EASTERN CHIMPANZEE

Pan troglodytes schweinfurthii

Geographic Distribution and Habitat
The Eastern chimpanzee is found in the rainforests of the Democratic Republic of Congo (DRC), Rwanda, Tanzania, and Uganda. There is also a small population in Burundi and there may still be some chimpanzees in Central African Republic and South Sudan, but this cannot be confirmed.

The famed Australian primatologist Colin Groves proposed that the Eastern chimpanzee subspecies be split into the Eastern chimpanzee and Southeastern chimpanzee (P. t. marungensis). The Southeastern chimpanzee lives everywhere south of DRC’s Ituri province in DRC, Uganda, Rwanda, Burundi, and Tanzania, while the “original” subspecies is found to the north and west of Ituri.

Although there are notable differences between the two groups, most organizations still consider them to be a single subspecies. Due to that and the lack of studies pertaining specifically to the Southeastern chimpanzee, this profile will treat both groups as members of the same subspecies, but their differences will be noted when applicable.

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

Size, Weight, and Lifespan
Eastern chimpanzees are the smallest recognized subspecies of chimpanzees, although the Southeastern variant is even smaller. Eastern chimps also have much less sexual dimorphism than the other subspecies, with both males and females having roughly the same head-to-rump length of 27.5–35.5 inches (70–90 cm). Males are only a bit heavier than females, weighing in at an average of 88.2 pounds (40 kg) compared to the average females weight of 72.8 pounds (33 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
Chimpanzees are robust apes with black hair covering most of their bodies. Their bare skin is exposed on their hands, face, and feet. Chimpanzees have pink skin early in life, which turns black as they mature. Eastern chimpanzees tend to have darker skin than the other subspecies. Once one looks past the hair, it is easy to see that chimpanzees are our closest relatives, having only diverged from humans 4–8 million years ago. Chimpanzee physiology is not all that different from human physiology.

Their arms are significantly longer than ours, with long fingers and short thumbs, adapted for climbing trees. Chimpanzees are too heavy to gracefully climb like the monkeys who inhabit the same areas, but they can climb well enough to forage for the best fruit. Another major difference between humans and chimps is how our legs are built. Human thighs slope inward while chimpanzee thighs slope outward. This allows humans to close their legs together and walk upright, while chimps can only walk upright in short spurts. The chimpanzee foot features an opposable big toe spaced far apart from the other toes allowing them to grasp the branches and maintain balance as they travel through the trees.

The most noticeable differences between the Eastern chimpanzee and the other subspecies are seen in the head. Eastern chimpanzees have a particularly round head and thinner, more subtle brow ridges. Additionally, Eastern chimpanzees have distinctly shorter limbs than other chimpanzees.

Photo courtesy of ©Innocent Ampeire. Used with permission.

What Does It Mean?

Activity Budget:
A way to quantify animal behavior by observing an animal over an extended period and documenting activity. An activity budget demonstrates how much time an animal spends in various activities such as eating, resting, sleeping, and moving.

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

Anogenital:
Relating to the anus and genital region.

Diurnal:
Active during daylight hours.

Estrous:
The recurring reproductive cycle in many female mammals, including estrus, ovulation, and changes in the uterine lining.

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

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

Polygamous:
Having more than one mate.

​Sexual dimorphism:
Distinct difference in size or appearance between the sexes of an animal in addition to difference between the reproductive organs themselves.

Subspecies:
A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.

Visit the Glossary for more definitions

Diet
Chimpanzees, like humans, have a set of teeth that is designed to eat both plants and meat. They mostly eat fruit, which makes up between 60% and 80% of their diet depending on the season. Another 20% of their diet is dedicated to leaves. The rest of the chimpanzees diet is made up of flowers, bark, honey, insects, and other meat.

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. Chimpanzees were long thought to be herbivores, but that changed in the 1960s when Jane Goodall observed the apes launching coordinated hunts of colobus monkeys and other mammals.

Behavior and Lifestyle
Eastern chimpanzees are some of the most well-studied animals on Earth. Much of what we know about chimpanzee behavior today comes from, or was inspired by, the work of Jane Goodall. She discovered a great deal of previously unknown behaviors while studying Eastern chimpanzees, the most notable of which is the creation and use of tools, which she observed in 1960.

