**Background:** The paper addresses the growing need for formalized quality-of-life (QoL) assessments in zoo animals. Historically, such assessments have been used predominantly for geriatric, large, charismatic mammals to aid end-of-life decision-making, including euthanasia timing. However, the concept of QoL is complex, abstract, and lacks a universally agreed definition, even in human medicine. The review argues that QoL should be considered equivalent to animal welfare over time and should be applied to all species and life stages, not just the aged and charismatic. It notes that while some zoos and industry associations have developed tools, these are often ad hoc, lack validation, and are limited in scope. The paper aims to summarize the QoL concept, assessment methodologies, challenges, and future directions for zoo animals.
**Methods:** This is a narrative review that synthesizes literature from human and veterinary medicine, animal welfare science, and zoo management. The author discusses various definitions of QoL, types of assessment tools (generic vs. disease-specific), and methods (e.g., Likert scales, multi-indicator checklists, observer-reported outcomes). The review draws on examples from published studies, including a study on phacoemulsification cataract surgery in 21 penguins where 20 staff members subjectively evaluated QoL using a 4-point Likert scale, and a study by Föllmi et al. (2007) on 70 geriatric zoo mammals using a scoring system for pain, discomfort, and QoL. The paper also references the EAZA Best Practice Guidelines for Elephants and the Animal Welfare Assessment Grid (AWAG) applied to various species. The author emphasizes the importance of validity and reliability, citing Yon et al. (2019) on elephant welfare tool validation.
**Key Results:** The review does not present original data but synthesizes findings from multiple sources. Key points include: (1) QoL assessments in zoos have focused on end-of-life decisions for large mammals, with tools like the one described by Vogelnest and Talbot (2015) used on 90 animals, leading to euthanasia in 4 cases and improved scores in 9 after individualized care plans. (2) Observer-reported outcomes (OROs) are the only feasible method for animals, but they are subject to biases; for example, Föllmi et al. found interobserver differences related to career experience and specialty. (3) The penguin study showed 81% of birds had subjective QoL improvement post-surgery based on averaged scores. (4) The paper highlights that many existing tools lack validation and that there is a need for species-specific guidelines, especially for non-mammalian species. (5) The author recommends using a combination of generic and disease-specific assessments, incorporating both negative and positive welfare indicators, and establishing baseline data for individuals and species.
**Clinical Implications:** The review has several clinical implications for zoo veterinarians and animal managers. First, it underscores the importance of moving beyond ad hoc, end-of-life-only assessments to proactive, longitudinal QoL monitoring for all animals. This can help detect welfare issues early, inform husbandry improvements, and guide treatment decisions. Second, the paper advocates for the use of validated, multi-indicator tools that include behavioural, physical health, physiological, and pain indicators, while acknowledging the inherent subjectivity of OROs. Third, it emphasizes the need for transparency in decision-making, separating QoL assessments from other factors like conservation value or economic costs. Fourth, the review calls for more research to develop and validate tools for understudied taxa (e.g., birds, reptiles) and to establish species-specific benchmarks. Finally, it suggests that integrating QoL assessments into routine practice can improve staff communication, consensus-building, and animal welfare outcomes, ultimately enhancing the social license of zoos.