Objective methods for assessing the cumulative lifetime experience of non-human animals would be valuable. We develop the hypothesis that biological age is a common currency that integrates the overall quality of an animal's lifetime experience across a range of types of exposure. Ageing is the result of the accumulation of somatic damage, and its rate is determined by the balance between experiences that cause damage and experiences that mitigate damage or promote repair. Negative affective states are associated with somatic damage via both direct causal and indirect pathways. Based on these premises, we predict that individuals that are biologically old for their chronological age will, on average, have experienced worse lives than individuals that are biologically younger, both in terms of their overall health and affective experience. Biological age is, thus, an attractive measure of cumulative experience because it requires no subjective decisions either about how a given exposure impacts an animal, or about how different dimensions of welfare should be weighted in an overall assessment. Biological age can be measured objectively using biomarkers. We argue that two biomarkers, namely leukocyte telomere length and hippocampal volume, are valid biomarkers of cumulative experience in humans, with potential for use in non-human vertebrates. We discuss how these biomarkers could be used to assess cumulative experience in animals, highlighting some of the limitations. We conclude that biomarkers of biological age offer a viable objective solution to the assessment of cumulative experience and their application in an animal welfare context deserves further exploration.

This paper proposes a system that uses intrinsic study data to provide a clear visualisation of the stresses involved during the animal's life history that can be applied to all types of studies, even those not requiring invasive techniques. Thus, it provides an opportunity for researchers to identify and refine key events which impact on the welfare of an animal, and to explain clearly the totality of any necessary harms when justifying the research. Assessment of animal welfare depends on measurement of a number of parameters which will vary according to species, the animal's environment and the scientific procedure, all of which are inter-related. Currently, there are few tools to assess the effects of lifetime events on welfare or even, in some cases, to recognise that they have an impact on the level of suffering. A matrix to assess the combined effects of environment, experimental and contingent events on welfare has been applied, retrospectively, to programmes of work involving macaques (Macaca mulatta and M. fascicularis). Lifetime records, available for animals from their birth in the breeding colony through to experimental use in vaccine efficacy evaluation studies, were analysed as a robust validation test for the assessment matrix and refinement of the way in which information on these events is captured. A meaningful assessment method is required prospectively for project licence applications and retrospectively for licence review or decisions on re-use. The analysis will provide information that would support the application of refinements that would optimally enhance the lives of experimental animals.