| Abstract
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Donkeys survive under conditions inhospitable to horses and cattle. Energy intake and expenditure was studied using 12 donkeys at Cornell (4 male, 2 female, body weight (BW) 200 18 kg) and at the Institute for Crops Research in the Semi-Arid Tropics in southern Niger (6 female, BW 138 10 kg). Feeding trials and total fecal collection were used to determine digestibility of wheat straw, grass hay and millet stover. Chewing speed and eating efficiency were measured by observation. Maximal aerobic capacity (VO2max), cost of standing, walking and working against a draft load were measured via indirect calorimetry. Experiments at Cornell used a motorized treadmill and an open-flow, N2-dilution calorimety system; experiments in Niger used a closed face mask system (Oxylog) on donkeys working outdoors in teams. Jugular cathers provided blood for metabolite analysis. Responses to undernutrition, with and without exercise, were studied by feeding wheat straw or timothy hay for 3 months each in a cross-over design. Animals consumed 1.1, 2.3 or 2.1% of BW daily of straw, hay and stover, respectively, digesting food energy with 45, 61 and 33% efficiency. Nigerien donkeys could consume more fiber than predicted from body size. Donkeys consumed 9 1 g of dry matter per minute, chewing 53 2 times per minute. Donkeys appear ed to process food faster than can cattle and nearly as fast as horses, despite the donkey's smaller mouth. VO2max was 110 5 ml/kg/min at 8.2 m/s on a 10% incline, with a maximal heart rate of 223 2 beats/min. Lactate accumulation at VO2max was only 10 1 mmol/l, with no hyperglycemia. Net cost of walking was 1.5 J/m/kg BW on the treadmill, and 1.3 J/m/kg in Nigerien donkeys out-of-doors. Efficiency of working varied from 21% to 35%. Exercise provoked increased blood cortisol and hematocrit, low lactate accumulation and hypoglycemia. Undernutrition caused a 30% decrease in resting metabolic rate, elevated resting FFA, depressed insulin, blood urea nitrogen and cortisol, and an 11% loss of body weight. Exercise in the underfed state led to higher heart rate, higher energy expenditure, higher blood lactate and transiently lower blood glucose. Donkeys did not have a lower resting metabolic rate or substantially greater digestive efficiency than horses, nor did they walk or work with exceptional efficiency. Daily maintenance energy requirement was estimated at 150 kJ/kg, identical to that of a same-sized horse. Other factors not studied, such as low levels of voluntary activity, grazing selectivity, or an opportunistic feeding strategy may account for donkeys' high survival rate under harsh nutritional conditions.
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