These animals are mainly nocturnal, particularly in summers. In the day, they dig into the sand in shady locations and rest in these depressions, which also protect them from sandstorms. Addax herds contain both males and females, and have from five to 20 members. They will generally stay in one place and only wander widely in search of food. The addax have a strong social structure, probably based on age, and herds are led by the oldest female. Herds are more likely to be found along the northern edge of the tropical rain system during the summer and move north as winter falls. They are able to track rainfall and will head for these areas where vegetation is more plentiful. Males are territorial, and guard females, while the females establish their own dominance hierarchies. Due to its slow movements, the addax is an easy target for predators such as lions, humans, African hunting dogs, cheetahs and leopards. Caracals, hyenas and servals attack calves. The addax are normally not aggressive, though individuals may charge if they are disturbed. The addax are amply suited to live in the deep desert under extreme conditions. They can survive without free water almost indefinitely, because they get moisture from their food and dew that condenses on plants. Scientists believe the addax has a special lining in its stomach that stores water in pouches to use in times of dehydration. They also produce highly concentrated urine to conserve water. Pale colour of the coat reflects radiant heat, and the length and density of the coat helps in thermoregulation. In the day the addax huddle together in shaded areas, and in cool nights rest in sand hollows. These practices help in dissipation of body heat and saving water by cooling the body through evaporation. In a study, eight addax antelopes on a diet of grass hay ("Chloris gayana") were studied to determine the retention time of food from the digestive tract. It was found that food retention time was long, taken as an adaptation to a diet including a high proportion of slow fermenting grasses; while the long fluid retention time could be interpreted to be due to water-saving mechanisms with low water turnover and a roomy rumen. The addax live in desert terrain where they eat grasses and leaves of what shrubs, leguminous herbs and bushes are available. Primarily grazers, their staple foods include "Aristida", "Panicum", and "Stipagrostis", and they will only consume browse, such as leaves of "Acacia" trees in the absence of these grasses.