Slate published this piece recently about a selfie-taking Celebes crested macaque and the copyright law consequences of the photograph. In the article, several people weigh in on whether the photograph is part of the public domain and free to use or not. I think the analysis is spot on, but it only briefly touches on an important aspect of copyright law - how tenuous can authorship get before copyright protection is no longer granted? It's a tough question with no bright line rule.
Copyright protects "original works of authorship", which requires a minimum amount of original creative expression. For the selfie, the photographer will have a difficult time showing that the photograph was part of his creative expression since he did not intend for a monkey to steal his camera. The bar for minimum creative expression is relatively low, however, and he would have had an argument that authorship exists if he had intentionally left the camera out to achieve such photographs.