In winter, if environmental factors are unfavorable, the Pacific pocket mouse may hibernate underground until spring brings better conditions. But if adequate food supplies are available, the mouse will remain active during winter. Currently, habitat destruction due to urban expansion, road construction and agriculture poses the greatest threat to this critically imperiled mouse. Without sufficient habitat, the Pacific pocket mouse is left with nowhere to hide during the hard winters.

Pacific pocket mice were feared extinct for nearly 20 years before the species was “rediscovered” in 1993. It initially received an emergency listing from the U.S. Fish and Wildlife Service, then was fully protected under the Endangered Species Act in 1994. But when the Service listed the Pacific pocket mouse, the agency declined to designate any critical habitat. The agency maintained that determining to do so would not be “prudent” because a critical habitat designation would only further threaten the species by identifying and publishing the location of the sole remaining Pacific pocket mouse population. Such twisted reasoning continues to hold sway; currently, the Pacific pocket mouse still doesn't have its much-needed critical habitat.

The Center is working to secure this habitat. In 1998, when the U.S. Marines persisted with construction activities in known Pacific pocket mouse habitat, the Center gave notice of intent to sue Camp Pendleton unless the Marines first consulted with Fish and Wildlife. And in 2000, the Center, Endangered Habitats League, and the National Resources Defense Council petitioned the Service to map out and protect critical habitat areas for the pocket mouse.