Wild burros, feral donkeys descended from the African wild ass, are exceptionally hardy herbivores that have successfully colonized the arid and semi-arid landscapes of the American Southwest. Their ability to thrive in these low-productivity environments stems from a specialized diet and a suite of physiological and behavioral adaptations.
Primary Forage in Arid Environments
Wild burros employ a mixed feeding strategy, acting as both browsers and grazers, which allows them to adapt their diet significantly based on seasonal availability. When grasses are present, they will graze on them, consuming Poaceae grasses found in the desert floor. This flexibility is a distinct advantage in environments where forage availability fluctuates dramatically throughout the year.
Their diet is predominantly composed of tough, high-fiber vegetation, including forbs (flowering herbaceous plants) and woody browse like shrubs and trees. This capability to consume woody plants is due in part to physiological and cranial musculoskeletal adaptations that allow them to utilize different parts of shrubs. For instance, in the Sonoran Desert, burros rely heavily on mesquite (Prosopis spp.) and various grasses.
Burros consume plants that are unpalatable or toxic to other livestock, showcasing their broad dietary niche. Examples include certain woody shrubs and the Creosote bush. During periods of severe drought or when seeking salts, they may strip the bark from trees, such as Acacia, to access moisture and minerals.
Digestive Adaptations for Low-Quality Diet
The burro’s success in processing low-quality desert plants is rooted in its unique digestive system. Like all equids, wild burros are non-ruminant hindgut fermenters, meaning they use a large cecum and colon to break down fibrous material. However, they possess a higher efficiency in digesting poor-quality fiber compared to horses.
A key adaptation is a slower rate of food passage through the digestive tract. This increased mean retention time allows microbes in the hindgut more time to ferment and extract nutrients from highly fibrous, low-protein forage. This mechanism ensures that even the coarsest vegetation provides sufficient energy for survival.
Burros also exhibit a lower resting metabolic rate, estimated to be about 20% less than that of horses, which reduces their overall energy demands. They also have the ability to internally recycle nitrogen, a process not found in horses. This recycling helps them conserve and reuse the limited protein available in the sparse desert diet, enhancing their tolerance for nutritional stress.
Water Requirements and Intake Strategies
Survival in the desert is intrinsically linked to water management, and wild burros display remarkable tolerance for dehydration. They are significantly more thirst-tolerant than horses, a trait that allows them to roam further from permanent water sources. Depending on temperature and forage moisture content, burros can typically go to water once every two to three days.
When they do reach a water source, they are capable of rapid rehydration, able to drink between 20 and 30 liters in a very short time without adverse effects. Physiologically, they conserve water by decreasing their resting metabolic rate during periods of dehydration, minimizing water loss from respiration and heat regulation.
Behaviorally, burros actively contribute to water availability through a practice known as well-digging. Using their hooves, they excavate holes in dry riverbeds or washes to access subsurface water. These burro-dug wells provide a water source for themselves and can be utilized by dozens of other native desert vertebrate species.