Energy, Water, and Solute Balance of the Ostrich Struthio camelus

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Abstract
The basal metabolic rate of adult ostrich (0.113 ml O₂ ) is only 58% of the predicted value for a 100-kg nonpasserine bird. The basal metabolic rate of ratite birds appears to be best summarized as BMR (ml O₂ ) = 389 , which is parallel to and has about 60% of the intercept of the relationship for nonpasserine, carinate birds. The resting metabolic rate of ostrich (0.26 ml ; 2.3 x basal metabolism) was similar to the predicted basal rate. The maximal factorial metabolic scope of ostrich is at least 28 x basal. The daily energy turnover rate of ostrich with ad lib. water was 12,700 kJ , which is equivalent to 0.26 ml O₂ . Daily energy intake dropped to 1,360 kJ with water deprivation, but daily energy turnover remained high at about 15,000 kJ . The digestive efficiency of ostrich with ad lib. water for lucerne was 43% of the ingested energy. Ostrich appear to digest a significant fraction of the cellulose in their diet. The daily water turnover rate of ostrich consuming dry lucerne with ad lib. water was 8.3 liters . Drinking was the main source of water gain (7.9 liters ), and urine (2.5 liters ) and fecal water (2.9 liters ) were the main avenues for water loss. The cutaneous evaporation of adult ostriches (0.6 liters ) constituted 40% of the total evaporative water loss. Minimal respiratory water loss was 0.82 liters (1.3 mg/ml O₂ consumed). Ostrich could not maintain body mass when water deprived. The rate of water intake for the ostrich without drinking water was 0.83 liters ; most of this was metabolic water. The rate of water loss was at least 2.21 liters . Plasma sodium concentration increased, but potassium and osmotic concentrations were unchanged after 7 days of water deprivation. Birds with ad lib. water and water deprived had a similar urinary solute loss (osmotic clearance = 1.25 liters ). However, urine flow rate declined with water deprivation from 2.5 to 0.5 liters , and urine osmotic concentration increased from 163 to as high as 800 mOsm (U/P ratio of 2.5), to conserve water. Rapid rehydration by drinking of 12-14 liters of water (17% body mass) in 3 h diluted the body water pool as expected, but the decreases in ionic and osmotic concentrations of the plasma were less than expected. The moderate plasma dilution during rapid rehydration would be insufficient to cause hemolysis of red blood cells. The water economy of the ostrich is similar to that of other large savannah and desert animals, such as antelope and the camel, although the partitioning of water loss differs. The ostrich has a lower evaporative water loss and a higher fecal and urinary loss than mammals of similar mass.