Indicators of functional integrity in production animals

An ability to describe the quality of lived experience across the course of an animal's life is of increasing interest in studies on wellbeing in farm animals. Integrity of physical functions is an important aspect of the tripartite model of wellbeing: feeling, functioning and interacting well. This review examines measures of functional integrity as potential indicators of functional (eudaimonic) wellbeing. Copious literature shows that coping with day-to-day fluctuations in the environment influences (1) periodicity of biorhythms, (2) allocation of resources to growth, defence and repair, and (3) rate of biological aging. Here we review opportunities for assessing functional integrity through analysis of (1) the dynamic status of physiological and behavioural variables, (2) the accretion of resources in production tissues (milk, eggs, body mass, offspring) and physical structures (fibre, hoof, bone, tooth, placoid scale, otolith, beak), and (3) biological age. A reduction in periodicity of biorhythms, complexity of behaviours, or uniformity of trajectories of growth and production is often associated with poorer current and future health outcomes and shorter functional longevity. These analytical approaches are grounded in statistical measures of stability in complex dynamic systems. Additionally, the cumulative effect of wear and tear on biological age can be assessed via telomere length, DNA methylation, biomarkers of allostatic load, hair cortisol, and physiological functional capacity. Timeseries of behaviour, physiology and accretion provide a richer record of functional integrity than do cumulative indicators of biological age. Indicators of integrity are needed for each phase of the animal's life course including attainment of developmental competencies, stability of mature performance, and rate of decline with advancing age, to provide a comprehensive record across the whole of the animal's life. The utility of measures may differ between real-time and retrospective assessments and between application to every individual versus sentinels. Further validation is required for interpretating measures of functional integrity as indicators of the functional aspect of wellbeing, together with determining their practicality and reliability before translation to commercial settings is feasible. A single type of indicator of functional wellbeing (e.g. DNA methylation) is unlikely to have utility for all life stages, species, production systems, and environments.