1. Lester E. Fisher Center for the Study and Conservation of Apes, Lincoln Park Zoo, Chicago, IL 60614, USA
2. Department of Psychology, Language Research Center, Neuroscience Institute and Center for Behavioral Neuroscience, Georgia State University, Atlanta, GA 30302-5010, USA
3. Center for Learning and Memory, University of Texas at Austin, 100 East 24th Street, Austin, TX 78712, USA