To whom the play signal is directed: a study of headshaking in black-handed spider monkeys (Ateles geoffroyi)
Journal of Comparative Psychology (2011) - Comment
pubmed: 21341911  doi: 10.1037/a0020547  issn: 1939-2087  issn: 0735-7036 

Sergio M. Pellis, Vivien C. Pellis