The 33-year-old chimpanzee, Santino, has a habit of sneaking up on visitors and hurling stone projectiles at them. Here, he is slowly moving toward visitors, with two projectiles in his left hand. (This image was taken 31 seconds before the throw.)
To make like he was just wandering about, Santino picks up an apple from a water moat, just 15 seconds before hurling the stones at visitors.
This image shows Santino just 1 second before the throw.
Santino not only hid projectiles behind logs and rocks, he also manufactured hiding spots from hay (projectile shown in lower part under the hay heap). All projectiles were placed near the visitors' area, and helped lull visitors into a false sense of security, allowing him the chance to fling his missiles at crowds before they had time to back away.
Santino, a dominant male at the Furuvik Zoo in Sweden, with a pile of hay.
The X in the left of the picture marks the position of the first hay heap. The arrow on the left points to the protruding rock structure that was used as concealment. The other two arrows point at the two logs that also served as concealing obstacles.