When winter streams in the Sonoran Desert dry up, equids like feral horses and wild donkeys start digging wells up to six feet deep.
"They will be sniffing the ground and then they will suddenly start digging and within just, you know, half an hour, they will be a meter or so deep, drinking water," said lead author Erick Lundgren, a postdoctoral fellow at Aarhus University in Denmark.
New researchin the journal Science shows the broad ecological impacts of those watering holes.
At four groundwater-fed streams near the Big Sandy River and Santa Maria River between Wikieup and Wickenburg, scientists mapped available water and set up camera traps near equid wells.
In the case of one fully dried-up stream, the wells provided 100% of available surface water. Even at sites that retained some water near their headwater springs, that proportion averaged 74%.
Around 60 animal species benefited from the wells, using them either as solitary water sources or as alternatives to pools frequented by predators. Lundgren and his colleagues encountered several while trying to do their jobs.
"There would be rock squirrels and yellow warblers and hooded orioles and ravens angry at us and trying to take drinks while we were measuring. They'd be like right at our feet chipping at us. It was amazing," he said.
Lundgren, who is based in the western U.S., says ecologically essential riparian plants, which are adapted to seasonal flooding that no longer follows old patterns, sometimes use the wells as nurseries.
"This specific disturbance may mimic the types of disturbance that were once ubiquitous before we dammed so many rivers."
The wells provide friendlier environs for such seedlings, which can struggle to compete with herbaceous cover found near riverbanks.
Lundgren says arid regions cover about one-third of the world and are expanding, but scientists do not yet have a firm grasp on how equids affect their surroundings.
"One thing that was, I think, important about our study on this well digging behavior, is it fills in a little piece of the picture of how big animals influence these dry land ecosystems."
Well-digging has been reported among some zebra and a Mongollian wild ass called a khulan, and is known to occur among elephants.
