Under water restriction a donkey can lose in the region of twenty to thirty per cent of its body mass and remain functional, continuing to eat and work. On reaching water it drinks enormous volumes rapidly, restoring the deficit in minutes rather than over hours. The large hindgut acts as a reservoir that buffers the sudden intake.
The physiological trick is where the water is taken from. Donkeys preferentially draw on gut and tissue water while defending plasma volume, so the blood does not thicken to the point where circulation fails. Horses under comparable stress lose plasma volume sooner, which is why they falter earlier in arid conditions.
This tolerance is routinely overstated into a claim that donkeys do not need much water. They do; they simply survive going without for longer. Working donkeys denied regular water suffer real harm, and the trait describes an emergency reserve rather than a lower requirement. Donkeys also mask pain and illness more than horses do, which compounds the risk of undertreatment.