Rock-throwing macaques

25februari2009
Source: nrc.nl
At a research institute in Inuayama (Japan) lives an interesting group of Japanese macaques. The macaques throw rocks, especially when they are excited. This behaviour was compared to behaviour in nine other macaque groups.
Every now and then the monkeys throw a stone, approximately 10 centimetres in size, backwards. They do this standing with three feet on the ground, and they throw up to 15 metres far. It is not common for small monkeys or other animals to throw rocks. Throwing is behaviour that is typically found with great apes, and only humans can do this well. Apart from humans, only chimpanzees can throw overarm, primarily because they can stand on two legs. Hitting a target, however, is restricted to humans, because chimpanzees are unable to aim.

The macaques only throw rocks when they are emotionally excited (e.g. when airplanes are flying low or when there are fights in the group). This is, by the way, not aggressive behaviour, as there is no targeting involved. According to the researchers, it is a kind of non-vocal emotional display, in other words, stirring commotion. This behaviour agrees with theories about the origin of rock throwing in humans. Targeted throwing would have evolved from the ‘pointless’ untargeted throwing that macaques demonstrate.
The rock throwing does not happen very often though: During 450 hours of observation, a rock was thrown only 83 times. However, in nine other (control) groups only one throw was observed during 1500 hours of observation, and this does constitute a big difference.