In Mediterranean and Tropical livestock production systems, animals are faced with sometimes drastic variations in the availability of food resources, for example duringmore or less predictable and extended droughts, leading to episodes of thermal, water and nutritional stress. In such conditions, grass and more generally biomass production is limited, either temporarily or over a longer period. In order to survive, ruminants, who are dependent on this resource must adapt either directly (individual physiological adaptation) or indirectly (with adjustments of management practices). By individual physiological adaptation, we are referring to the overall beneficial regulation of the physiological processes implemented by an individual subjected to new conditions and which allow it to respond in a more or less effective manner (dynamic process). Among the range of physiological adaptations, one of the main levers is the ability to adjust feeding behaviour, based on the implementation of mechanisms related to food choice and intake as well as spatial mobility. In conditions of extreme shortage, to compensate for the consequent negative energy balance, another hysiological compensation mechanism on which ruminants rely is the mobilisation and reconstitution of body reserves. In addition to body condition, other traits such as the animals’ reproductive performance are negatively affected by such food and nutrient deficiency events. Underfed females adapt their behaviour by changing the nature and frequency of estrus and mating. The reproductive behaviour of males is indirectly affected via the attractivity of females. Understanding the complex cascade of these physiological mechanisms (either singly or in combination), at both individual and herd levels, is an integral part of efforts to make good use of them in an adaptation strategy for these farming systems at various levels of organisation.