Chimpanzees are the closest living evolutionary relatives to humans, and are found in the wild mainly in west and central Africa. Their natural intelligence has long fascinated scientists, who recognise similarities to human behaviour in the use of tools and altruistic behaviour within social groups. Observations of human-like behaviour have included mourning and romantic love, danced responses to natural beauty such as a sunset over a lake, 'pretend play' as young chimps cradle rocks or sticks like dolls, and empathy towards other species; chimps have been seen feeding turtles. Although they have no vocal cords, they communicate with one another using gesture, sound, facial expression and body language using the same areas of the brain as that used by humans for speech.
A number of experiments have been undertaken to teach chimpanzees to respond to human speech, and to use human sign language. Some disagreement continues over the results, although memory tests and recognition of shapes and numbers have shown a level of intelligence at least equal to that of the most able humans. Washoe the chimp (1965-2007) was the first non-human to use American Sign Language, and used at least 350 signs spontaneously and repeatedly. She later taught the signs to her adopted son Loulis.