Stimuli of thirst in donkeys (Equus asinus)

A study of the stimuli of thirst was conducted on six feral donkeys. Donkeys were found to be stimulated to drink by overnight water deprivation, by the diuretic furosemide, and by hypertonic saline infusion, all in the absence of heat stress or work. Donkeys compensate accurately for the fluid deficit caused by overnight water deprivation. After 19 hr without water, they drank 8.8 +/- 2.4 (mean +/- SE) liters within 60 min. Their undeprived overnight intake was 8.4 +/- 1.5 liters. However, latency was longer and water intake was less than that of ponies with the same changes in blood parameters, suggesting that donkeys have a higher thirst threshold than ponies. Further, plasma volume fell less in donkeys, but osmotic changes were similar to those reported in ponies exposed to the same deprivation. Donkeys infused with 250 ml of 15% NaCl drank 0.7 +/- 0.6 liters of water within 45 min, and osmolality increased from 287 to 297 mosmol/kg water; they drank no water in the same time period when infused with 250 ml 0.9% NaCl (p less than 0.05). Donkeys injected IV with 2 mg/kg furosemide drank 3.8 +/- 1.1 liters within 3 hr. Plasma protein increased from 6.9 to 7.8 g/dl. When injected with 0.9% NaCl they drank 1.0 +/- 0.5 liters (p less than 0.05). In sum, the positive thirst responses of these donkeys to cellular and extracellular dehydration were similar to those earlier demonstrated in ponies, but the results suggest a less sensitive response, albeit combined with a better internal defense of blood volume.