Santino evidently knows he's going to get upset, so he plans ahead.

The 30-year-old chimpanzee, who has lived in a Swedish zoo most of his life, sometimes gets agitated when zoo visitors begin to gather on the other side of the moat that surrounds his enclosure, where he is the dominant -- and only -- male in a group that includes half a dozen females.

He shows his displeasure by flinging stones or bits of concrete at the human intruders, but finding a suitable weapon on the spur of the moment perhaps isn't so easy. To prepare, Santino often begins his day by roaming the enclosure, finding stones and stacking them in handy piles.

On some days, he's barraged visitors with up to 20 projectiles thrown in rapid succession, always underhand. Several times he has hit spectators standing 30 feet away across the water-filled moat.

The behavior, witnessed dozens of times, has made Santino something of a local celebrity.

It also made him the subject of a scientific paper, published yesterday, documenting one of the more elaborate examples of contingency planning in the animal world.

"Many animals plan. But this is planning for a future psychological state. That is what is so advanced," said Mathias Osvath, director of the primate research station at Lund University and author of the paper in the journal Current Biology.

In order to decrease his agitation, which was fueled in part by high testosterone levels characteristic of dominant males, the animal was castrated last fall.