ST. LOUIS – It’s not every day that St. Louis and Chicago agree to a mutually beneficial trade.

This week, the Saint Louis Zoo and Brookfield Zoo in Chicago swapped Jontu and Zachary, two male western lowland gorillas, for conservation purposes.

The western lowland gorilla is the most common of four subspecies of gorilla. However, it’s a critically endangered species, and they are facing complete extinction in the wild.

Jontu, who was born at the Columbus Zoo in 1997, came to the Saint Louis Zoo in 2005 as part of a “bachelor gorilla group” to strengthen western lowland gorilla populations in North American zoos. According to a statement from the Saint Louis Zoo, Jontu will provide stability for a family of gorillas at Brookfield.

Zachary, born at the Brookfield Zoo in 2015, is joining two other male gorillas, Joe and Bakari, at the Saint Louis Zoo’s Jungle of the Apes exhibit. Zachary joins this “bachelor gorilla group” of adult males who live together until they’re ready to become part of a breeding group.

These bachelor groups are commonly found in the wild, the zoo says.

These moves are based on recommendations from the Association of Zoos and Aquariums’ Species Survival Plan.

Both Jontu and Zachary are being kept in private areas at each zoo as they acclimate to their new homes.