The Complex Social Nature of Chimpanzees

Hello to all you chimpanzee enthusiasts! We’re back with another article to talk to you about chimpanzees and their social lives! When it comes to the importance of sociality, chimpanzees are no less dependent on their friends as we are. That being said, there are some clear differences between humans and chimpanzees and the ways in which we live our social lives. Today’s article will look at those two sides of the coin in more detail.

For decades, scientists have debated on the subject of chimpanzee mental capabilities and to what extent they may possess theory of mind (Gallup, 1970). Theory of mind is an important social-cognitive skill that involves the ability to think about mental states, both of the self and of others. A recent study in the journal PNAS (Proceedings of the National Academy of Sciences) titled “Great apes use self-experience to anticipate an agent’s action in a false-belief test”(2019), is an important advance in efforts to determine to what extent our closest relatives can understand others’ perspectives. Indeed studying this topic in chimpanzees also helps to clarify which aspects of the human mind were already present 6-9 million years ago in our common ancestor and which evolved uniquely in the human lineage after the divergence from the other apes. There have been many such studies that have attempted to investigate similar topics, however all of these take place in captive settings where chimpanzees, though they may live long healthy lives, are ultimately not exposed to their natural, innate settings in which their brains and bodies were adapted for. Answering these questions in a natural setting is close to impossible because there are too many factors to control and the ethical questions that arise are numerous and indeed valid. In the wild however, over many decades of behavioral research, scientists have learnt much about our closest living relatives just by watching them go about their daily lives. We are constantly learning and discovering new information about the ways in which chimpanzee society functions and the way in which the chimpanzee mind works in their natural habitats. Chimpanzees are now well known for their cooperative skills, these are most notably observed in group hunting, territory defense and the formation of alliances (Boesch, 1994).

Another one of the many interesting topics of social cognition is social learning. Defined simply it is the ability to transfer information between individuals. In humans social learning has led to a diverse cultural world. Do you think our non-human primates have culture as well? This question has been a hot topic of debate for the past decades among primatologists, psychologists and anthropologists alike. The main findings are that even if not as starkly diverse as human culture, there are clear site-specific and even multi-generational “traditions” in chimpanzee populations. These cultural variations pertain to some of the first observations made by Jane Goodall herself. Some examples of current differences researchers have identified are: tool use, aimed throwing, nest building, grooming, rain dancing and courtship rituals. Differences can also mean a complete absence of a certain type of behavior amongst separate chimpanzee populations or even communities. One of the most interesting findings has been that chimpanzees belonging to one community sometimes develop different behavioral patterns from their neighboring communities even when they share the same habitat and have access to the exact same materials (Luncz et al., 2012). This in some cases could be due to “lessons” between mother and offspring, teaching the ways of the community. However chimpanzees learn all throughout their adolescence from all group members. To finish up the short briefing on chimpanzee culture we want to provide you with one more “hands-on” example. An interesting study studying one community of chimpanzees in the Kibale forest in Uganda showed that chimpanzees there use sticks to extract honey from a log whilst individuals in the Budongo forest of Uganda only 200km away use chewed up leaves as “sponge-like” tools to extract the sweet honey from logs (Brunette, 2020).

Chimpanzee Eating Tortoise (Site-Specific Behavior)

Now we will dive deeper into another aspect that is important and vital for social relationships to form and develop: Emotions. Emotions are said to be indicated via facial expressions (Ekman & Friesen, 1971). These expressions carry a variety of information about motivation and intention. In humans the study of and effort to make sense of these facial expressions has spanned decades (Ekman, 1997). Scientists have now even created a pretty impressive tool for facial coding for Chimpanzess called ChimpFACS, which can be used as a tool to help us understand them better. Identifying prototypical expressions can help propel the research of chimpanzee social cognition as it can provide a standard for scientific work (Parr et al., 1998). Nonetheless, there is still a major problem to get photographic data of emotional expressions as the situations they occur in are highly charged and involve fast movements. However despite these hurdles, research into emotions and social phenomena is producing interesting findings. For example, using these tools researchers have quantified chimpanzee empathy, showing that individuals look out for one another and provide help when needed. An example of this in a captive setting is at the Tchimpounga Chimpanzee Rehabilitation Centre where female chimps have been known to adopt orphaned infants. This also takes place in the wild, at Tai Chimpanzee Project in Cote d’Ivoire there is the amazing story of adult male Fredie and his band of orphans that he cared for and protected, most into adolescence. On a final note, it is important to note that chimpanzees who are deprived of social interactions or their normal living environment often succumb to depression and other negative emotional states. Though sanctuaries around the world do their best to create healthy living environments for chimpanzees, the best place for a chimp is in the wild where they belong.

