What if I told you there were populations of chimpanzees that made spears to hunt, lived in caves, and loved playing in water? These are behaviors usually associated with ancient humans, not chimpanzees. However, recent research has revealed that there are populations of western chimpanzees (Pan troglodytes verus) that engage in all of these behaviours, and it is challenging our current understanding of chimpanzee taxonomy. In other words, they may not be chimpanzees!
Western chimpanzees are currently understood as one of four subspecies of chimpanzee. The other three subspecies are eastern chimpanzees (Pan troglodytes schweinfurthii), central chimpanzees (Pan troglodytes troglodytes), and Cameroon-Nigeria chimpanzees (Pan troglodytes ellioti). Collectively, these subspecies are members of one of the most well-studied wild animals on the planet. There are currently several field sites across Africa where long-term, continuous behavioural and ecological studies have been conducted (e.g., Whiten et al., 1999). Through these studies, we now have a detailed understanding of our closest relatives’ social structure, mental capacity, and culture. The more we understand about our closest relatives, the more we are forced to relegate our differences from them to human-constructed continuums of complexity. However, researchers are now realizing that there is incredible behavioral, ecological, and genetic diversity within chimpanzees, that was previously unknown.
The East African chimp bias
Until recently, scientists knew very little about chimpanzee subspecies variation. An overwhelming majority of wild field studies focused on eastern chimpanzees (Pan troglodytes schweinfurthii), the subspecies of chimpanzee first studied by Jane Goodall at Gombe in Tanzania (e.g., Goodall, 1962; Goodall, 1967; Goodall, 1968). Studies of this subspecies primarily informed our scientific understanding of wild chimpanzee behaviour and ecology. Over the decades, this resulted in what primatologist, Dr. Jill Pruetz, called an “East African chimp bias” (Pruetz, 2009). This means that eastern chimpanzee behaviour is typically used to represent normative chimpanzee behaviour. Researchers have shown that western chimpanzees do not fit the eastern chimpanzee stereotype (e.g., Humle & Matsuzawa, 2002; Boesch, 2009; Pruetz & Bertolani, 2009; Bogart & Pruetz, 2011).
One of the major causes of this initial bias was habituation. Habituation is the process of conditioning an animal to behave neutrally to a human’s presence, and as I discovered on two recent field surveys in Cameroon, behavioral studies are impossible to conduct on unhabituated populations of chimpanzees. However, the process of habituation presented (and still presents) primatologists with logistical and ethical concerns. Logistically, habituation is difficult because chimpanzees lose their fear of our presence only after several years of repeated neutral contact with researchers (Fedigan, 2010). Habituation presents ethical concerns because humans in general are a threat to chimpanzee existence in the wild (Kondgen et al., 2008; Hughes et al., 2011). It is evolutionarily advantageous for them to avoid us, as opposed to tolerate us (Fedigan, 2010). Despite all of these problems, eastern chimpanzees were the first subspecies habituated, while most other populations remained unhabituated, thus creating the East African chimp bias.
What’s so different?
Recent primatological research on western chimpanzees has revealed behaviors never before observed in other subspecies. This seems to be particularly evident in the behaviour of the Fongoli chimpanzees in Senegal. They live in a woodland-savanna mosaic environment, which is atypical because most known populations of chimpanzee live in a forested environment. They have been observed to use caves as places to socialize and sleep (Pruetz, 2007), predict movement of fire (Pruetz & LaDuke, 2009), construct spears to hunt other primates (Pruetz & Bertolani, 2007), share plant foods (Pruetz, 2009), feed disproportionately on termites compared to other populations (Bogart & Pruetz, 2011), travel and forage at night (Pruetz & Bertolani, 2009), soak themselves, and play in water (Pruetz & Bertolani, 2009). These discoveries have forced scientists to rethink behavioral variation among chimpanzee subspecies because eastern chimpanzees have never been observed to behave in any of those ways (Pruetz, 2009). Furthermore, researchers at other sites in West Africa have also collected data bolstering the claim that western chimpanzee behavior is unlike eastern chimpanzee behavior (e.g., Whiten et al., 2001; Boesch, 2009; Stumpf & Boesch, 2010).
So, are western chimpanzees so different from eastern chimpanzees that they should be classified as a separate species of the genus Pan? Are they more different from eastern chimpanzees than they are from other closely related primates, such as bonobos? It is hard to quantify behavioral differences, but it appears as though western and eastern chimpanzees are more behaviorally similar to each other than eastern chimpanzees are to bonobos (Pruetz, 2009). Also, it is still unclear to what extent the ecological differences between western and eastern chimpanzee habitats are causing these behavioural differences. Understanding these questions will require future behavioral studies but current genetic evidence may help us better understand the causes of chimpanzee variation.
Genetic evidence indicates that western chimpanzees have been separated from the other chimpanzee subspecies for almost 500,000 years (Gonder et al. 2011). In contrast, central and eastern chimpanzees may have consisted of a single population as recently as 100,000 years ago (Gonder et al., 2011). Other genetic studies have confirmed that central and eastern chimpanzee populations are more closely related to each other than they are to western chimpanzees (Becquet et al., 2007). To put those numbers in perspective, bonobos (Pan paniscus) diverged from all chimpanzee subspecies only ~875,000 years ago (Won & Hey, 2004). Furthermore, research has revealed that subspecies genetic differences between western and central chimpanzees are much stronger than seen between any human populations (Fischer et al., 2004). This could indicate that there has been strong directional selection and/or genetic drift within western chimpanzee populations over the past 500,000 years. Is that enough genetic divergence and difference to reclassify Pan troglodytes verus?
A new Pan?
Whether western chimpanzees should be considered a different species within the genus Pan now seems to be largely dependent on definition. There are several different ways to classify a species. Some researchers define a species as any organisms that can actually, or potentially interbreed in the wild and produce fertile offspring. Under this definition, western chimpanzees would not be reclassified because all subspecies of chimpanzee can interbreed and produce fertile offspring, even though they are reproductively isolated in the wild. However, species are not always classified under this definition. Species can also be classified based on similarities and differences in DNA, morphology, and ecological niche. Current behavioral, ecological, and genetic data have revealed that there is an incredible and unexpected amount of chimpanzee subspecies variation that may require us to restructure African great ape taxonomy. But regardless of whether researchers decide to reclassify western chimpanzees, better understanding of chimpanzee variation has enabled us to gain new insight into our closest relatives, and explore our origins as a species in greater detail. Spear-making, cave-dwelling, water-loving western chimpanzees offer us a new opportunity to understand early human evolution. Let’s continue to explore.
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Images: top: Male chimpanzee: Hans Hillewaert / CC-BY-SA-3.0; bottom: Chimps from Fongoli, photo by Joshua Marshack.