A?o Nuevo State Reserve is the site of the largest mainland breeding colony in the world for the northern elephant seal, and the interpretive program has attracted increasing interest every winter for the past 19 years. People who hope to see the seals during the winter breeding season are urged to get their reservations early. The males battle for mates on the beaches and the females give birth to their pups on the dunes. Most of the adult seals are gone by early March, leaving behind the weaned pups who remain through April. The elephant seals return to A?o Nuevo's beaches during the spring and summer months to molt and can be observed during this time through a permit system. During the breeding season, December through March, the reserve is closed except for official guided walks. Since 1975, such closures and guided walk programs have allowed a natural development of the elephant seal herd. The mainland herd has increased during that time from only 35 males to more than 4,000 animals, including more than 1,500 pups born there last winter.

Guided elephant seal walks take about two-and-a-half hours and cover about three miles, much of it through loose sand. The walks are conducted by docents, state park volunteer naturalists from the surrounding community, who take an intensive 65-hour training course given by California State Parks, prior to the breeding season. During the guided walks, they and Reserve staff will lead visitors among the huge marine mammals and explain both the story of the elephant seals and the other natural and cultural history of the area.