WESTERN CHIMPANZEE

Pan troglodytes verus

Geographic Distribution and Habitat
Western chimpanzees, also known as West African chimpanzees, are found in western Africa in the countries of Ivory Coast (Côte d’Ivoire), Ghana, Senegal, Guinea, Liberia, Mali, Guinea-Bissau, Sierra Leone, and Burkina Faso. This subspecies of the common chimpanzee also used to inhabit Benin, Gambia, and Togo, but has gone extinct in those areas.

Whereas most other chimpanzees live in dense jungles, Western chimpanzees live in forest-savanna mosaics—open grasslands mixed with dry gallery forests. The local chimps have developed a vast arsenal of novel behaviors and adaptations to survive in the unique environment.

Western chimpanzee geographic range. Map credit: Luís Fernández García/Creative Commons

Size, Weight, and Lifespan
Western chimpanzees are intermediaries in size between other subspecies of chimpanzee. They are much smaller than the Central chimpanzee (P. t. troglodytes) and are slightly larger than the Eastern chimpanzee (P. t. schweinfurthii). The male Western chimpanzee has a head-to-rump length of 30–38 inches (77-96 cm) and a weight of 103.5 lb (47 kg). Females are only slightly smaller, with a length of about 27.5–36 inches and a weight of 92 lb (41.6 kg).

Chimpanzees can have a wide range of lifespans. Many chimps in captivity have lived to well over 50; however, a more typical life expectancy for a captive chimpanzee is between 30 and 40 years. In the wild, most chimpanzees that survive their infancy will live between 15 and 25 years.

Appearance
Humans and chimpanzees diverged between 4 and 8 million years ago and are our closest relatives. As such, we have a great deal in common between our species. Chimpanzees are robust apes covered in thin black hair, except for their faces, hands, and feet, which are bare. This bare skin is pink on newborns, but turns black as they get older. Like humans, chimpanzees may go partially bald later in life and develop gray hair, particularly around their chin.

The arm span of a chimpanzee is approximately 50% longer than their body length. (By comparison, human arm spans are usually equal to or slightly above our height). Their hands feature long fingers and short thumbs, which are specialized for climbing trees and picking fruit. Interestingly, fossil records show that human hands are actually more primitive than chimpanzee hands. That is to say, our hands have gone relatively unchanged since we separated from chimpanzees, while their hands have developed a structure more suitable to their environment.

Chimpanzee legs are far different from human legs. While human thighs slope inward, allowing us to keep are legs together and walk upright, chimpanzee thighs are sloped outward. This means they have to keep their feet farther apart. This fact, along with our stronger pelvic muscles and spinal adaptations, is why we can walk upright while chimps can only do so for short moments. Chimps have an opposable big toe to help grasp branches.

Western chimpanzees have rounder muzzles and more prominent brows than Eastern and Central chimpanzees. Researchers also report that they have dozens of unique nonmetric skull features. Nonmetric skull features are anomalies in the skull that cannot be numerically measured, but can be observed and described by experts.

What Does It Mean?

Agonistic:
Of, relating to, or being aggressive or defensive social interaction (such as fighting, fleeing, or submitting) between individuals usually of the same species.

Alpha male:
The dominant male animal in a particular group.

Anogenital:
Relating to the anus and genital region.

Diurnal:
Active during daylight hours.

Estrus:
A recurring period of sexual receptivity and fertility in many female mammals.

Fission-fusion society:
A society in which the members of a social group break off into smaller groups and then rejoin as a larger group.

Gestation Period:
The time of pregnancy from conception until birth.

Natal group:
The group into which an animal is born.

Visit the Glossary for more definitions

Diet
Chimpanzees are omnivores with a set of teeth adapted to both grinding plants and tearing flesh. About 50–60% of the chimpanzee diet is made up of fruit, depending on the season. Another ten percent is dedicated to leaves. Western chimps tend to feed on termites and ants more often than other subspecies, using tools to scoop the insects out of their nests. Chimpanzees are also known to eat tree bark, gum, honey, flowers, and algae when available.