Chimpanzee culture is passed down from generation to generation. As a result, behaviors can vary wildly even within the same subspecies. In one study, to showcase these differences, two adolescent Eastern chimpanzees were presented with the challenge of retrieving honey out of small hole. One chimp used a stick as a spoon to scoop out the honey, while the other chimp from another group used a damp leaf as a sponge to soak up the honey and suck on the leaf. This study showed how chimpanzee behaviors are not necessarily innate, but are rather taught and learned over generations.

Fun Facts

The Bili ape, a mysterious ape that is reported to eat lions, howl at the moon, and share many traits with gorillas, is in fact an Eastern chimpanzee. Due to the density of the rainforest and the political instability of the region, it is hard for researchers to study the animal and discern fact from legend.

Daily Life and Group Dynamics 
Chimpanzees are most active during the day. Depending on the season, they will spend somewhere between a third and a half of their time foraging. Activity budgets also show that chimpanzees spend about 25% of their time socializing, 12% traveling, and 20% resting.

Eastern chimpanzees live in large groups of 50 to 100 individuals. However, the main group is not always together and they may break off into smaller groupings for months at a time. Depending on their habitat, a healthy group may control anywhere between 2.5 and 10 square miles of territory. Although chimpanzees are mostly peaceful, they can be fiercely territorial and will engage in violent battles to gain control of a group or land rich in resources.

This became evident in the 1970s in Gombe Stream National Park in Tanzania, where Jane Goodall observed what would be known as the “Gombe Chimpanzee War.” The war, the only known chimpanzee war to have occurred, revealed just how complex politics can be in chimpanzee society. The war began after the death of an alpha male divided the group into a northern and southern faction. Between 1974 and 1978, the northern faction killed off the majority of the southern group and conquered territory. The victorious group, however, weakened by the civil war, quickly lost much of their territory to a separate group.

It is not uncommon for chimpanzee societies to mirror human societies. A chimpanzee leader must play politics just as a human one would. With groups sometimes reaching over a hundred individuals, no chimp can rule without help. Alpha males must solidify their power either by intimidating rivals through brute strength, or by winning support from useful but subordinate allies. A truly successful alpha male may rule for several years.

The Kasakela chimpanzee community (the victors of the Gombe War) is the most well-studied chimpanzee society in the world. The history and lineages of the group have been documented since 1961. In addition to the war, several of their stories have garnered world-wide attention, with one example being the rivalry between two brothers—Freud, a calm and clever ape who ruled from 1993 to 1997, and Frodo, a brutal and powerful chimp who usurped Freud and ruled from 1997 to 2002. Both of their reigns were ended by diseases that did not kill them, but weakened them enough for others to take advantage.

Freud and Frodo’s youngest brother, Ferdinand (born in 1992), would eventually fill the power vacuum left by his two brothers and become the longest-reigning ruler in the group’s history, ruling for 8 years from 2008 to 2016. Ferdinand himself was eventually overtaken by his nephew Fudge, who currently rules the group as of the writing of this profile. To add more drama to the story, one of Fudge’s top allies in overthrowing Ferdinand was Faustino, Ferdinand’s older brother.

All of these chimps have names beginning with “F” because they come from the F-family, a lineage that goes back to a female chimp named Flo. Other families in the Kasakela group include the G-, P-, T-, and S-families.

Communication
Chimpanzees may communicate emotions and ideas through sound, physical displays, contact, or smell. Communication is generally centered around food, threats, and politics. For example, a chimp who is attempting to take over or maintain control of a group will make the loudest noises he can. With his hair standing on end (making himself seem larger), he shouts out barks and hoots while slamming branches and banging his feet on tree trunks.

Thirty-two different vocalizations have been deciphered and researchers are working to learn more. The most famous chimpanzee call is the pant-hoot, which is a series of inhaling and exhaling “hoo-hoo” calls. These calls can be made by chimps finding food, maintaining contact with out-of-sight individuals, and warning others of danger nearby. Other calls may include submissions grunts, screams as a distress call, and soft grunts used to stay in contact during foraging.

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
After a pregnancy of about 8 months, chimpanzees are born totally helpless and unable to support their own weight for their first two months of life. The mother will carry her baby close to her chest for about six months before the baby can ride on her back. Over the next two years, the baby will gradually grow more independent and curious. Chimpanzees are fully weaned after about 5 years. It is not uncommon for a young chimp to be prematurely weaned 3 or 4 years into life if his mother gives birth to another baby in that time. In this case, the older sibling is still cared for, but he may struggle in developing confidence and social skills.

