanzees and bonobos. Human Nature. Volume 4 (1), 47-79.

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