Hunger vs thirst in the burro (Equus asinus)

Abstract The burro ( Equus asinus ) can withstand deprivation of both food and water for up to 48 hours even in summer heat. This ability depends in part on his capacious cecum and colon where food is stored and is digested by bacterial action for hours after his last meal. In eight such periods of deprivation maximum temperatures ranged from 35 to 45°C and estimated increases in osmotic pressure in two burros ranged up to 17%. Given free choice of hay and water at the end of deprivation they chose hay always when estimated increase in osmotic pressure was less than 10%. In one case the second 24-hour period included an eight-hour walk. In this case the estimated increase in osmotic pressure was 17% in both and both chose water first. Their ability to eat dry hay after long periods of dehydration is essential to their survival in hot deserts where they can graze for 24 hours before returning to a water source.