Females are able to mate at some time between 12 and 14 years old while males will likely begin mating at 15 years old. Males tend to stay with their natal group for life while maturing females will leave their group for a new one in what is usually a gradual, months-long process.

Chimpanzees mate year-round. When a female is ready to mate and enters estrus, her anogenital region will swell up. Chimpanzees live in polygamous societies. Any persistent male will find opportunities to father offspring, but males who have higher social rankings tend to be more successful at mating.

Photo credit: Ikiwaner/Creative Commons

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 parents 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.

Conservation Status and Threats
The International Union for Conservation of Nature classifies the Eastern chimpanzee as Endangered (IUCN, 2016) with a declining population due to hunting, pet trade, habitat destruction and fragmentation, and disease. Many smaller chimpanzee populations have become isolated and unable to contribute to the species gene pool. Without help, these groups will be forced into an inbreeding depression and die out.

The threats chimpanzees are facing changes with the country. For example, some rebel groups in Burundi have taken to protected forests to hide from government forces resulting in increased hunting and decreased research. Meanwhile, Rwanda has done well in protecting their chimpanzees from poaching, but must do extra work to protect them from human diseases.

Conservation Efforts
The IUCN developed a 10-year action plan for the years of 2010 to 2020 for the conservation of the Eastern chimpanzee. The plan’s goals include the reform of laws to protect priority habitats, increased enforcement of forest protection laws, encouraging agroforestry and other methods of responsible land management, increased education of local communities, and the development of new strategies to prevent the spread of disease to chimps.

The Jane Goodall Institute (JGI) has numerous projects designed to aid the conservation of chimpanzees and other African animals. One major part of their mission is to help create new economic opportunities for locals that do not require destroying habitats. These projects include funding honey production, coffee farming, and seed money for startup businesses. The JGI also emphasizes education of local schoolchildren and the training of new scientists from surrounding areas.

In documentaries, you will often see researchers and crew members wearing face masks. This is meant to prevent the transfer of disease. Since chimpanzees and humans are so genetically similar, we are also vulnerable to many of the same illnesses.

References:​

  • Plumptre, A.J., et al (2010). Eastern Chimpanzee (Pan troglodytes schweinfurthii):Status Survey and Conservation Action Plan 2010–2020. IUCN/SSC Primate Specialist Group, Gland, Switzerland. 52pp. http://static1.1.sqspcdn.com/static/f/1200343/20456530/1348925838260/ECCAP.pdf?token=Cdf%2Fbmy%2FvtA723Z3lJxCorgZ%2BqU%3D
  • http://pin.primate.wisc.edu/factsheets/entry/chimpanzee/taxon
  • http://www.theprimata.com/pan_troglodytes/
  • https://www.iucnredlist.org/species/15937/102329417
  • https://www.newscientist.com/article/mg22229682-600-only-known-chimp-war-reveals-how-societies-splinter/
  • https://www.janegoodall.org/our-work/our-approach/
  • https://news.janegoodall.org/2016/06/16/fathers-day/
  • https://www.sciencemag.org/news/2015/07/humans-have-more-primitive-hands-chimpanzees
  • https://en.wikipedia.org/wiki/Kasakela_chimpanzee_community

ABOUT NGAMBA ISLAND CHIMPANZEE SANCTUARY

Founded in 1998, Ngamba Island Chimpanzee Sanctuary is home to orphaned and confiscated chimps, rescued from the illegal pet and bushmeat trade. Despite their initial trauma, chimps living at Ngamba have a safe and semi-natural environment in which to recover and eventually thrive over their long lives of up to 60 years. The island offers 95 acres of natural forest where the chimps roam and forage daily. The goals of the sanctuary are to:

  • Provide a safe home for rescued chimpanzees while also caring for the environment and other wildlife on the island
  • Provide a high-quality educational experience for visitors
  • Benefit local communities

Learn more about Ngamba Island Chimpanzee Sanctuary.

OUR GRATITUDE

Our sincere thanks to Innocent Ampeire of Ngamba Island Chimpanzee Sanctuary for providing some of the lovely photos in this profile. Innocent is the photographer and generously permitted our use of the photos. Thank you, Innocent!

Written by Eric Starr, May 2019