A third population of Pacific pocket mice, once thought extinct, now lives at Camp Pendleton

A third reintroduced population of Pacific pocket mice, one of North America's most endangered mammals, has been established at Marine Corps Base Camp Pendleton in Southern California, San Diego Zoo Wildlife Alliance said. The new population lives at North San Mateo, a training area on the base.

The milestone marks 20 years of the Alliance's recovery program. Since it began in 2006, more than 500 pocket mice have been released into new and existing populations across the species' native range.

Rediscovered in 1993 after being written off

The Pacific pocket mouse is the smallest mammal in North America, about the size of a human thumb. It was once thought to be extinct until a small remnant population turned up in Orange County in 1993. The species was then given emergency protection under the Endangered Species Act, and today only three original wild populations survive.

Its habitat is narrow. The mice historically lived within about 2.5 miles of the Pacific Ocean, mostly on sandy soils, and all known populations are threatened by fragmented, degraded habitat and their small size, according to the U.S. Fish and Wildlife Service.

Bred in care to dig, hide and survive

In 2012 the program added conservation breeding, aiming to produce mice that are both genetically diverse and able to behave as wild animals do. The first reintroduced population was established at Laguna Coast Wilderness Park in Orange County in 2016, and a second at Camp Pendleton in 2024.

“Pacific pocket mice are ecosystem engineers, and their impact goes far beyond their small size.” — Debra Shier, Associate Director of Recovery Ecology, San Diego Zoo Wildlife Alliance

Their digging improves how water moves through the soil and how nutrients cycle, and their burrows shelter other species, Shier said. Restoring the mice therefore helps restore part of how coastal scrub works.

Partners include the U.S. Fish and Wildlife Service, the California Department of Fish and Wildlife, the U.S. Geological Survey and the Marine Corps. The Alliance has not said how many mice were released at the new site or how the earlier populations are breeding; that data would show whether the third site can sustain itself. It is one of many species recovery stories that rely on breeding in care.