Western chimpanzees: one of the closest living relatives of humans

One Earth’s “Species of the Week” series highlights an iconic species that represents the unique biogeography of each of the 185 bioregions of the Earth.

Among the dense mountain forests of West Africa is the closest living relative of human beings. The western chimpanzee (Pan troglodytes verus), along with bonobos, share more than 98% of their DNA with humans and many of the same activities such as using tools for hunting and plants for medicinal purposes, and even rocks to play sports.

Unlike any other chimp

Once, the western chimpanzee population spanned from southern Senegal east to the Niger River. Today, they are only found in Liberia, Sierra Leone, and Guinea-Bissau, with the largest population in Côte d'Ivoire.

Males have a head-to-rump length of 77–96 centimeters (30–38 in) and a weight of 47 kilograms (103.5 lb). Females are slightly smaller, with a length of about 70–91 centimeters (27.5–36 in) and a weight of 41.6 kilograms (92 lb).

Black hair covers their entire body, hands, and feet, and opposable thumbs allow for complex motor functions. While this species is not that visually different from the central and eastern chimpanzees, western chimpanzees have been isolated for over half a million years and are now genetically and behaviorally very distinct.

Hunting tools

Omnivorous, western chimpanzees have a set of teeth adapted to both grinding plants and tearing meat. Over half of their diet comprises fruit, while the other half is dedicated to hunting prey.

Western chimpanzees use wooden spears for hunting and teach younger chimpanzees how to make and operate them successfully. Another unique quality of their hunting is that male and female western chimpanzees differ in their prey.

Males prey more on other primates, such as green monkeys, patas monkeys, and Guinea baboons. Senegal bushbabies account for 75% of females' prey and are the only western chimpanzees observed to hunt banded mongooses. Both sexes have been known to hunt bushbucks.

Additionally, western chimpanzees feed on termites and ants, using tools to scoop the insects out of their nests. With such an expansive diet, this species serves as a top predator, seed disperser, and pest control in their ecosystem.

Other human-like behaviors

Western chimpanzees have been observed using plants as medicine. When sick, they eat certain types of leaves to settle their stomachs and kill parasitic worms.

The chimpanzees have also been observed collecting fermented alcoholic sap from palm trees in leaves. They seem to enjoy the drink and its intoxicating effects because when an alcohol source is discovered, they often return to it.

They have also been known to throw large rocks into hollow tree stumps or against trees. While it is uncertain why western chimpanzees do this, it has been theorized by their seemingly competitive nature in the act that it is a type of sport.

Furthermore, western chimpanzees have been successfully taught sign language. Washoe, a wild-born chimp who lived with two researchers from the University of Nevada for a time, learned 350 signs and passed them down to her son.

A gender-balanced society

Unlike other chimpanzee species, western chimpanzees live in a more gender-balanced hierarchy than in a patriarchal society. Females often team up with each other in conflicts with males.

Both males and females mate with multiple partners throughout their lifetime. A female gives birth after eight months of gestation to a single infant, which the mother will take care of until it reaches around four years old. However, both sexes will adopt orphan infants.

Protecting this species

Today, western chimpanzees are critically endangered due to the loss of their habitat by deforestation and the expansion of human agriculture. As a result, all chimpanzees are protected by national and international laws, but enforcement remains an ongoing problem.