The chimpanzee (Pan troglodytes) belongs to the Pongidae family of the Primate order. They have a wide distribution, extending across central Africa from Senegal in the west to Tanzania in the east. There are three subspecies of common chimpanzee recognized. They are the western subspecies, P. troglodytes verus, the central species, P. t. troglodytes, and the eastern subspecies, P. t. schweinfurthi. A fourth subspecies from Nigeria and Cameroon may soon be added.
North of the Zaire River, the common chimpanzee has been known of since the 17th century. Captive chimpanzees have been studied since the 1920s, beginning with German psychologist Wolfgang Kohler conducting behavioral research. Most famously, Jane Goodall’s research at Gombe Stream National Park in Tanzania has been going on for nearly 40 years and is the longest study of a wild species ever conducted.
Chimpanzees are well-known for their strength and ability to walk upright on two feet or bipedally for short distances. They are human’s closest living relatives, sharing more than 98% of the same DNA and with similarities in immune functions and blood composition. Human and chimpanzee lineages separated about 6 million years ago. We are also their closest relatives of all of the living primates, except bonobos. This means that they can contract any infectious disease that a human can be infected by and has led to exploitation by biomedical research institutions. Bonobos and chimpanzees are thought to have shared a common ancestor from whom they split approximately 3 million years ago. Chimpanzees diverged from gorillas 7.5 million years ago and orangutans 16 million years ago.
Chimpanzees live in a variety of habitats, from densely forested jungles to savannas that are believed to closely resemble the environment that human ancestors would have had to endure. Chimpanzees are well suited for life in the trees, where their quadrupedal dexterity and opposable thumbs allow them easy movement, and on the ground, where they knuckle-walk like the other African apes. Male chimpanzees are approximately 1.2 m (4 ft) tall and weigh 60 kg (132 lbs). Females are 1.1 m (3.5 ft) tall and weigh 47 kg (103.6 lbs). Chimpanzees live an average of 30 to 40 years in the wild and longer in captivity.
Chimpanzees have stout bodies with relatively long arms. They have a round head with large, projecting ears and small, forward-facing nostrils on a prognathous muzzle. The top of a chimpanzee skull is flat and is without a sagittal crest. Chimpanzees have large teeth when compared with humans, but have the same dental formula: 2123/2123 = 32. Chimpanzee senses are similar to human senses; however, they have more effective olfactory functions.
There are 13 different categories for chimpanzee calls that range from soft grunts to loud pant-hoots and shrieks. Chimpanzee calls are distinguished from bonobos by being a lower pitch.
Chimpanzees exhibit a high degree of sexual dimorphism. Males are larger and stronger, with characteristically large testes and prominent genitalia. Sexually mature females can be recognized by the pink swellings of perineal skin that are present during the estrous cycle. The estrous cycle is when a female is sexually receptive and fertile. This period lasts for 2 to 3 weeks and occurs every 4 to 6 weeks. Captive females reach sexual maturity at 8 to 9 years of age and give birth for the first time at 10 to 11 years of age. Wild females mature and give birth 3 to 4 years later than captive females.
Male chimpanzees engage in some sexual activity at 2 years of age. They begin to engage in more mature courtship rituals around 3 to 4 years old. These courtship rituals consist of displaying an erect penis, swaggering with hair bristling, and branch shaking and leaf stripping. Receptive females respond by approaching the males and presenting their sexual swellings for inspection.
There is no breeding season for chimpanzees, but females are sexually attractive and receptive only during their estrous cycles, which occur throughout the year. At the beginning of the estrous cycle, females will mate up to six times a day with different individuals. Near the end of the estrous cycle, more exclusive partnerships, called consortships, are formed. A consortship, consisting of one receptive female and a male, will often disappear into the forest for several days or even for weeks or months. Sexual activity during this time in the estrous cycle likely results in pregnancy.
The gestation period of a chimpanzee is 8.5 to 9 months long. After birth, the infant is helpless. The female will not be receptive again for 3 to 4 years, when her offspring has been weaned. Unlike bonobos, chimpanzees do not engage in sexual activity during pregnancy or lactation.
Within a few days of birth, infants are able to cling to their mother’s underbelly, and they will ride on her back at 5 to 7 months of age. Young chimpanzees are weaned around 3 years of age, and by 4 years of age are walking separately from but always near their mothers. Female chimpanzees leave their natal group when they reach sexual maturity, but males remain in their birth group and maintain close relationships with their mothers and maternal siblings.
Chimpanzees live in colonies of up to 150 individuals. The range of a community is 10 to 50 sq km, with a core area of 2 to 4 sq km. The core area is inhabited 80% of the time. Colonies may have overlapping ranges, but not core areas. Males will regularly patrol the boundaries of their territory. They will chase and attack intruders.
