Western Chimpanzee, Pan troglodytes verus
WESTERN CHIMPANZEE
Pan troglodytes verus
Geographic Distribution and Habitat
The Western chimpanzee is found in West Africa with a fractured distribution between Senegal to Ghana. They may also range as far East as Western Nigeria, but we haven’t collected the genetic data to know if this is accurate. Populations of Western chimpanzees have been confirmed in Togo and Benin but have recently gone locally extinct in these countries. They were also believed to have gone extinct in Guinea-Bissau, but are now known to persist in the country albeit in the low hundreds. The Western chimpanzee primarily persists in Côte d’Ivoire (also called the Ivory Coast), Guinea, Liberia, Mali, and Sierra Leone, although the populations in these countries have also experienced significant declines.
Western chimpanzees inhabit both dry and moist lowland tropical forests, savannah woodlands, and the mixed gallery forests in between. They are also found in fallow agricultural areas (farmed land that is temporarily being left alone to allow regrowth of some native plants to maintain the soil’s quality), typically in wild or cultivated oil palm plantations.
Overall, Western chimpanzees live in forests with higher and more consistent food availability compared to other chimpanzee subspecies, which researchers attribute to many of their social and behavioral differences from other chimp subspecies.
Initial studies of chimpanzees were based in East Africa, starting with Jane Goodall’s famous research in Tanzania in 1960, shaping our understanding of the species as a whole. It wasn’t until almost 20 years later in the late 1970s that field sites in West Africa began initiating studies on the regional chimpanzee populations and noticing the differences between the subspecies.
While still debated, scientists have settled on the four current chimpanzee subspecies based on genetic, morphological, and behavioral differences. The two West African subspecies (the Western chimpanzee and the Nigeria-Cameroon chimpanzee) are much more similar to each other genetically than the East African subspecies (the Eastern chimpanzee and Central African chimpanzee) and are estimated to have split from each other between 500,000 – 1.6 million years ago. The differences between Western chimpanzees and Eastern and Central African chimpanzees are so distinct that some scientists think they should be considered their own species, but this is a highly argued subject.
While researchers may not agree on the nuances of chimpanzee taxonomy, they do agree that the four currently recognized chimpanzee subspecies all face unique threats and require different conservation approaches.
Size, Weight, and Lifespan
When standing upright (bipedally), Western chimpanzees are about 39 – 59 inches (99 – 150 cm) in height. When standing quadrupedally (on all fours), they are about 23.6 – 33.5 inches (60 – 85 cm) tall. Their head-to-rear length is 27.2 -37.8 inches (69 – 96 cm).
Chimpanzees are sexually dimorphic, with males being larger with an average weight of 102 pounds (46.3 kg), while females typically weigh around 91.7 pounds (41.6 kg). They fall in between Eastern and Central African chimpanzees in size, with Central African chimps being larger and Eastern chimpanzees being smaller. Western chimpanzees are also less sexually dimorphic compared to the other subspecies.
The average lifespan for chimpanzees is around 30 years, with males having shorter lifespans than females. Many individuals only make it to around 15 years, but there have been records of chimpanzees making it into their 60s, even in the wild.
Appearance
Chimpanzees have a robust build, with a protruding brow ridge. In all subspecies, chimpanzee infants are born with pink faces and ears that contrast their dark fur which becomes darker with age until blending in with their fur color. Western chimpanzees are noted to have darker skin around their eyes than the rest of their face even in infancy, appearing as a mask, and this contrast often persists into adulthood. All chimpanzees are also born with a small white tuft of tail fur, which they lose in adulthood.
Diet
Chimpanzees are omnivorous with a highly variable diet. The overall composition of their diet varies between populations and seasons. Broadly, they mostly eat fruit, plus other plant matter such as bark, leaves, and stems. Chimpanzees have been known to crop-raid and eat rice, papaya, mango, oranges, cassava, maize, sugarcane, cashews, and beans grown on local farms. While crop-raiding often receives a negative response from farmers, cashew farmers take less issue with the chimps. Chimpanzees don’t eat the cashew nut like we do, but rather eat the fruit (technically a pseudofruit), which is difficult for farmers to sell due to its thin easily damaged skin. While the chimpanzees do sometimes damage cashew trees, they also tend to leave the nuts in easily collectible piles, essentially doing part of the farmers’ job for them. Western chimpanzees have also been observed using modified leaves (or “leafy tools”) to drink fermented sap from tapped palm trees (with an average alcohol by volume of 3.1%).
