San Diego, United States - August 21, 2025 Endangered Pacific pocket mice, once believed to be extinct, are now the focus of groundbreaking conservation work led by the San Diego Zoo Wildlife Alliance. The tiny rodents, native to Southern California, were rediscovered in the mid-1990s. Since then, the zoo has launched a conservation breeding and reintroduction program aimed at saving the species from extinction. While the program has achieved success, the population still faces the danger of losing genetic diversity, which threatens survival and reproduction. A new study published August 21 in the journal Science shows that genetic rescue—mixing mice from different populations—creates healthier, genetically stronger offspring. The findings challenge concerns about outbreeding depression, which has limited the use of this strategy in conservation efforts. “When species are restricted to small, isolated populations, genetic erosion can lead to poor health. Our study examined the trade-offs between genetic erosion and outbreeding depression in Pacific pocket mice, and we find that the benefits of genetic rescue outweigh the risks of keeping these populations in isolation,” said lead author Aryn Wilder, Conservation Genetics researcher at San Diego Zoo Wildlife Alliance. Researchers found that when mice from different populations were mixed, diversity loss seen in wild groups was reversed. Wilder explained, “The genetically healthier population had higher survival and reproductive success. Although different numbers of chromosomes carried by the mice from different populations increase the risk of incompatibilities in the mixed breeding program, the non-mixed mice have even lower fitness, indicating a greater risk of extinction if the populations remain isolated.” The Alliance has brought in 49 mice from the wild, producing over 700 individuals, including 94 births in the past year. In 2024, the organization and its partners released Pacific pocket mice to a second site, where 100 pups were born in the wild. With two-thirds of the world’s species in decline, the study’s implications extend beyond this single species. The research underscores the importance of zoos and managed care facilities in preventing extinction and highlights genetic rescue as a powerful tool for conservation.