Foraging for food is not an entire group activity. Chimpanzee males will forage in a group of three to six individuals, while females forage alone or with their offspring. Chimpanzees eat ripe fruit for up to 4 hours a day. They also eat soft pith, tree seeds, galls, resin, bark, and young leaves. They have been observed to eat up to 20 different species of plant in 1 day and over 300 different species in 1 year. They are also known to use some plant species for medicinal purposes.
Chimpanzee diets also consist of up to 5% animal protein, which is taken in by foraging for insects and hunting small mammals, including other primates. Females spend twice as much time searching for insects, and males are more active in hunting small mammals, such as monkeys, pigs, small antelopes, and occasionally birds. Different populations of chimpanzees may eat very different diets, depending on their habitats and the traditions of the group. For instance, the chimpanzees of Gombe eat a high percentage of palm oil nuts, while those at nearby Mahale do not. Chimpanzees do not store any food.
Jane Goodall’s discovery in the early 1960s that chimpanzees actively engaged in and were successful at hunting small mammals, especially primates, such as colobus monkeys and other mammals, shocked the world of primatology. Since then, different hunting strategies have been observed in various communities, but the most frequently employed technique is based on subgroup cooperation. When prey is spotted, the lead hunter will head silently into the forest to where the monkeys are roosting. The lead hunter is often, but not always, the alpha male. The rest of the hunting troop, a subgroup of the entire troop, follows the lead hunter into the forest. Usually, two of the members will sprint to the periphery of the monkey troop, climb to their level, and act as side blockers. Several other blockers run ahead on the ground to get in front of the fleeing monkey troop and climb high into the canopy to prevent the less heavy prey from getting to an unreachable vantage point, effectively closely the trap.
While this may sound very strategic and organized, a successful hunt is often chaotic. The panicking monkeys dart throughout the canopy, with some of the males staying behind to delay the chimpanzees, even attempting to fight them. When a monkey is captured about 40% of the time at Gombe, the entire chimpanzee troop is brought to a frenzy of excitement. The one who possesses the carcass at this point, which is not always the individual who actually captured the prey, will share the meat with his friends and allies, both male and female. This individual is often not the alpha male, and there is a notable change in the social dynamics of the group during the distribution of the spoils of the hunt.
One possible byproduct of predation, or failed attempts at predation, has been infanticide. In seven instances with the Mahale Mountain National Park chimpanzee group, over several years, the kidnapping, killing, and eating of a male infant followed the stress caused by a failed hunt. The mothers of the victims were fairly new transfers to the group. Therefore, the infanticide might be explained as a result of uncertain paternity and the alpha male’s natural desire to get rid of any new infants he suspects are not his own. However, in one case, the mother had been with the group for more than 5 years.
Questions remain as to whether or not the newcomers were seen as threat from the time they joined the group and were intended to be killed. The adult males may have seen the failed hunt as an opportune time to snatch an infant from an excited and distracted mother. Or it is possible that the infants were only viewed as prey in the excitement and disappointment of a failed hunt. Infanticide has finally been accepted as an event that occurs regularly in many species. Our own relationship to chimpanzees and other primates makes it a difficult subject for many to discuss and study without bias. Interestingly, infanticide has not been observed or suspected to occur in bonobo societies.
In chimpanzee troops, adult males at the top of the social hierarchy are a formidable and overpowering force. The alpha male and several subordinate males dominate and control the troop, often by force and aggression. Physical displays and confrontations reestablish their position on an almost continual basis, which can make life for lower-ranking individuals very stressful. They reestablish and reinforce their positions several times a day. For example, when a chimpanzee troop reaches a food source, the alpha male will usually put on a physical display of his strength by crashing around, reaffirming his dominance by threatening the others. Feeding will then proceed under his watchful eyes, in a hierarchical order, with the least-dominant individuals waiting their turn for and often receiving the least amount of nourishment.
The coalition of adult males is usually intolerant of neighboring troops and are much more territorial
than bonobos. Chimpanzees are known to patrol their territorial range and chase or attack any intruders. Jane Goodall observed a war, the “4-year war,” between two troops that had originally been one at Gombe. The original troop systematically searched for and attacked all of the individuals from the splinter group. It is believed that the entire splinter group was killed during the 4 years.
Despite the overwhelming evidence for an aggressive existence, chimpanzees are very affectionate with each other. They are known to frequently embrace their friends when nervous or after a stressful encounter and to reinforce bonds and curry favor from dominant individuals. They will also embrace the individual they were at odds with in order to diffuse the tension of a fight and reconcile. Frans DeWaal has done extensive research on the coalition-building and reconciliation habits of chimpanzees.
