O'Connell, M. 2017. To Be a Machine. London: Granta.

O'Toole, A. J., et al. 1998. The perception of face gender: The role of stimulus structure in recognition and classification. Memory and Cognition 26:146–60.

O'Toole, A. J., J. Peterson, and K. A. Deffenbacher. 1996. An «other-race effect» for classifying faces by gender. Perception 25:669–76.

Oakley, K. 1950. Man the Tool Maker. London: Trustees of the British Museum.

Orbach, D., and P. Brennan. 2019. Functional morphology of the dolphin clitoris. Presented at Experimental Biology Conference, Orlando, FL.

Ortiz, A. 2020. Diego, the tortoise whose high sex drive helped save his species, retires. New York Times, January 12, 2020.

Palagi, E., and E. Demuru. 2017. Pan paniscus or Pan ludens? Bonobos, playful attitude and social tolerance. In Bonobos: Unique in Mind and Behavior, ed. B. Hare and S. Yamamoto, pp. 65–77. Oxford: Oxford University Press.

Palagi, E., et al. 2020. Mirror replication of sexual facial expressions increases the success of sexual contacts in bonobos. Scientific Reports 10:18979.

Palagi, E., T. Paoli, and S. Borgognini. 2004. Reconciliation and consolation in captive bonobos (Pan paniscus). American Journal of Primatology 62:15–30.

Paresky, P. B. 2019. What's the problem with «traditional masculinity»? The frenzy about the APA guidelines has died down. What have we learned? Psychology Today, March 10, 2019.