This study sought to quantify the rate of energy expenditure ( _ VO 2 ), the total cost of transport (COT tot ) and the net cost of transport (COT net ) in camels Camelus dromedaries and donkeys Equus asinus during level locomotion. _ VO 2 of camels and domestic donkeys were measured at exercise speeds between 0 and 4.17 m s À1 . Resting _ VO 2 for camels was significantly (Po0.05) lower than predicted, while donkeys exhibited resting values similar to mammals of the same body mass. In both camels and donkeys _ VO 2 increased in a nearly linear fashion over the range of exercise speeds. The minimum COT tot of camels in the walking and pacing gaits were not significantly different (P= 0.27). Similarly, donkeys exhibited no significant difference (P = 0.09) in the minimum COT tot while walking and trotting. In both camels and donkeys, the minimum COT tot was significantly (Po0.05) lower than the predicted COT tot for mammals of the same body mass. The COT net in both camels and donkeys was determined to be gait dependent and significantly (Po0.05) lower than the predicted minimum COT net values for walking and running. The low COT seen in camels and donkeys results in energy and water savings. IntoductionThe one-humped camel Camelus dromedarius and domesticated donkey Equus asinus are large desert-adapted ungulates of great economic importance to nomadic inhabitants of hot arid regions in Africa and the Middle East. While camels have proven to be remarkably suited for transporting loads long distances, slow-footed donkeys have demonstrated an outstanding capacity to carry loads in mountainous terrain. For desert-adapted species, selective pressures should favour locomotory adaptations that lower energy expenditure and reduce water loss. Therefore, we thought it would be of interest to determine the effect that locomotion has on energy expenditure ( _ VO 2 ) in exercising camels and donkeys.Working camels and donkeys are often required to move at a range of slow to moderate speeds not of their choosing. However, when migrating, wild African ungulates choose to move at a narrow range of speeds near the mid-range within each gait (Pennycuick, 1975). At this preferred range of speeds, the energetic cost of transport (COT), the energy (J) required to move a unit of body mass (kg) a given distance (m), is minimized in some species (Hoyt & Taylor, 1981;Alexander, 1989;Langman et al., 1995;Griffin et al., 2004). The COT can be calculated as the total cost of transport (COT tot ) or the net cost of transport (COT net ) depending on the level of analyses. The COT tot reports how much total energy is expended for locomotion, while the COT net provides an estimate of the energy required by active muscles during locomotion above that required for standing quietly. We sought to determine whether economy in energy expenditure and muscle performance existed in large desert-adapted ungulates while exercising at a wide range of speeds.

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