Dar and Tatu were in the outdoor enclosure grooming each other. All of a sudden I heard Tatu quietly laughing. I tried to see what was so funny but it just looked like Dar was grooming her face. I sat back down and continued reading. The next thing I knew they were racing past me engaged in a high-speed game of chase; chimpanzee smiles on both of their faces. Dar was in the lead but Tatu was close behind and trying to grab his feet. They ran back and forth along the terrace for several minutes laughing, smiling, and signing CHASE to each other.