As the environments in which livestock are reared become more variable, animal robustness becomes an increasingly valuable attribute. Consequently, there is increasing focus on managing and breeding for it. However, robustness is a difficult phenotype to properly characterise because it is a complex trait composed of multiple components, including dynamic elements such as the rates of response to, and recovery from, environmental perturbations. In this review, the following definition of robustness is used: the ability, in the face of environmental constraints, to carry on doing the various things that the animal needs to do to favour its future ability to reproduce. The different elements of this definition are discussed to provide a clearer understanding of the components of robustness. The implications for quantifying robustness are that there is no single measure of robustness but rather that it is the combination of multiple and interacting component mechanisms whose relative value is context dependent. This context encompasses both the prevailing environment and the prevailing selection pressure. One key issue for measuring robustness is to be clear on the use to which the robustness measurements will employed. If the purpose is to identify biomarkers that may be useful for molecular phenotyping or genotyping, the measurements should focus on the physiological mechanisms underlying robustness. However, if the purpose of measuring robustness is to quantify the extent to which animals can adapt to limiting conditions then the measurements should focus on the life functions, the trade-offs between them and the animal's capacity to increase resource acquisition. The time-related aspect of robustness also has important implications. Single time-point measurements are of limited value because they do not permit measurement of responses to (and recovery from) environmental perturbations. The exception being single measurements of the accumulated consequence of a good (or bad) adaptive capacity, such as productive longevity and lifetime efficiency. In contrast, repeated measurements over time have a high potential for quantification of the animal's ability to cope with environmental challenges. Thus, we should be able to quantify differences in adaptive capacity from the data that are increasingly becoming available with the deployment of automated monitoring technology on farm. The challenge for future management and breeding will be how to combine various proxy measures to obtain reliable estimates of robustness components in large populations. A key aspect for achieving this is to define phenotypes from consideration of their biological properties and not just from available measures.

-In a systemic approach, the breeder can be considered as the decisional component of the livestock system, whereas animals are usually depicted to be part of its biotechnical component. The animal itself is a biological system whose ability to survive, grow, reproduce and cope with the environnement and livestock practices play a major role in the ability of the livestock system to sustain. In such a conceptual representation of the system, the reproductive females draw a peculiar attention since they determine in a great part the productivity and the durability of the system through their abilities to maintain their own production level (milk production, numeric productivity) and to save their reproductive efficiency (repeated pregnancies and lactations) over years. Considering the animal level and its lifespan, it is clear that the abilities to adapt rely on behavioural and physiological regulatory processes. The study of the biological mechanisms involved in the adaptation to undernutrition is particularly interesting since regulatory processes implied in energy metabolism may interfere directly or indirectly with the reproductive function, and consequently, with the durability of the livestock system. A biological significance of such relationships between nutrition and reproduction is given that they allow the female to be informed about the associated risk of entering a productive process facing the uncertainty of the nutritional context. Although the general mechanisms implied in the ability to adapt to the underfeeding constraint are conserved among ruminants, the thresholds (or priorities) may largely differ according to the breed within the same species. Hence, in order to evaluate the ability of the ruminant livestock systems to perpetuate in hard environments (maintaining their production levels) or to assess sustainable objectives (opening bushy landscapes by increasing grazing pressure), animals' inherent adaptive potentialities have to be well known. Résumé -Capacités adaptatives des femelles et durabilité des systèmes d'élevage. Synthèse bibliographique. Dans une représentation systémique du système d'élevage, l'éleveur incarne la composante décisionnelle du système, tandis que l'animal constitue, avec la ressource, sa composante biotechnique. L'animal lui-même peut être considéré comme un système biologique soumis à un environnement contraignant, dont les aptitudes à survivre, croître, se reproduire et s'adapter jouent un rôle fondamental dans la pérennité du système d'élevage. Les femelles reproductrices tiennent une place particulière dans cette représentation car non seulement elles déterminent une large part de la productivité du système par leur propre niveau de production (production laitière, productivité numérique), mais elles en assurent également sa reproductibilité au cours du temps (investissement reproductif). A l'échelle de l'individu et de sa durée de vie, les modalités d'adaptation reposent sur des processus de régulations comportementales et physiologiques. De tels pro...