Meat is a prized meal among chimpanzees. When they can, they will eat birds or other mammals. They may lead raiding parties where they hunt a group of monkeys and share their catch with one another. Male and female Western chimpanzees have displayed different behaviors when feeding. Males are more likely to eat monkeys while females eat smaller primates such as bushbabies. Researchers have recently observed a remarkable behavior where Western chimpanzees will craft a spear and stab it into a tree hole in hopes of catching a bushbaby. This behavior has not been observed in any other subspecies of chimpanzee.

Behavior and Lifestyle
The differences between subspecies of chimpanzees can be most clearly seen in their behaviors, and the Western chimpanzees is incredibly unique in this regard. In addition to hunting with spears and eating more insects, Western chimpanzees have also been observed predicting the movement of forest fires, using water to cool down on hot days, and regularly gathering in caves to socialize and sleep. While Western chimpanzees—like other chimpanzees—are diurnal, they have also been observed hunting at night. No other subspecies of chimp has been observed taking part in these behaviors. One can attribute the number of unique behaviors to their unique habitat, as Western chimpanzees are the only chimps that live in savanna woodlands. It follows that a unique environment would be met with unique adaptations.

It should be noted that differences between chimpanzee behavior may not be due to subspecies differences, but rather cultural ones. Chimps learn most of their behavior from adults when they are young (research suggests that older chimps are often unable or unwilling to learn new behaviors). Because of this, we can see marked behavioral differences not just among different subspecies, but among different groups of the same subspecies. How one Western chimpanzee collects ants can be completely different from a Western chimpanzee only a few miles away.

Fun Facts

Western chimpanzees separated from the other chimpanzees around 500,000 years ago. For comparison, bonobos are only separated from them by 800,000 years. This has led some researchers to suggest Western chimps be defined as their own separate species.

Daily Life and Group Dynamics 
Chimpanzees live in complex societies, which is no surprise given their complex brains. They live in fission-fusion societies, where individuals may break away from the main group for months at a time before rejoining. Western chimps spend about half of their time foraging for food, a quarter of their time resting, and the rest of their day traveling and socializing.

Western chimpanzee groups usually consist of 16 to 29 individuals, which is very small in comparison to their Eastern and Central cousins whose groups can number over a hundred. These groups are ruled by an alpha male who must routinely prove himself as the strongest and/or smartest member of the group to fend off challengers. Chimpanzees regularly play politics by forging and breaking alliances in order to gain power.

These fights for power become more common when a female in the group enters estrus and the males engage in testosterone-fueled battles for mating rights. Most of these battles feature displays of intimidation and loud noises, but rarely end in actual violence as the risk often does not outweigh the reward. However, death and debilitating injuries are always possibilities in these power struggles.

Western chimpanzees and their political games were recently spotlighted on the BBC documentary “Dynasties.” The premiere episode of the series featured David, an alpha male Western chimpanzee in Senegal, who struggles to maintain control of his group of 32 chimpanzees.

Communication
Chimpanzees have a vast repertoire of barks, hoots, and grunts to communicate with one another. Researchers have learned at least 32 different calls, with other call meanings still being disputed. The most well-known chimp vocalization is the pant-hoot, a series of inhaling and exhaling “hoo hoo” sounds meant to signal many things including food, location, greetings, and threats. Context is key is when deciphering chimpanzee communication.

Other vocalizations include barks, which are usually used during agonistic interactions; pant-grunts, which signify submission to another; screams, which may be a call for help or submission; whimpering from infants; laughing during play; warning calls, which seem to be specific for each threat; and soft grunts made during travel and feeding.

Visual communication through facial expressions and body language is often better understood by researchers. A chimp who opens his mouth and bares his teeth is frightened, but a chimp who opens his mouth while hiding his teeth is looking to intimidate others. Likewise, an aggressive chimp’s hair may stand on end to make him look bigger.

