Inspiring systematic inclusion of individual affective states to enhance the quality of wild animal research

This paper was published online in October 2026 by Wildlife Biology. It was authored by Wild Animal Initiative Researcher Janire Castellano Bueno and our former Science Director, Vittoria Elliott.

Abstract

Studies on animals continue to attract criticism, often relating to not accounting for variation, for issues with data quality, reproducibility and generalisability, yet one source of variation remains rarely systematically incorporated: differences in individuals' affective states. In this paper, we suggest that evaluating affect should be considered standard good practice in ecological and behavioural research with wild animals, alongside assessing common variables such as sex, age or dominance rank. While related concepts such as personality and stress physiology capture some aspects of individual variation in wild animals, affective states, discussed here as integrated products of animals' past and current experiences, offer a complementary, welfare-grounded framing. They influence how individuals perceive and respond to experimental treatments, environmental conditions and human activities, and can systematically alter behavioural, physiological and ecological outcomes. We first place affective states within existing discussions about replication, bias and rigour in animal research, and argue that unaccounted differences in affect can generate unexplained variation, apparent outliers, selective exclusion of individuals and difficulties in reproducing results across sites or studies. We show how behavioural, somatic, physiological and cognitive indicators can be used to evaluate welfare and infer affective states in both captive and free-ranging animals, increasingly with minimally invasive or remote methods. By systematically collecting welfare-relevant data throughout a project, affective state can be used as an explanatory or control variable in statistical models, to detect sampling biases, and to interpret behavioural patterns. Finally, we discuss ethical and logistical implications. Integrating welfare assessments has the potential to support the 3Rs by reducing unnecessary repetition, increasing the amount learned from each animal and helping to identify welfare problems earlier. Despite challenges such as limited validated, species-specific indicators and added analytical complexity, routinely accounting for affective states is a feasible step toward more robust, and ethically defensible research with wild animals.

Learn more about integrating welfare into animal research methodology:

Paper: Research sense: Incorporating animals’ sensory capacities in animal care and study design