Cacchione, T., Call, J., & Zingg, R. (2009). Gravity and Solidity in Four Great Ape Species (Gorilla gorilla, Pongo pygmaeus, Pan troglodytes, Pan paniscus): Vertical and Horizontal Variations of the Table Task. Journal of Comparative Psychology, 123(2), 168-180. doi:10.1037/a0013580.