Researchers Find That Wild Donkeys and Horses Dig Desert Watering Holes Vital For Entire Ecosystem E. Lundgren Research on feral horses and wild donkeys in the American southwest show they dig desert wells with their hooves in the soft sand of riverbeds, thus creating a network of extra fresh water sources for the creatures that are native to the area. This find has thrown a wrench in the prevailing wisdom that feral equids, who were introduced by the Spanish, are pests that should be removed—as the scientist behind the research suggests they could be fulfilling a vital function once performed by now-extinct mammals from the Pleistocene.