This story fascinates me. Researchers trained monkeys to "pay" for treats using pebbles, and then monitored what happened when they treated one monkey better than another - giving a grape instead of a cucumber, or giving a monkey a treat for free.

" If one monkey got a grape in return for her pebble but the other only a less desired piece of cucumber, the shortchanged monkey would often refuse to hand over the pebble in exchange or might decline to eat the cucumber — both very unusual behaviors.

These refusals were often accompanied by emphatic body language, like dashing the pebble or the cucumber on the floor, Dr. Brosnan said. The expressions of exasperation were twice as common if the monkey offered a cucumber saw her companion being given a grape without even having to hand over a pebble."

I just got this picture in my head of a pissed off monkey hurling the cucumber to the ground, like "Screw you researcher! You know where you can put your cucumber!"