Now that you know a bit about the mental and cultural capabilities of chimpanzees let’s take a look into social structure and the story behind hierarchies! Be warned the social structures of chimpanzees are not as simple as they appear and can sometimes even seem as interwoven as those in Game of Thrones (disclaimer: do not take this comparison too literal). Indeed the mixture of friends, foes, community and power dynamics at play make for a lot of interesting phenomena to study and understand.

Let's start at the top of the ladder - with the alpha-male. Becoming the alpha-male in a group does not come with a simple guaranteed approach, but rather it depends on the personality of the individual chimpanzee and the standing of that chimpanzee's mother throughout his life. However what most alpha-males do have in common is a strong coalition with other subordinate chimpanzees. These other individuals are vital in helping him both gain and then maintain the power he possesses. These allies can be playmates from juvenile years, brothers or even “new” friends looking to increase their standing in the group. Sometimes however there are new coalitions formed to overthrow an old leader. Much of a chimpanzee male’s life is dedicated to climbing up or being knocked down the chimpanzee social ladder (read about the ‘Fall of Ferdinand’ here). All in all it's an ever changing power struggle, and some of the most exciting moments to watch in chimpanzee society are when places on the hierarchy are challenged (Brittany Cohen-Brown, 2018).

Now what about female chimpanzees? How do they fit into the hierarchy? First of all, it's important to know that there is also an alpha-female. A major difference between male and female alphas is that females aren’t as likely to use aggression and violence to get to their goals. They rather prefer to use long-time relationships and personality traits to either maintain their place or gradually move up the hierarchy. However, this is not to say it's all calm and peaceful, female chimpanzees are capable of using their force, especially when displeased by lower-ranking females. One major benefit being the female alpha is that she is more likely to be more successful when it comes to reproduction and access to food. As mentioned before the offspring of higher-ranking females tend to achieve a higher status themselves than chimps born to lower-ranking mothers (Brittany Cohen-Brown, 2018).

Another interesting piece of information about chimpanzees is that they do not have nuclear families but are rather promiscuous. This comes with a certain advantage: Not being able to know who the father is. This reduces stress that females and their children are faced with and serves as an indirect protection for them. However in very rare cases in intra-community settings do male and sometimes even female chimpanzees kill the offspring of others, due to rivalry (males) or food competition (females). Another thing to note is that male chimpanzees stay in their natal community for their whole life, while females tend to leave and join other communities around puberty (de Waal, 2005).

As we are talking about males and females now let's quickly take a look at “parenting”. Firstly, it should not be understood in the same way that we humans see parenting. For the most part male chimpanzees are not involved in raising the offspring. This is due to the promiscuous dynamics in a community mentioned above. Yet, interestingly more recent studies that can identify paternity in the group show that some fathers seem to be able to identify their offspring and behave in a somewhat positively biased way towards them (Murray et al., 2016). However, the origins of that are not fully understood by research as of now. In general, mothers do all of the hard work! When it comes to mother-child relationships there are different degrees of parental care and mothering styles, comparable to us humans. Some mothers are very protective and others are more relaxed.

Mother and Child

With that all being said, let's take a look into one of the more sinister social aspects of chimpanzees by investigating a phenomenon that was once thought to be uniquely human – war. This phenomenon is common in all communities of chimpanzees that have neighbouring communities. There are several reasons that these aggressive interactions may occur or increase: territory defense, competition for food or territory, predation and more. One famous instance of this is the “Gombe War'' that waged for four years in the 1970’s. It commenced with the death of the original alpha-male which then split the community into two groups the “North” and the “South”. The larger of these two groups then began to invade the territory of the others - if and when they found a rival they attacked him and left him to die of his wounds. Ultimately, this led to the annihilation of the entire group.

Even rarer than this is interspecies violence. Here at Loango the first two observations of two separate encounters between chimpanzees and gorillas were made in 2019. In both situations the chimpanzees outnumbered the gorillas and instigated the attacks. The result: two infant gorillas were killed. What caused this outburst of violence against the gorillas? We assume that it might be due to interspecific competition, or the possibility that the infant gorillas were seen as prey or even that the chimpanzees saw the gorillas as intruders much in the way they view neighbouring chimpanzee groups. If you are interested in more information on that feel free to take a look at our publication in Nature: https://www.nature.com/articles/s41598-021-93829-x

We hope that all this information gives you a rough idea of how interesting and complex research into chimpanzees and their social structures is and how surprising some observations can be. Also, there is much uncharted territory still left to explore!

