Updated: Oct 1
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)
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