New ‘Development’ Burrowing into Orange County, California

Successful Captive Breeding of the Endangered Pacific Pocket Mouse Results in Its Relocation into the Species' Historic Range

The population of Orange County, California is growing, but its newest group of residents won’t block any views of the ocean. Fifty endangered Pacific pocket mice, which make their homes underground, are about to be relocated into an area of Laguna Coast Wilderness Park, part of OC Parks.

This is the first relocation for the Pacific pocket mouse recovery program, which is managed by staff at the San Diego Zoo Institute for Conservation Research, in cooperation with the U.S. Fish and Wildlife Service. The relocation comes exactly four years after 30 Pacific pocket mice were brought in from their native habitat (in June 2012) for a captive breeding program at the San Diego Zoo Safari Park. 

“This collaborative effort between federal, state, and regional partners has been instrumental in helping this critically endangered mammal take this important step towards recovery,” said Mendel Stewart, field supervisor for the U.S. Fish and Wildlife Service’s Carlsbad office. “We also appreciate the stewardship of the U.S. Marine Corps and Center for Natural Lands Management for their ongoing management of this species’ habitat and supporting the captive breeding effort.”

Pacific pocket mice are critical to their ecosystem function, because they are seed eaters that disperse the seeds of native plants throughout their habitat. They also dig burrows that hydrate and increase nutrient cycling in the soil that encourages growth of native plants.

The Pacific pocket mouse is the smallest mouse species in North America, with adults typically weighing between 6 and 7 grams (about one-fifth to one-quarter of an ounce). This nocturnal species was thought to be extinct in the 1980s, but it was rediscovered in 1993.

In 2011, together with conservation partners, San Diego Zoo Institute for Conservation Research decided that the species couldn’t be recovered by keeping the Pacific pocket mouse in the wild. In June 2012, 30 adult Pacific pocket mice were taken from the three remaining wild populations along the California coast to participate in a breeding program at an off-exhibit area at the San Diego Zoo Safari Park. The breeding program was designed to increase the overall population, while maintaining genetic diversity in the species. In the wild, the three Pacific pocket mouse habitats are separated by human development, so there was no chance for these populations to interbreed.

Pacific pocket mice have very strict habitat requirements—they live only within 4 miles of the coast. Their historic range is a stretch of coastal land extending from the El Segundo dunes near Los Angeles International Airport to the Mexico border. The area within Laguna Coast Wilderness Parks is part of the native habitat for this species.

“It’s thrilling to be a part of the reintroduction of the Pacific pocket mouse into its historic range, and to know that their return will also bring about growth for the native plant species that live here,” said Lisa Bartlett, chairwoman of the Orange County Board of Supervisors and vice chair of the Foothill/Eastern Transportation Corridor Agencies.

Once permits and approvals were issued for the reintroduction of the Pacific pocket mouse to OC Parks, there were several steps to get the area—and the animals—prepared for the relocation. In May, biologists fenced off a 1-acre area of coastal sage scrub habitat. The fencing provides some protection against terrestrial predators, including coyotes and snakes, and it also serves to dampen dispersal following the release, which will facilitate mating interactions. With the fence in place, biologists set traps and worked overnight to confirm the types of species already living in the area—which were other native heteromyidae, or burrowing rodents, including the kangaroo rat and the California pocket mouse.

To make the site “move-in ready” for the relocated Pacific pocket mouse population, biologists built 50 acclimation cages with small-grade mesh cage above ground and an underground burrow built from biodegradable materials, mimicking the nests that Pacific pocket mice build themselves, with one main chamber and two tunnels for exits. They were built with the tunnels facing downhill, so the nests will not flood during rains.

The 50 Pacific pocket mice will be held in these acclimation cages for one week. Researchers will fill the chambers with seeds from California native plants—similar to the diet they received at their breeding facility—to help them adjust to living in their new habitat. Researchers will replenish the animals’ food supply daily and will continue to provide supplemental feedings in the habitat after the temporary cages above the man-made burrows are removed.

The gestation period for a Pacific pocket mouse is 23 days, and the species can reach sexual maturity in about 41 days. Because of this, pocket mice can reproduce and give birth or father pups within the same breeding season—which runs approximately from March to September.

Some of the 50 Pacific pocket mice that will be reintroduced into Orange County are expected to raise pups in their burrows this summer, and hopes are high that those pups will go on to make burrows and give birth to other pups. Biologists from San Diego Zoo Global will be monitoring the Pacific pocket mice to ensure that the animals are acclimating and thriving in the new area. 

“This is a historic moment for the Pacific pocket mouse—establishing a fourth population in Laguna Coast Wilderness Park—and it is so exciting,” said Debra Shier, associate director of applied animal ecology for the San Diego Zoo Institute for Conservation Research. “It’s a huge responsibility to pull animals out of the wild and into captivity; we prefer to keep them in the wild. But for the Pacific pocket mouse, it got to a point we couldn’t recover the species by keeping them in the wild.”