The three male cubs, born March 6, were found to weigh from 4.2 pounds to 4.8 pounds when examined by the veterinarians Saturday, the San Francisco Chronicle said. Sumatran cubs have a 30 percent to 40 percent mortality rate.

Leanne, the cubs' 230-pound, first-time mother, is doing an "excellent" job at taking care of them, Jacqueline Jencek, the zoo's chief veterinarian, told the newspaper.

Zoo staff had left the new family alone for the first several days after the births and at first only confirmed one cub was born. The other two were discovered when Leanne got out of her nest box for a drink of water, the Chronicle reported.

Leanne and the cubs' father, George, are part of the zoo's breeding program.

The zoo was the site of a tragic attack Christmas Day when a Siberian tiger got out of its enclosure and attacked three young men, one fatally. That tiger, Tatiana, was shot and killed by police.