After years of conservation work that included painstakingly tracking, breeding and releasing almost 100 endangered Pacific pocket mice, researchers from San Diego Zoo Global have discovered that mice reintroduced into Orange County, California’s Laguna Coast Wilderness Park have begun to breed on their own.

The science team made the discovery late last month—just weeks after representatives from San Diego Zoo Global, the U.S. Fish and Wildlife Service and the California Department of Fish and Wildlife released a new group of 25 Pacific pocket mice into a 1.6-acre fenced section of the park. Not only have the released mice that were raised at an off-site breeding facility given birth to offspring, but the second generation of mice—born in the wild habitat—have also begun to produce offspring, without human assistance.

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