08/28/2026
The U.S. border wall in Arizona has 13 openings the size of a sheet of printer paper cut into 70 miles of continuous steel bollards. Border Patrol calls them cat doors. They measure 8.5 inches wide and 11 inches tall. A trail camera filmed a female mountain lion squeezing through one of them and then waiting on the other side while her three spotted cubs followed through the four-inch gaps between the bollards beside it. The largest cat in the American Southwest crossed the border through a hole the size of a piece of office paper because that hole was the only option the wall gave her.
Eamon Harrity at Sky Island Alliance and researchers from Wildlands Network placed 36 wildlife cameras across 100 miles of the Arizona-Sonora border between August 2022 and July 2024. The stretch they studied included roughly 70 miles of 30-foot steel bollard wall, most of it built during the Trump administration, and 30 miles of older waist-high vehicle barriers and barbed-wire cattle fencing. They recorded 9,240 wildlife events involving 20 species. They counted every crossing attempt. They counted every failure.
At the vehicle barriers, 65 percent of wildlife interactions resulted in a successful crossing. Bears walked over them. Deer jumped them. Turkeys flew past. At the 30-foot bollard wall, 9 percent succeeded. An 86 percent reduction. For bears, deer, and wild turkeys, the wall produced a 100 percent reduction. Not one bear, not one deer, and not one turkey was documented crossing the bollard wall during two years of continuous camera monitoring. The four-inch gaps between the steel bollards stopped everything larger than a bobcat. Bobcats, foxes, hares, raccoons, and skunks could compress their bodies enough to squeeze through four inches of space. Mountain lions could not. Deer could not. Bears could not.
The 13 cat doors changed the math for three species. Mountain lions, javelinas, and coyotes were 16.7 times more likely to cross the border wall when they encountered one of the small openings. The cat doors were not designed for mountain lions. An 8.5-by-11-inch opening was intended for bobcats and smaller animals. Nobody expected a 130-pound mountain lion to compress itself through a hole smaller than a laptop screen. The cameras showed it happening anyway.
Harrity described the openings to the Sierra Club: they are too small to be used by humans, but they do make a big difference for some animals. The engineering logic was simple. A hole small enough that no adult human can fit through it but large enough that a bobcat can is a hole that does not compromise border security while allowing some wildlife connectivity. The mountain lions did not read the engineering specifications. They saw an opening and went through it because the alternative was the four-inch bollard gap that their shoulders could not fit between or the 30-foot wall that they could not climb.
The 13 openings are spread across more than 70 miles of continuous wall. That is roughly one opening every five and a half miles. A mountain lion walking the wall looking for a way through must travel an average of nearly three miles before encountering a cat door, assuming it is walking in the right direction. If it is walking the wrong direction, the next opening may be 10 miles away. The cameras documented animals pacing along the wall, hitting the bollards, turning, pacing farther, and either finding an opening or giving up.
The study was published in Frontiers in Ecology and Evolution in November 2024. Its conclusion was direct: small wildlife passages improved crossing rates for several species but were too scarce and too small for bears, deer, and turkeys to use at all. More frequent and larger openings are urgently needed.
Image is for illustration purposes only
Source: Harrity et al. (2024), Frontiers in Ecology and Evolution / Sky Island Alliance / Wildlands Network / Sierra Club / High Country News / Courthouse News.