Efforts to restore the red-legged frog to Southern California, where it had all but disappeared, seemed doomed when the COVID-19 pandemic struck and restrictions were put in place at the U.S.-Mexico border. But scientists were able to airlift coolers of frogs’ eggs from a tiny population on a remote ranch in Mexico and race them across the border to plant them in American ponds. Biologists have been using artificial intelligence to confirm that the batch not only hatched but went on to breed in a remarkable experiment to restore an ecosystem spanning two countries. The red-legged frog is the latest species to see success from binational cooperation along the near-2,000-mile border. The efforts come as the Trump administration builds more walls along the border, raising concerns about the impact on wildlife. At 2 to 5 inches long (5 to 13 centimeters), red-legged frogs are the largest native frogs in the West and once were found in abundance up and down the California coast and into Baja California in Mexico. The species is widely believed to be the star of Mark Twain’s 1865 short story, “The Celebrated Jumping Frog of Calaveras County,” and their crimson hind legs were eaten during the Gold Rush. But as the red-legged frog declined in numbers, the bullfrog — with its even bigger hind legs — was introduced to menus during California’s booming growth in the late 19th and early 20th centuries. The red-legged frog population was decimated by the insatiable appetite of the bullfrogs and the disease the non-native species brought in, but also because it lost much of its habitat to drought and human development in the shape of homes, dams and more. Brad Hollingsworth, a herpetologist or expert in reptiles and amphibians at the San Diego Natural History Museum, couldn’t estimate the number of red-legged frogs that remain, but said they have disappeared from 95% of their historical range in Southern California. In 2006, Hollingsworth and others visited Baja where they had heard of a small population of red-legged frogs. Anny Peralta, then a student of Hollingsworth at San Diego State University, joined them. They found about 20 frogs, and Peralta was inspired to dedicate her life to their recovery. Peralta and her husband established the nonprofit Fauna del Noroeste in Ensenada, Mexico, which aims to promote the proper management of natural resources. In 2018, they started building ponds in Mexico to boost the frog population that would later provide eggs to repopulate the species across the border. But just as they were preparing to relocate the egg masses, the COVID-19 pandemic hit. Peralta and the U.S. scientists scrambled to secure permits for the unusual cargo and a pilot to fly the two coolers of eggs closer to the border. The rest of their journey north was by road, after the eggs passed a U.S. border guard inspection. Over the past five years, Hollingsworth and his team have searched for sounds to prove their efforts to repopulate ponds in Southern California worked. On Jan. 30, he heard the quiet, distinct grunting of the red-legged frog’s breeding call in an audio flagged by AI. Based off the latest count in August, more than 100 adult frogs are believed to be in Southern California ponds.