The Pacific pocket mouse weighs as much as three pennies. Image: San Diego Zoo Wildlife Alliance

An endangered mouse is slowly returning to the Golden State. Pacific pocket mice ( Perognathus longimembris pacificus ) are about as large as a human thumb and about as heavy as three pennies. Unsurprisingly, they are the smallest mammals in North America and are one of many pocket mouse subspecies. But they are also one of the most endangered .

Pacific pocket mice have lived in and around Los Angeles and San Diego but before 1993, the species was believed to be extinct. When a small population was found in Orange County, the tiny rodent was listed as endangered.

Today, there are only three original populations in the wild, but groups are being reintroduced into the wild.…