Thats all we have for you today. Stay tuned for upcoming articles!

Best wishes

The Ozouga-Blogging-Team

References:

1. Gallup, G. G., Jr. Chimpanzees: Self-recognition. Science 167, 86-87 (1970)

2. Kano, F., Krupenye, C., Hirata, S., Tomonaga, M., & Call, J. (2019). Great apes use self-experience to anticipate an agent's action in a false-belief test. Proceedings of the National Academy of Sciences of the United States of America, 116(42), 20904-20909. https://doi.org/10.1073/pnas.1910095116

3. Boesch, C. Cooperative hunting in wild chimpanzees. Animal Behavior 48, 653-667 (1994).

4. Luncz, L., Mundry, R., & Boesch, C. (2012). Evidence for Cultural Differences between Neighboring Chimpanzee Communities. Current Biology, 22(10), 922-926. doi: 10.1016/j.cub.2012.03.031

5. Matt Brunette, JGI Canada Volunteer (2020, June 9). Do Chimpanzees Have Culture? Jane Goodall. https://janegoodall.ca/our-stories/chimpanzees-and-culture/

6. Ekman, P., & Friesen, W. V. (1971). Constants across cultures in the face and emotion. Journal of Personality and Social Psychology, 17(2), 124–129. https://doi.org/10.1037/h0030377

7. Ekman P. Should we call it expression or communication?. Innovations in Social Science Research, 1997, vol. 10 (pg. 333-44)

8. Parr LA, Hopkins WD, de Waal FBM. The perception of facial expressions in chimpanzees (Pan troglodytes), Evolution of Communication, 1998, vol. 2 (pg. 1-23)

9. Cohen-Brown, B. (2018, July 11). From Top to Bottom, Chimpanzee Social Hierarchy is Amazing! Jane Goodall’s Good for All News. https://news.janegoodall.org/2018/07/10/top-bottom-chimpanzee-social-hierarchy-amazing/

10. de Waal, F. A century of getting to know the chimpanzee. Nature 437, 56–59 (2005). https://doi.org/10.1038/nature03999

11. Murray Carson M., Stanton Margaret A., Lonsdorf Elizabeth V., Wroblewski Emily E. and Pusey Anne E. 2016 Chimpanzee fathers bias their behaviour towards their offspring, R. Soc. open sci.3160441160441

12. https://releasechimps.org/chimpanzees/intellect-and-emotion

13. http://www.conservenature.org/learn_about_wildlife/chimpanzees/chimp_mothering.ht

Food and Friends in Loango

We previously discussed what chimps get up to on a daily basis, in our Daily Lives of Chimps article. Today, we intend to do some myth-busting and discuss the feeding and food assimilation patterns of chimpanzees. We will also look into whether chimpanzees demonstrate prosocial behaviors like the sharing of food, tool use, and teaching.

For starters (pun intended), we would like to clarify a myth that we likely all have encountered at some point in our life. That is, chimpanzees and other apes love bananas! We will just remind you of the depictions in the Disney movie: “The Jungle Book”. But it’s not just there that we encounter this image. Mainstream media have portrayed chimpanzees as ‘banana lovers’ for decades. Nonetheless, there have more recently been efforts by zoos to remove bananas from the diets of their chimpanzees due to health concerns, such as high sugar levels and the associated risks that follow it. Those might be rotting teeth or even diabetes. Yet, the assumption that chimpanzees and other apes like bananas is not completely unfounded, they really do enjoy them. One probable reason for that belief might be a study from back in the 1930s with captive monkeys. There they found that when given the choice between multiple food types the preference for bananas ranked pretty high, just below grapes, which are the first place (Katz & Katz, 1936). This information has for some reason made its way in many feeding guides for captive chimpanzees. They do like bananas, there is no debate about that. However one must always distinguish between captive chimpanzees and chimpanzees in the wild! Bananas are a plant species domesticated by humans which means that in the wild the chances of coming across this tasty treat is relatively low to non-existent.

Chimpanzees are essentially frugivores (fruit comprises ~⅔ of their diet), however, they also feed on leaves, insects, and other food types such as honey and meat. In Loango they access honey mainly from underground bee nests by using tools specifically designed for this purpose (Estienne and colleagues, 2017). Meat consumption is achieved by mammal predation, though still under investigation it seems that there may be an aspect of seasonality to hunting activity in different chimpanzee populations. Some other factors that influence the diet of chimpanzees are the presence or absence of prey species, competing predators, or human interference (McGrew, 1983).

