Interest in the welfare of zoo animals is strong, both within the professional zoo community and among the general public. Maintaining the highest standards of animal welfare is a key priority for keepers, curators and zoo veterinarians, and zoo animal welfare science has advanced considerably in recent years. In the past, zoo animal welfare centred on the avoidance of negative states, typified by the Five Freedoms, and an absence of poor welfare was thought to indicate good welfare (Farm Animal Welfare Council 1979, Melfi 2009, Mellor 2016). Monitoring methods were often confined to resource-based measures such as assessment of the provision of enrichment, appropriate nutrition or veterinary care. In contrast, today it is recognised that zoos should actively promote positive welfare states and that assessment of both the physical and psychological wellbeing of individuals is critical. The World Association of Zoos and Aquariums (WAZA) animal welfare strategy reflects this and highlights the fact that sound animal welfare principles should be integrated into all activities and intrinsically linked to the conservation mission of modern zoos (Mellor and others 2015). Walraven and Duffy (2016) explain the steps taken by the Taronga Conservation Society to embed this focus on animal welfare into staff culture, develop …