BY Peggy Scott
Photography by Ken Bohn
Videography by Maria Bernal-Silva
No one is more aware of the high demand for California real estate than the Pacific pocket mouse. Once found from Los Angeles all the way to the Mexican border at the southern edge of San Diego County, the Pacific pocket mouse found its habitat being taken over by humans, until the rodents seemingly disappeared. They were thought to have gone extinct, until a tiny remnant population was rediscovered in 1993 in Dana Point, California. Further research revealed that four small Pacific pocket mouse populations remained: one on Dana Point and three in military training areas on Marine Corps Base Camp Pendleton. If the species was going to survive, it was going to need a boost in its numbers—and the San Diego Zoo Institute for Conservation Research stepped in to help.
Little But Mighty (Important)
One of 19 subspecies of pocket mice, the Pacific pocket mouse Perognathus longimembris pacificus was federally listed as endangered in 1994. “Pocket” refers not to the animal’s pocket-sized stature, but rather to its fur-lined, external cheek pouches, which are used to temporarily store seeds while foraging. Given their diminutive size—only 5 inches long from nose to tail tip, and weighing less than a quarter—the importance of these mice could be overlooked. And as rodents, some might consider them pests. But as Brown Endowed Associate Director of Applied Animal Ecology Debra Shier, Ph.D., noted, Pacific pocket mice play an important role in their habitat. “They are eaters and dispersers of native seeds, movers of soil, and prey for other animals,” she said. She added that the mice dig burrows, which aerate the soil and increase nutrient cycling, encouraging growth of native plants.
Given the small number of Pacific pocket mice remaining in the wild, a translocation program to populate areas of former habitat was originally deemed too risky. Instead, the Pocket Mouse Conservation Breeding Facility was established at the San Diego Zoo Safari Park in 2012. “It’s a huge responsibility to pull animals out of the wild and into zoo care; 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,” Debra said. Here, our Applied Animal Ecology and Reproductive Physiology and Genetics researchers are studying pocket mouse behavior, ecology, stress, and genetic variation to ensure that we successfully produce the most fit offspring for release, and learn as much as we can about them along the way. So far, successful breeding has almost quadrupled the number of mice at the facility.
Running the Numbers
Once the mice did what comes naturally and reproduced, the next step was to take some of the rescued rodents back to their native habitat. Once permits and approvals were issued for the reintroduction, it was time to get the area “move-in ready” for the mice—and to get the mice ready to move! This is the first reintroduction for the Pacific pocket mouse recovery program, which we are managing in cooperation with the U.S. Fish and Wildlife Service and California Department of Fish and Wildlife, and it comes exactly four years after the mice were brought into zoo care for the conservation breeding program at the Safari Park.
In May of this year, biologists fenced off a one-acre section of coastal sage scrub habitat in Laguna Beach, California. The fencing provided the mice some protection from terrestrial predators such as snakes and coyotes, and also helped keep the mice relatively close together, which makes it more likely they will mate. Once the fence was in place, the rodent-wrangling researchers set live traps and worked overnight to confirm the types of species already living in the area, which were other native heteromyids—or burrowing rodents—including the Dulzura kangaroo rat and the related California pocket mouse, deer mice, and cactus mice.
Then in June 2016, a major milestone was reached—50 Pacific pocket mice were released into the protected 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,” Debra said. Home improvements for the reintroduced Pacific pocket mouse population consisted of 50 acclimation cages, with small-grade mesh above ground and an underground burrow built from biodegradable materials. These mimic the nests that the species would build for itself, with one main chamber and two exit tunnels.
The 50 mice lived in these acclimation accommodations for a week, during which the researchers filled the chambers with seeds from California native plants to help them adjust to living in their new habitat. The food supply was replenished daily, and this grocery delivery service continued even once the temporary cages were removed. The mice were carefully monitored to ensure that the animals were acclimated and would thrive in the new area.
Check in, Check up
In July, the researchers did a post-release assessment on the mice to look at survival, stress, and health; conducting a “roundup” of sorts that involved catching as many of the reintroduced rodents as they could. More than 200 live traps were placed throughout the fenced habitat, containing seeds to entice the mice to enter. The mice were weighed, they were checked for reproductive viability, and a fecal sample was collected to check for stress levels.
Because each mouse had a tiny chip implanted in its tail prior to its release, researchers could identify each one and record the checkup results, creating a unique health record. The results of the roundup have Debra feeling “cautiously optimistic.” “We captured 24 of the 50 mice released in June,” Debra said. “Pacific pocket mice can be trap shy, so we can assume a few more are out there—our current estimate of survival is about 50 percent, which is quite good.”
As Debra pointed out, the first few months of such a project are critical—and so far, so good. “I was thrilled to see that after a month in the wild, our mice looked so healthy. They are maintaining their weights and pelage (fur), which are good indicators of overall health and suggest that they are transitioning well to life in the native habitat.” A successful transition could also result in the project’s ultimate goal: more Pacific pocket mice. “We are seeing both females and males come into reproductive condition,” Debra said. “This is especially exciting because producing offspring would move the population toward stabilization and ultimately, growth.”
Sometimes, the best laid plans of mice and men can go quite smoothly!