Calder is invited to make sculptures for an African habitat designed by Oscar Nitzschke for the Bronx Zoo. Calder conceives of treelike sculptures to be made in steel so they can withstand the abuse of the wild animals. Although the habitat is never realized, Calder creates five
models for the project: Sphere Pierced by Cylinders, Hollow Egg, Four Leaves and Three Petals, Leaves and Tripod, and The Hairpins.
In 1937, Calder completed Devil Fish, his first stabile enlarged from a model. He received two important commissions: Mercury Fountain (1937) and Lobster Trap and Fish Tail (1939). His first retrospective was held in 1938 at the George Walter Vincent Smith Gallery in Springfield, Massachusetts, followed by another in 1943 at the Museum of Modern Art in New York.