The baby has not been named yet, the zoo said in a news release. Zookeepers also aren't sure yet if it is a boy or girl.
The baby currently clings closely to its mother's belly to nurse and nap, the zoo said. In a few weeks, the baby should begin interacting with his or her siblings, male Anderson, 3, and females Izzy, 2, and Luella, 1.
An older sister named Calvin now lives at the Fresno Chaffee Zoo in Fresno, Calif.
The birth has caused a change in the swamp monkeys' family dynamics, the zoo's news release said.
Brie and Bangi's other offspring are curious about the baby, the zoo said.
“At first, Luella seemed upset that she couldn’t get all of her mom’s attention, but she seems to have adjusted to the new baby," zookeeper Erin Fairchild said in the news release.
Zoo visitors won't get to see the baby now because the zoo is closed for the season. But the baby should be jumping and swinging around the swamp monkey exhibit by the time the zoo opens next April for its regular season, the news release said.
Swamp monkeys are native to central African forests, where they feed on fruits, leaves and insects, the zoo said. They typically live in swampy areas and are good swimmers, sometimes diving underwater to avoid predators.