Chimpanzees are also known for sometimes eating meat. This can range from insects to mammals (including bush babies and red colobus monkeys), although researchers have noted this part of their diet is not nutritionally important, only supplementary. Hunting and (more rarely) sharing of meat can have significant social implications for chimpanzees. Hunting is almost exclusively done by males, which serves to strengthen their social bonds and they will even share meat with their friends, or high-ranking females. Males will also steal meat from lower-ranking individuals to assert their dominance.
Behavior and Lifestyle
Chimpanzees are mostly active during the day, making them diurnal. They move mostly on the ground by knuckle-walking but are also apt at moving and swinging through the trees. They are highly territorial, and encounters with other groups can become violent and even fatal. As chimpanzee habitat shrinks, encounters and subsequent altercations between groups are likely to increase. Male and female Western chimpanzees will patrol the periphery of their territory once a month to make sure no other groups are infringing on their space. If intruders are seen, the males will often attack, although for Western chimpanzees these altercations are less likely to be lethal. In other subspecies of chimpanzee, it is strictly the males that go on patrols.
Chimpanzees are also known for their ability to use tools (such as the modified leaf mentioned earlier). Tool use is reflective of how animals can have their own culture, as it is something that is learned from other individuals and is not innate knowledge shared throughout the species. This means that not all chimpanzee groups will use the same tools, even within the same subspecies.
Western chimpanzee groups that live in savannah landscapes utilize their own behavioral adaptations, shaped for their unique environment. Caves provide shelter from the sun when tree cover is not an option. During the rainy season, groundwater is a valuable resource, and individuals will even soak in shallow pools when they have the chance. While typically active during the day, savannah chimpanzees will also forage at night when temperatures are cooler, especially during the full moon which makes it easier to see.
Western chimpanzees live alongside several other primate species, including red colobus, baboons, and bushbabies (galagos). They also live alongside other iconic African wildlife, including elephants, lions, leopards, buffalo, and hippos.
Chimpanzees live in large, multimale-multifemale groups with a fission-fusion group structure, meaning individuals in the group associate with different subgroups within the main group, which will change throughout the day. Group sizes can range from 16 – 82 individuals, with home ranges typically of 5.8 – 25.1 square miles (15 – 65 square km). Daily range varies between habitat types, with groups found in savannah landscapes ranging farther than those in forests, making up for the wider spread of resources in the prior landscape. Their daily range can be anywhere from 3 – 34 square miles (8 – 89 square km). While many primates move together as a group within their home range, because of chimpanzees’ fission-fusion social structures, daily ranges may differ among individuals. Males typically have longer day ranges than females, except for females that are in estrus and traveling along side males. Female Western chimpanzees in particular travel more often with males compared to other subspecies, making females more likely to have a daily range that matches or comes close to the males. Western chimpanzees in savannah habitats will sometimes even move together as a whole group.
Like other chimpanzee subspecies, they are male-dominant, with males having the strongest social bonds and even grooming each other more than females. Males will group together to hunt and defend the group. Males form a linear dominance hierarchy, with a single alpha male. Many studies have regarded female chimpanzee relationships as weak due to female emigration (and a general lack of studies focusing on females). However, more recent studies have shown that in Western chimpanzees, female alliances are more common, and females will groom each other more often than females of other subspecies. Some recent studies have also suggested that female Western chimpanzees may also have linear hierarchies like the males do. Because females are more social with males and each other compared to other chimpanzee subspecies, Western chimpanzees have been dubbed “bisexually bonded” by some researchers to better reflect their social habits.
When chimpanzees are ready to go to sleep at night or take a quick nap during the day, they’ll build a nest in the trees to rest in. Some Western chimpanzees have been known to sometimes nest on the ground. These nests are crucial for researchers in helping determine the presence of chimpanzees in an area, as the chimpanzees themselves are often elusive.
