After a week of domestic living in Laguna Beach, 50 of North America's smallest mice — each roughly the weight of three pennies — have been ushered back into the wild.
Biologists released the group of Pacific pocket mice into Laguna Coast Wilderness Park on Monday night, a milestone moment in the four-year effort to rescue the mice species from extinction, according to the Zoological Society of San Diego.
The effort began with 22 mice, collected from three different wild populations — all separated, meaning they could not interbreed with each other — to a breeding program at the San Diego Zoo Safari Park.
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In addition to swelling in numbers, the mice at the breeding center underwent training to prepare them for predators. The mice listened to recorded pocket mouse calls while exposed to a snake behind Plexiglas. They also experienced a simulated threat in the form of a taxidermy owl on zip line.
In the week leading up to the release, the mice adjusted to their new home in acclimation cages inside the Laguna Coast park, living in human-made nest chambers and burrows.
"I can't be happier with how they behaved when we pulled off their top cages," Debra Shier, associate director of applied animal ecology for the San Diego Zoo Institute for Conservation Research, said at a Tuesday news conference. "They investigated the area, they sand-bathed, they foraged and they were wary — which is really critical for a prey species."
The mice are expected to improve the park's ecosystem, as their seed-eating and burrow-digging habits will bolster the growth of native plants.
Scientists still plan to keep tabs on the mice, which are marked by microchips, and the San Diego Zoo Institute for Conservation Research will provide them with extra food until September.
September also marks the approximate end of the summerlong breeding season for the mice, meaning some of the newly released animals may be rearing pups in the coming months. Just a few decades ago, scientists believed these pocket mice to be extinct, until rediscovering the species in 1993.
"This little mouse also reminds us that we should celebrate hope," said Michael Long, division chief in listing and recovery of the Pacific Southwest Region Office of the U.S. Fish and Wildlife Service. "The hope that led biologists to continue to search for a diminutive mouse that many thought was gone from the face of the earth forever."