Abello, M. T., Velasco, M., Esteban, F. 1999. A training programme for a male gorilla at Barcelona Zoo. International Zoo News 46, 418-420.

Training program for voluntary ejaculation is described. In May 1998 we began the training program with Xebo. At the time of this writing (March 1999) Xebo understands ten different orders. ... So we hope that soon we will be able to get semen samples in this non-invasive way.