Primates and humans share a sense of fair play
Logan knows when he's been handed a raw deal. He and Liam, the monkey next to him, just did the exact same trick and Liam was rewarded with a grape. Logan got a cucumber. Any monkey knows grapes are more prized than that boring old cucumber slice Logan received. So he simply tosses that cucumber slice out of his cage.
For more Science Nation, go to http://www.nsf.gov/news/special_reports/science_nation/index.jsp