San Diego Zoo Global Staff Check Health of Pacific Pocket Mice, One Month After Release at Laguna Coast Wilderness Park

One month after 50 endangered Pacific pocket mice were released into Laguna Coast Wilderness Park, reintroducing the species into its historic range, scientists and researchers from San Diego Zoo Global conducted a health check on the animals. Staff set out 237 traps during four different nights and captured Pacific pocket mice to check their weights, overall heath and signs of reproductive activity. The traps were set at dusk and checked at midnight, and they were reset for a second check at 3:30 a.m. All the traps were closed following the second check.

During the trappings, 24 of the 50 individuals were caught. The weigh-ins and overall health checks show that the animals are adapting well to their new habitat. In addition, males were reproductively ready and several females showed physical signs of approaching breeding cycles. 

“It’s exciting that we were able to observe 48 percent of the Pacific pocket mouse population in Laguna Coast Wilderness Park during our first round of trappings,” said Debra Shier, associate director, applied animal ecology division, San Diego Zoo Institute for Conservation Research. “We never expected to capture all of the mice, as some of them are trap-shy; others are trap-happy—and we caught them several times. Overall, we’re really pleased with what we’re seeing.”

Each of the released Pacific pocket mice has a small microchip placed under the skin at the base of its tail, allowing staff to identify the animal. When an animal was removed from the trap, it was moved into a clear bag, so biologists could observe the animal for overall health and any signs of injury. The Pacific pocket mouse was then held by the skin on its neck while the tail was scanned to identify the animal and collect a fecal sample. The fecal samples will be tested for hormone levels, which are used to assess stress. All observations made about the animal were entered into a file about that individual Pacific pocket mouse.

Prior to release, man-made underground nest chambers and burrows built from biodegradable materials were installed for each of the 50 Pacific pocket mice. During supplemental feeding visits in the last month, and when reviewing remote camera videos, researchers noted that the animals are using these burrows, but they are also seeing that more burrows have been dug—another sign that the mice are adapting to the relocation site.

Staff from the San Diego Zoo Institute for Conservation Research visit the 1-acre release site three times a week to provide supplemental food for the 50 Pacific pocket mice—a mix of non-native seeds and locally grown native seeds. To prevent non-native plant seeds from germinating in this sensitive habitat, these seeds have been autoclaved. The non-native seed is the same seed that is fed to mice at the Pacific pocket mouse breeding facility in Escondido, Calif., which is home to more than 100 animals.

Pacific pocket mice are critical to their ecosystem function, because they are seedeaters that disperse the seeds of native plants throughout their habitat. They also dig burrows that hydrate and increase nutrient cycling in the soil, which 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 the same as three pennies). This nocturnal species was thought to be extinct in the 1980s, but it was rediscovered in 1993. This is the first Pacific pocket mouse relocation since a captive breeding program for the species was started in 2012.