We humans generally love to share our meals, but what about chimpanzees? Do they like to eat with kin and friends?

Unlike us humans, sharing food outside of kinship or mating relationships is rare across the animal kingdom. Yet, chimpanzees and bonobos are two of the rare exceptions. Insights into these behaviors in the non-human great apes might help us better understand the evolution of human cooperation. So let's take a closer look!

Some studies have shown that chimpanzees are more likely to share food with their friends and that neither high dominance status nor harassment by beggars influence their decision (Samuni and colleagues, 2018) though there is some variation, the rate of sharing is quite high in general (Silk and colleagues, 2013). Research also suggests that chimpanzees selectively share meat with receptive females. Such transfers have been reported to enhance immediate mating opportunities at Gombe (Stanford, 1998) and future mating prospects, but not immediate ones, in the Taï Forest (Gomes & Boesch, 2011). Furthermore, males selectively transfer meat to males that have also transferred the meat to them and to their allies. This has been observed in the Taï Forest of Côte d’Ivoire and in Ngogo in the Kibale Forest of Uganda (Mitani & Watts 1999, 2001; Boesch & Boesch-Achermann 2000; Mitani 2006). After group hunts sharing is observed quite frequently (Silk and colleagues, 2013). Tolerated theft (Blurton Jones, 1984) or sharing under pressure (Wrangham 1975; Stevens & Stephens 2002; Stevens & Gilby 2004) has been observed at Gombe wherein males give up parts of their kills to avoid the costs of defending a food item against persistent beggars.

By now we have just covered the general mechanisms at play when it comes to food sharing. But what does it actually look like when sharing takes place? Chimpanzees use a lot of gestures when begging for meat, for example, they stretch out their hands, and they also use vocalizations to demonstrate their desires i.e. with grunts and whimpers. Excitement over food might also be displayed via embracing, touching, kissing and even sociosexual behavior. So far, we have covered the observable part of behavior. But let's now take a look at what's happening “inside” of chimpanzees in these contexts. Oxytocin or more commonly known as “the love hormone” studied in humans is also found in the chimps and can be used to shed some light on their prosocial behavior. Samuni and colleagues (2018) found high levels of oxytocin, when meat and other valued foods were shared or after hunting with others. This suggests that oxytocin is a key hormone involved in cooperation in general. On a side note, the effects are highest for meat and honey, so there seems to be something special about them. But that is a story for another day.

Sharing also takes place in the form of teaching skills and sharing tools. For most wild chimpanzees, tool use is an important part of life -- but learning these skills is no simple feat. The capacity for teaching and sharing tools is predominantly observed in mother-child relationships. Apart from that, tool donors experienced significant reductions in tool use and feeding, while tool recipients significantly increased their tool use and feeding after tool transfers. Since tool transfers are costly to the teacher, tool donors also showed sophisticated cognitive strategies that effectively buffered them against potential costs. Tool transfers, for example, have been documented during honey extraction from underground beehives by our chimpanzees in the Loango National Park. Given the complexity of these tool tasks, tool transfers from skilled chimpanzees to less competent conspecifics constitute a form of teaching.

In video: Rekambo chimps extracting honey from underground beehives

We would like to end this article with an interesting quote that not only covers teaching in chimpanzees but also its probable impact on the evolution, “Teach a chimpanzee to fish for insects to eat, and you feed her for a lifetime. Teach her a better way to use tools in gathering prey, and you may change the course of evolution.” (Musgrave, 2019)

Yours sincerely,

The Ozouga-Blogging-Team

References:

1. (1936), Some Problems concerning the Feeding Behaviour of Monkeys. By Professor DAVID KATZ, Ph.D., and ROSA KATZ, Ph.D.*. Proceedings of the Zoological Society of London, 106: 579-582. https://doi.org/10.1111/j.1469-7998.1936.tb08519.x

2. Samuni L., Preis A., Mielke A., Deschner T., Wittig R. M. and Crockford C. 2018. Social bonds facilitate cooperative resource sharing in wild chimpanzees. Proc. R. Soc. B. 285: 20181643 20181643 https://doi.org/10.1098/rspb.2018.1643

3. Estienne, Vittoria & Stephens, Colleen & Boesch, Christophe. (2017). Extraction of honey from underground bee nests by central African chimpanzees ( Pan troglodytes troglodytes ) in Loango National Park, Gabon: Techniques and individual differences. American Journal of Primatology. 79. 22672. 10.1002/ajp.22672.

