Research on problem-solving and tool use by primates and birds led to a frequently cited book, Animal Tool Behavior, published in 1980. A second edition, co-authored with Rob Shumaker and Kristina Walkup, was published in May 2011. He published a novel titled Thirteen Gold Monkeys in 2013 that is based on the reintroduction of golden lion tamarins in Brazil, and another, Ape, that is based on research, conservation, cognition and personhood of chimpanzees in Rwanda.