Dehydration and rehydration in donkeys: the role of the hind gut as a water reservoir.

Body fluid distribution was measured in three donkeys, fully hydrated, following dehydration and after being rapidly rehydrated. In twenty other donkeys that were slaughtered to supply food for predators in a safari zoo, the water content in the different compartments of the gastrointestinal tract (GIT) was assessed. Prior to being slaughtered, four of the animals were fully hydrated, four dehydrated and 12 dehydrated, rehydrated and then slaughtered and examined, four at 1, for at 3 and four at 5 hours following drinking. When the body mass of dehydrated donkeys dropped to 75.4 +/- 2.4% of their initial value, total body water of the animals (HTO space), extracellular volume (SCN space) and plasma volume (EB space) were reduced to 76.6 +/- 5.3%, 80.9 +/- 10.6% and 73.2 +/- 8.3% of their initial values, respectively. The amount of water retained in the GIT of the fully hydrated donkeys amounted to 15.9 +/- 3.4 1, 19% of total body water. In the dehydrated donkeys it was only 7.4 +/- 1.3 1. The calculated total intracellular volume in the dehydrated donkeys was only 14% lower than in the fully hydrated animals. When drinking was allowed, dehydrated donkeys consumed 17.6 +/- 2.4 1 of water. The increase in the water content in the gut of newly rehydrated donkeys matched this volume, 80% of which was retained in the hind gut. During the five hours after drinking, only slow and moderate changes in the volume of the hind gut were recorded. Changes in plasma volume were also suppressed. It is suggested that the hind gut of the donkey, similar to the rumen of goats and sheep, plays a role as a water reservoir that helps maintain the osmotic stability of the body.