4. Washington University in St. Louis. "Chimpanzees more likely to share tools, teach skills when task is complex: Study has implications for evolution of technology and culture in humans." ScienceDaily. ScienceDaily, 24 December 2019.

5. Silk, Joan & Brosnan, Sarah & Henrich, Joseph & Lambeth, Susan & Shapiro, Steven. (2013). Chimpanzees share food for many reasons: The role of kinship, reciprocity, social bonds and harassment on food transfers. Animal Behaviour. 85. 941–947. 10.1016/j.anbehav.2013.02.014.

6. Stephanie Musgrave, Elizabeth Lonsdorf, David Morgan, Madison Prestipino, Laura Bernstein-Kurtycz, Roger Mundry, Crickette Sanz. Teaching varies with task complexity in wild chimpanzees. Proceedings of the National Academy of Sciences, 2019; 201907476 DOI: 10.1073/pnas.1907476116

7. McGrew, W.C. Animal foods in the diets of wild chimpanzees (Pan troglodytes): Why cross-cultural variation?. J. Ethol. 1, 46–61 (1983). https://doi.org/10.1007/BF02347830

8. Stanford, CB. Chimpanzees and Red Colobus. Cambridge, Massachusetts: Harvard University Press; 1998.

9. Gomes CM, Boesch C. Wild chimpanzees exchange meat for sex on a long-term basis. PLoS One. 2011; 4:e5116. [PubMed: 19352509]

10. Mitani, JC. Reciprocal exchange in chimpanzees and other primates. In: Kappeler, PM.; van Schaik, CP., editors. Cooperation in Primates: Mechanisms and Evolution. Heidelberg: Springer-Verlag; 2006. p. 101-113.

11. Mitani JC, Watts DP. Demographic influences on the hunting behavior of chimpanzees. American Journal of Physical Anthropology. 1999; 109:439–454. [PubMed: 10423261].

12. Mitani JC, Watts DP. Why do chimpanzees hunt and share meat? Animal Behaviour. 2001; 61:915– 924.

13. ​​Blurton Jones NG. A selfish origin for human food sharing: tolerated theft. Ethology and Sociobiology. 1984; 5:1–3.

14. Wrangham, RW. Ph.D. thesis. Cambridge University; 1975. Behavioural ecology of chimpanzees in Gombe National Park, Tanzania.

15. Stevens JR, Stephens DW. Food sharing: a model of manipulation by harassment. Behavioral Ecology. 2002; 13:393–400.

16. Stevens JR, Gilby IC. A conceptual framework for nonkin food sharing: timing and currency of benefits. Animal Behaviour. 2004; 67:603–614.

17. Boesch, C.; Boesch-Achermann, H. The Chimpanzees of the Taï Forest: Behavioural Ecology and Evolution. Oxford: Oxford University Press; 2000.

Daily Lives of Chimps!

Hello fellow reader,

Today we would like to introduce you to our chimpanzee community, and show you what a typical day in the life of a chimpanzee looks like. We invite you to a thought experiment: Just imagine one night going to bed human and waking up as a chimpanzee in our Rekambo community. What would your day look like? What would you eat? and where would you sleep? Let's find out and explore a day in the life of a chimpanzee.

Chimpanzees are definitely early birds, starting their day at sunrise which is almost constant throughout the year in Gabon due to its close proximity to the equator. Their typical day starts around 6:30 am.

What’s the first thing you think of when you wake up in the morning? Food of course! There is nothing better than a good breakfast. This is why chimpanzees often set up their nests close to a tree with the fruits they eat. It would be fair to say that most of their waking day revolves around food, which in Loango comprises more than 150 plant species and several animal species as well. Depending on the season, a chimpanzees’ diet mainly consists of fruits, leaves, seeds, pith, flowers, insects, and meat from monkeys, duikers, and tortoises. Furthermore, chimpanzees display innovative techniques to access difficult to obtain food sources like honey from underground bee nests (Estienne and colleagues, 2017a, 2017b, 2019; Moscovice and colleagues, 2007).

