On 30 January 1999, she left for Japan[2] where she did her doctorate in primatology, under the guidance of Japanese primatologist Tetsurō Matsuzawa, at the Primate Research Institute of the University of Kyoto.

[3] The tokens are used in the same way as humans use money and they may be stones that can crack nuts or leaves that can be used to catch water.

[4] The early recognition followed observation of captive chimps who were given tokens by researchers for good behaviour which they could use to gain food by placing them in a machine.

[4] Sousa carried out field work at the Cantanhez Forests National Park in Guinea-Bissau.

[6][7] Kyoto University's Primate Research group established a fund to pay for a student each year to continue in Sousa's field of study.