The NY Times reports on Elisabetta Visalberghi and colleages’ work demonstrating that capuchin monkeys pick the right size rock for nutcracking:
The researchers left stones of various sizes and types around and observed as the monkeys chose from among two or three. In almost all cases the monkeys had no trouble picking the most solid, heaviest stone, the researchers report in Current Biology. They seemed to know immediately that weight is absolutely necessary, Dr. Visalberghi said.
Sometimes it must be frustrating that behavioral ecologists have to subject every little step of a proposed behavioral pattern to hypothesis testing. The fact that you have to watch animals for hundreds of hours to test such hypotheses is one reason I study dead bones instead.
But the method leads to progress eventually. In this case, when monkeys learn to crack nuts, they apparently develop a mental search image for rocks of the right size.