Chimpanzees employ a suite of cognitive mechanisms like classifying food trees, remembering the quantity and frequency of fruit production on those trees across years, and planning return times to the feeding trees with efficient travel between them (Janmaat and colleagues 2013, 2014, 2016). Chimpanzees spend almost a third of their day eating and foraging, and it is considered to be the most performed daytime activity. Apart from eating, grooming is another activity that they spend almost a tenth of their day on. Grooming serves both the practical purpose of removing dirt and parasites but also helps the group members to establish and maintain their special social bonds.

Bravo! you’ve successfully made it through half of the day, now it’s time to get some rest. Chimpanzees spend a good portion of their day sleeping or resting, in addition to the overnight sleep. With one exception…... infant chimpanzees! Who knows no rest and are active all day as opposed to the human infants who tend to sleep most of their day. On the majority of days, before building their nest for the night, there is usually another short trip in search of the evening snack if it’s not in the vicinity. After this last burst of activity, the chimpanzees then start deciding on their nesting location and build accordingly. They do this every single night in a different location!

So what we’ve given you so far is the typical account for a rather normal day. However, there are some special days where the male chimpanzees in the group (and occasionally some females and juveniles) may collectively decide to go on patrol. This behavior sometimes leads to an intercommunity encounter (either vocal or visual) with another community of chimps! During a patrol, chimpanzees quite literally surveil the outskirts of their own and nearby territories looking for signs of other chimpanzee communities in the area. They do so by examining feeding signs or smelling leaves and frequently used travel paths. However, this activity requires great caution, as the scouting mission might fail and they could be outnumbered and maybe even killed by the community controlling the area (Wilson and colleagues, 2014). In Loango, patrolling behavior varies in its frequency throughout the year and this behavior also varies across different field sites.

(*snapping finger*) You are back in front of your screen! How did you enjoy your day as a chimpanzee? Interesting isn’t it?

Yet, that’s all for now! But keep an eye open for new articles that will be published soon.

Best wishes,

The Ozouga-Blogging-Team

References

1. Based on the accounts of researchers working with the Rekambo Community in the Loango National Park on a daily basis.

2. Pruetz, Jill & Mcgrew, William. (2001). What does a chimpanzee need? Using natural behaviour to guide the care of captive populations.

3. Moscovice, L. R., Issa, M. H., Petrzelkova, K. J., Keuler, N. S., Snowdon, C. T., & Huffman, M. A. (2007). Fruit availability, chimpanzee diet, and grouping patterns on Rubondo Island, Tanzania. American journal of primatology, 69(5), 487–502. https://doi.org/10.1002/ajp.20350

4. Estienne, V, Stephens, C, Boesch, C. Extraction of honey from underground bee nests by central African chimpanzees (Pan troglodytes troglodytes) in Loango National Park, Gabon: Techniques and individual differences. Am J Primatol. 2017a; 79:e22672. https://doi.org/10.1002/ajp.22672

5. Estienne, V., Mundry, R., Kühl, H.S. and Boesch, C. (2017b), Exploitation of underground bee nests by three sympatric consumers in Loango National Park, Gabon. Biotropica, 49: 101-109. https://doi.org/10.1111/btp.12354

6. Janmaat, K.R.L., Ban, S.D. & Boesch, C. Taï chimpanzees use botanical skills to discover fruit: what we can learn from their mistakes. Anim Cogn 16, 851–860 (2013). https://doi.org/10.1007/s10071-013-0617-z

7. Janmaat, K., Polansky, L., Ban, S., & Boesch, C. (2014). Wild chimpanzees plan their breakfast time, type, and location. Proceedings of the National Academy of Sciences, 111, 16343 - 16348.

8. Janmaat, K.R., Boesch, C., Byrne, R., Chapman, C.A., Goné Bi, Z.B., Head, J.S., Robbins, M.M., Wrangham, R.W. and Polansky, L. (2016), Spatio-temporal complexity of chimpanzee food: How cognitive adaptations can counteract the ephemeral nature of ripe fruit. Am. J. Primatol., 78: 626-645. https://doi.org/10.1002/ajp.22527

9. Wilson, M. L., Boesch, C., Fruth, B., Furuichi, T., Gilby, I. C., Hashimoto, C., Hobaiter, C. L., Hohmann, G., Itoh, N., Koops, K., Lloyd, J. N., Matsuzawa, T., Mitani, J. C., Mjungu, D. C., Morgan, D., Muller, M. N., Mundry, R., Nakamura, M., Pruetz, J., Pusey, A. E., … Wrangham, R. W. (2014). Lethal aggression in Pan is better explained by adaptive strategies than human impacts. Nature, 513(7518), 414–417. https://doi.org/10.1038/nature13727