Tool Use and Chimpanzee Culture
Chimpanzees are well-known for their ability to make and use tools, in the wild and in captivity. In fact, different tool-making traditions, which some scientists consider primitive cultures, have developed at different locations. These traditions are passed on from one generation to the next by observation, imitation, and even instruction. An example of this process at one location is the use of tree roots and rocks as a hammer and an anvil to crack open palm oil nuts. For this technique to be successful, the chimpanzee must be very selective about the hammer and anvil materials and also must master the cracking technique with strength and aim. They have been observed to carry a good hammer from one feeding site to another.
Some of the tools made and used by chimpanzees are as simple as a termite “fishing stick,” which requires the toolmaker to pick an appropriate length of stick, strip it of leaves, and poke it into the right termite mounds. As simple as this sounds, it is a skill that is learned. Some chimpanzees are better at it than others. The fisher must patiently wait for the termites to grab ahold of the wood before removing the stick with a bounty of protein attached. Chimpanzees make other tools from their natural environment, such as chewing leaves and moss and using the wad as a water sponge.
Another example of the various primitive cultures of chimpanzees at different locations includes the hand-clasp grooming technique. Grooming partners clasp hands overhead while facing each other, as if to hold up the arm for better access to the body. However, not all chimpanzees engage in this grooming practice. The habit seems to have originated with one troop and followed an emigrant into another troop, transmitting the culture to the troop that did not originally practice the hand-clasp grooming technique. The technique is even used by a captive group that seems to have learned it from a newly introduced wild chimpanzee.
Chimpanzee cognitive abilities are evident in the fact that they pass the self-recognition mirror test. For this test, a red dot is painted on an anesthetized individual. When the individual wakes up and is presented with a mirror, it begins to inspect its image and touch the dot on its own face, recognizing that the image in the mirror is its own and not another being. Other primates, such as monkeys, do not have this ability.
One of the most significant demonstrations of chimpanzee intelligence has been well documented through ape language studies. Sue Savage-Rumbaugh and her husband, Duane, work with apes at the Language Research Center at Georgia State University, in Atlanta. The experiments that Savage-Rumbaugh conducts include teaching symbolic representations, called lexigrams, of English words to chimpanzees and bonobos. As a result of this work, the apes have shown that they are able to combine meaningful lexigrams to demonstrate a rudimentary understanding of language. Other scientists and institutions are engaged in similar research using sign language.
Some scientists, such as Steven Pinker of York University in Toronto, who studies language acquisition in children, are not convinced the apes actually understand what they are doing. Pinker believes the language skills of Savage-Rumbaugh’s subject are little more than circus tricks. Fortunately, ape language acquisition research continues with positive results and star pupils in each new study.
Spontaneous communication, especially when not in regard to food or within an immediate experiment, is what most researchers believe is the best evidence of language comprehension by chimpanzees and bonobos. In one instance, one of Savage-Rumbaugh’s chimpanzees, Panshiba, wanted to communicate with her, alone. Panshiba took her aside to describe an event that the chimpanzee had overheard in another cage earlier in the day. An adolescent chimpanzee, Austin, had bitten his mother and caused a commotion. Panshiba appeared to want to gossip about the incident by telling Savage-Rumbaugh what had happened.
The emotional lives of chimpanzees are more difficult to research and document. However, they have been observed getting excited, rushing about, and shrieking when they encounter a waterfall or it begins rain. Researchers call this a “rain dance” and believe the chimpanzees are capable of an emotion comparable to human awe of nature. Unlike their bonobo relatives, chimpanzees are usually afraid of standing water, and they despise rain.
If chimpanzees share more than 98% of the same DNA as humans, sense their habitat with the same functionary capabilities, and even behave in similar ways to humans, it should logically follow that they have nearly equal emotional and intellectual capabilities. However, this would mean that they are more humanlike than many would like to believe or that we are more animal-like. Whatever position is taken, even those who exploit chimpanzees and their habitats cannot deny the fact that their numbers are dwindling due to the encroachment of humans for agriculture and forest products, as well as hunting them for the bushmeat trade and the live-animal market. Chimpanzee populations today equal a mere 5% of the numbers in existence at the turn of the century.
Like humans, chimpanzee infants are slow to develop and have long periods of dependence. Coupled with a fairly low birth rate, which is not able to keep up with death rates that are influenced by external forces, chimpanzees and the other African apes are in dire need of habitat protection and the enforcement of hunting and selling bans. We have learned so much about or closest relatives in the past decade, yet the knowledge we have amassed is negligible compared with what is still available through continued long-term, intensive studies. If we believe that chimpanzees and bonobos are an important model for our own evolution, we must preserve their well-being in order to continue researching their origins, as well as our own. As the natural conditions are destroyed, so to are the natural behaviors from which we wish to learn.
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