A tiny mouse that was once believed to be extinct is starting to make a comeback along the Southern California coast. A breeding and reintroduction program for the Pacific pocket mouse reached a new milestone this month, after San Diego Zoo Global researchers made a big decision to allow the endangered mice access to areas outside of their carefully managed, fenced-in sanctuary. To do this, the research team cut small holes into the perimeter fence surrounding their current 1.6-acre area at Laguna Coast Wilderness Park in Orange County, California. Their hope is to provide the rodents the opportunity to thrive by moving beyond their current territory, while also allowing other animals vital to the ecosystem to gain access to the fenced land. This major step comes less than two months after representatives from San Diego Zoo Global, the U.S. Fish and Wildlife Service and the California Department of Fish and Wildlife discovered that the second generation of Pacific pocket mice—born in the wild habitat—have begun to produce offspring, without human assistance. Researchers said they are thrilled with the continued success of this program—and with the increased breeding. Allowing the mice to leave their original area in the park will encourage them to continue growing their population. “This is a huge milestone for the project,” said Shauna King, research coordinator for San Diego Zoo Global. “The fact that they are reproducing in the wild, and do so well … that makes us very excited.” The Pacific pocket mouse breeding program is managed by San Diego Zoo Institute for Conservation Research staff, working closely with the U.S. Fish and Wildlife Service, the California Department of Fish and Wildlife, and OC Parks, for the Laguna Coast Wilderness Park population. Starting in 2012, 34 adult Pacific pocket mice were taken from the three remaining wild populations to participate in this breeding program at an off-exhibit area at the San Diego Zoo Safari Park. To prepare the mice for release at the Laguna Coast Wilderness Park, the breeding facility utilized air conditioning and humidifiers to mimic the coastal temperatures and humidity levels these mice would experience. The facility was also equipped with large skylights to make sure these nocturnal animals remained attuned to the rising and setting of the sun, which cues their activities. Scientists trained the mice to spot potential threats and recognize food that they would likely encounter in their new, wild surroundings. The Pacific pocket mouse is the smallest mouse species in North America, with adults typically weighing between 6 and 7 grams—about the same as three pennies. The subspecies, thought to be extinct in the 1980s, was rediscovered in 1993. It is currently listed as Endangered under the U.S. Endangered Species Act. Scientists consider these mice vital to their ecosystem, due to their function as seed dispersers for native plants throughout their habitat. They also dig burrows that hydrate and increase nutrient cycling in the soil, which encourages the growth of native plants.