Chimpanzees use multiple methods of communication. Vocalizations range from a variety of “barks” while hunting or to alert other group members of danger, to “pant hoots” used in a variety of social situations. Facial expressions are also commonly used to get various messages across. Bared teeth typically indicate fear, while a gaped-mouthed stare acts as a threat to the target of the expression. Chimpanzees also use gestures to communicate, from a simple hug or touch to show affection to elaborate branch-shaking displays to assert dominance. Recent studies on chimpanzee communication have revealed they used more nuanced gestural communication as well. These are being studied and are not yet fully understood by humans, who many not even see them.
When female chimpanzees become sexually mature, they will go into periods of estrous when they are sexually active, marked by large pink perineal swellings to advertise their sexual state. Female Western chimpanzees will display their swellings for longer than Eastern chimpanzees. Ovulation (the brief few days when a female may actually conceive) only occurs around the end of estrous, meaning most of chimpanzee’s sexual activity is nonreproductive. Having a longer period of advertised sexual activity may allow females to choose which male actually sires their offspring, as well as strengthen social bonds with other males in the group. Females give birth every 5-6 years to a single offspring. Inter-birth intervals (the time from one female giving birth to her next) are variable but have been noticed to be shorter for Western chimpanzees compared to their Eastern counterparts. Females will typically go back into estrous (and the associated swellings) about a year after giving birth.
Researchers describe four different mating strategies that chimpanzees use: opportunistic mating, where females may mate with many males in a close timeframe; consortship, where a male and female will go off for “alone time” over a few days or even weeks; and possessive mating, where a male, or group of males, will guard a female from other males.
Chimpanzees are male philopatric, meaning males will typically stay in their home group for their entire lives, while females will immigrate to a new group when sexually mature. Immigrant females will make their place in a new group by establishing relationships with the resident males. Resident females are generally unaccepting of new females and will at times attack them to discourage their entry to the group.
Like many primate species, chimpanzees hold the essential role of seed disperser. While eating fruits, they help spread seeds throughout the forest as they move (often conveniently pre-packaged in fertilizer). This helps maintain the biodiversity of the forest, and ensures chimpanzees and other animals that depend on these fruits will be able to utilize them in the future.
The Western chimpanzee is classified as Critically Endangered by the International Union for Conservation of Nature (IUCN, 2016), appearing on the IUCN Red List of Threatened Species. Their population size is projected to decrease, and they are the second-most threatened of the four chimpanzee subspecies.
Western chimpanzees are primarily threatened by habitat loss caused by the development of oil palm, rubber, eucalyptus, and sugar cane farming. Logging, mining for minerals, and extraction of oil also contribute to habitat destruction. This can also cause habitat fragmentation, where once wide, accessible habitat is reduced to small forest fragments separated by farmlands, roads, and other human developments. For chimpanzee groups in these fragments, their ability to access new food sources is severely limited, as well as access to potential mating opportunities. With agricultural developments expanding into chimpanzee habitats, it becomes more and more common for chimpanzees to crop raid, causing local farmers to perceive them as pests and retaliate.
Although chimpanzees are a protected species, they are still hunted for bushmeat, use in traditional medicines, and infants are sometimes captured to be sold in the pet trade.
Because humans and chimpanzees share 98% of our DNA, many diseases that affect us also impact chimpanzees, including coronaviruses and Ebola. With human settlements expanding into chimpanzee habitats, human/chimp encounters also increase along with the risk of transmitting diseases between us.
Western chimpanzees are listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. They are also Class A of the African Convention on the Conservation of Nature and Natural Resources. Although under protections of both local and international laws in all regions chimpanzees are found, these protections are poorly enforced and often subverted by determined poachers. Western chimpanzees are found in national parks throughout their range, which are protected, but the majority of the population is found outside of these areas.
Mediating human-wildlife conflict is a critical aspect of conservation biology, both for the wellbeing of the animals and the humans protecting their own livelihood. Many of the people in conflict with chimpanzees are also struggling with poverty and depend on their crops to feed and financially provide for their families. Conservationists are pushing to find ways to help prevent chimpanzees from crop-raiding, helping them find alternative forms of income, and changing local attitudes towards local wildlife.
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Written by Lina Rademacher, March 2025