Validation of hippocampal biomarkers of cumulative affective experience

•Recent knowledge on hippocampal structural plasticity is reviewed.•This knowledge is harnessed to develop biomarkers of cumulative experience.•Hippocampal plasticity is shown to have construct, content and criterion validity in mammals.•The biomarkers require further validation to be used in birds and fish.•We discuss some practical considerations to implement the biomarkers. Progress in improving the welfare of captive animals has been hindered by a lack of objective indicators to assess the quality of lifetime experience, often called cumulative affective experience. Recent developments in stress biology and psychiatry have shed new light on the role of the mammalian hippocampus in affective processes. Here we review these findings and argue that structural hippocampal biomarkers demonstrate criterion, construct and content validity as indicators of cumulative affective experience in mammals. We also briefly review emerging findings in birds and fish, which have promising implications for applying the hippocampal approach to these taxa, but require further validation. We hope that this review will motivate welfare researchers and neuroscientists to explore the potential of hippocampal biomarkers of cumulative affective experience.