A chimp may hug, pat, or brush the face of another chimp in an effort to reassure them or calm them down. Submissive individuals may bob their head, bow, or kiss another chimp to show their respect. Many different researchers are studying chimpanzee communication and several guides have been developed to translate their actions.

Reproduction and Family
Chimpanzees do not have a mating season, but females generally go into estrus when food is most available. It is obvious when a female chimp goes into estrus as her anogenital region swells up. Although the dominant male will often try to restrict mating, females mate with several males in their group. The chimpanzee gestation period lasts a little under 8 months.

Chimpanzees are completely helpless at birth and are unable to even support their own weight for their first two months. They are carried on their mother’s chest for 5 or 6 months before they are strong enough to ride on their mother’s back. Babies will only begin to show independence after about two years of life. Chimps are fully weaned after about 5 years. Around that same time, the mother will be ready to mate again and have another baby. Females reach sexual maturity between 12 and 14 years while males begin mating at around 15 years old.

When a female starts to reach adulthood, she will usually leave her family. The process is not sudden. They’ll move between their new group and their natal group for several months before settling into their new group. Males, on the other hand, tend to stay in their natal group for life.

Playtime. Photo courtesy of ©Jenny Desmond

Ecological Role
Like many fruit-eaters, chimpanzees play a special role in their environment as seed dispersers. Chimpanzees consume the fruit and defecate the seeds elsewhere in the forest. The seeds are allowed to grow into trees without having to compete with their parent plants for resources. Chimpanzees are capable of eating far larger seeds than most animals in their ecosystem, and many plant species depend on large animals such as chimps to spread their genes and prosper.

Seed dispersal is especially important to West African countries such as Liberia, which has a small agricultural industry requiring many people to depend on natural processes to produce food.

Conservation Status and Threats
The International Union for Conservation of Nature assesses the conservation status of the Western chimpanzee as Critically Endangered (IUCN, 2016), while the common chimpanzee species as a whole carries an Endangered status. The greatest threats to their survival are habitat destruction, poaching, and disease. The current population of Western chimpanzees is estimated to be between 21,000 and 55,000 and has declined by 80% in the last 25 years. The IUCN also believes that these threats will persist for years to come, despite conservation efforts. The Western chimpanzee is listed as one of the world’s 25 most endangered primate species in the IUCN’s 2018-2020 report.

The IUCN identifies the emergence of the palm oil industry in West Africa as a rising threat to the Western chimpanzee. The palm oil industry requires mass deforestation to grow plantations of palm trees and has had devastating effects on wildlife in southeast Asia. The IUCN states that 94% of chimpanzee habitat in Liberia and 84% of chimpanzee habitat in Sierra Leone are vulnerable to destruction for oil-palm plantations. These two countries, as well as Guinea, are seen as the last strongholds for the Western chimpanzee.

In Senegal, where less than a thousand chimps remain, a gold rush has taken hold of the region. Not only does this lead to the destruction of habitats to build mines, it also leads to the construction of towns to support the mines and the rise of prospective gold miners, which in turn leads to further deforestation as well as poaching.

As humans and chimpanzees come into contact more and more, chimps are increasingly threatened by disease. Due to our genetic similarities, there are many diseases that can transmit across our two species. Chimpanzees have not been exposed to these diseases before and so have not developed a proper immune system to combat them. Although some Western chimpanzees died due to an Ebola outbreak in the 1990s, none have yet reportedly died from the most recent outbreak; however, conservationists are still concerned, as the epidemic has already affected other subspecies of chimpanzees and gorillas.

Poaching is also a major threat to Western chimpanzees. The main reason for their poaching is for bushmeat. A 2015 study in Liberia found that 74 chimpanzees had been killed to be sold in food markets. In addition to hunting, the infants of slain chimps are captured and sold as pets, despite the fact that the chimpanzee pet trade is illegal in all chimpanzee-native countries.

