Under the programme, San Francisco Zoo works with federal agencies and the Midpeninsula Regional Open Space District; the juveniles are raised at the zoo before being released.
A snake subspecies found nowhere else
The San Francisco garter snake occurs exclusively in San Mateo County and a small part of Santa Cruz County. It is a subspecies of the common garter snake. According to the U.S. Fish and Wildlife Service's 2025 5-Year Review, it was listed as endangered at the federal level on March 11, 1967, and in California in 1971. Almost sixty years later, the wild population is estimated at just a few thousand animals, according to the zoo. The species needs habitats that are increasingly rare and fragmented, and it is also threatened by drought, disease, low genetic diversity, declining prey populations, invasive species, vehicle collisions and illegal poaching.
Since 2004, the Golden Gate National Parks Conservancy and the National Park Service have restored 33 acres of wetland, grassland and coastal scrub habitat in San Mateo County specifically for the snakes, including new ponds and raised paths to protect the animals, according to Midpen. According to the zoo, such habitat work also improves conditions for the threatened California red-legged frog, which the snakes feed on. Karine Tokatlian, a biologist at the Midpeninsula Regional Open Space District, said habitat fragmentation, drought, disease, invasive species, declining prey and poaching had reduced genetic diversity, which could mean fewer healthy young snakes are born per mother and the long-term viability of populations suffers, according to KQED.
How rearing at the zoo works
In a process known as head-starting, juveniles are raised in captivity during the stage of life when they are particularly vulnerable to dangers until they reach a minimum size intended to increase their chances of reaching reproductive age. For the programme, USGS biologists collected pregnant females from a healthy population in a Midpeninsula Regional Open Space District preserve in 2025 and brought them to the zoo; after giving birth, the mothers returned to the wild while the zoo's conservation team raised their offspring. The Midpeninsula Regional Open Space District's official project description from April 2025 already envisaged collecting no more than eight pregnant females per year for two years and returning one-third of their offspring to Cloverdale Ranch, with the rest distributed among other preserves. The zoo followed this pattern during the release in autumn 2026: after genetic testing, about one-third of the 62 snakes returned to their mothers' place of origin, while conservationists distributed the remaining two-thirds among populations that needed additional animals and greater genetic diversity. Laura Patterson of the California Department of Fish and Wildlife said information from statewide monitoring of snake fungal disease led by the agency had helped with site selection, and that the monitoring had been funded in part by a federal State Wildlife Grant. The zoo is not disclosing exactly where the animals were released to protect them from illegal collection, road deaths and disease.
The U.S. Fish and Wildlife Service's 2025 status review paints an incomplete picture: of twelve population complexes classified as presumed to still exist, the species has actually been documented in seven since 2020.
What could determine what happens next
First, releases for reintroductions would need to continue for years, according to a modelling study by Rose and colleagues: it considers at least 15 years necessary to keep the risk of quasi-extinction low. Second, a second round is already under way: more than 130 additional young snakes were born at the zoo in 2026 and are being raised for a release planned for 2027. Planned monitoring is intended to show how much the releases actually improve populations at the sites.