Conservation Efforts
In 2003, the IUCN led the creation of a conservation action plan to save the Western chimpanzee. An assessment of the plan in 2008 found that, while the plan had successfully increased funding and education in the region, it did not inspire meaningful policy change. An updated action plan is currently in the works and is expected to be publicly released some time in 2019.

Conservation organizations are encouraging more sustainable logging practices. However, most countries are reluctant to adopt policies that could hinder their developing economies. This problem, as well as other threats related to sustainability such as poaching, may be mitigated through financial incentives from international organizations such as the World Bank.

Improvement of healthcare throughout western Africa would also help conserve the chimpanzee population. Other than the obvious positive effects of better healthcare for humans, it would also decrease the transfer of diseases to chimpanzees. Additionally, scientists have developed an Ebola vaccine for apes, which could not only save apes but perhaps humans as well. However, there are several legal and ethical hurdles that must be passed before such a vaccine can be used in the wild.

Many organizations around the world exist to help chimpanzee conservation. Liberia Chimpanzee Rescue and Protection is a sanctuary and conservation organization based out of Liberia. They currently care for dozens of orphaned chimpanzees and work with the Liberian government to enforce wildlife protection laws. While the laws needed to protect chimpanzees already exist in many of the chimpanzees’ countries, these laws are poorly enforced.

Although the chimpanzee is one of the most well-studied animals on Earth, the amount of new information we are learning about these creatures every year shows we still have much to uncover. Researchers have observed what they call an “Eastern chimpanzee bias,” where the bulk of chimp research has come from the eastern subspecies.

The original thought was that chimpanzee were more-or-less the same throughout the continent, but recent findings have shown that is simply not true. A universal truth in conservation is that the more educated the local population is on an animal, the better that animal is protected. With that said, more research can only benefit a species as intriguing and charismatic as the Western chimpanzee.

ABOUT LIBERIA CHIMPANZEE RESCUE

Liberia Chimpanzee Rescue & Protection is the first and only chimpanzee sanctuary and conservation center in Liberia, rescuing chimpanzees who are victims of the illegal bushmeat and pet trades. LCRP is rescuing chimpanzees today and driving change for tomorrow.

The sanctuary currently cares for over 70 chimpanzees, most of whom are under five years old—and given this remarkable chance at a healthy and happy life, will live up to 60 years in the sanctuary’s care.

All of LCRP’s chimpanzee family members are orphans whose mothers and other family members were killed to be eaten; the young chimps were kept alive to be sold into the local and international pet trade. 

LCRP is a Liberian NGO, collaborating with local and international partners in caring for current chimpanzee residents and the development of long-term strategies to combat the illegal trade of chimpanzees and other protected wildlife.

Learn more about LCRP and support here their great work: Liberia Chimpanzee Rescue

References:

  • https://www.iucnredlist.org/species/15935/102327574
  • http://pin.primate.wisc.edu/factsheets/entry/chimpanzee/taxon
  • https://www.liberiachimpanzeerescue.org/
  • Hey, Jody. “The divergence of chimpanzee species and subspecies as revealed in multipopulation isolation-with-migration analyses.” Molecular biology and evolution vol. 27,4 (2009): 921-33. doi:10.1093/molbev/msp298
  • Thomas, Rosie. “Chimpanzee.” Dynasties, season 1, episode 1, 11 Nov. 2019, www.bbc.co.uk/programmes/p06mvmmr.
  • https://blogs.scientificamerican.com/guest-blog/are-western-chimpanzees-a-new-species-of-pan/
  • https://www.sciencemag.org/news/2015/07/humans-have-more-primitive-hands-chimpanzees
  • http://www.theprimata.com/pan_troglodytes/
  • https://www.newscientist.com/article/2124022-ebola-vaccine-promising-in-chimps-but-may-never-be-used/
  • https://corewalking.com/the-human-foot-and-the-chimp-foot/
  • https://alltheworldsprimates.org/Home.aspx

Written by Eric Starr, May 2019