Audubon’s American Cross Fox, American Chipmunk, and Wildcat are all part of his publication The Viviparous Quadrupeds of North America (1845-1854), which was his last major accomplishment and consisted of 150 watercolors and oil paintings of North American mammals. After completing his Birds of America, Audubon went on his final drawing expedition up the Missouri River to Fort Union in the summer of 1843. The resulting publication was the first major attempt to document all four-footed animals of North America. John James Audubon and his son, John Woodhouse Audubon, each created about half of the animal drawings. His oldest son, Victor Gifford Audubon, illustrated most of the backgrounds. Family friend and naturalist John Bachman contributed the scientific information, wrote the texts for all three volumes, and gave the work its scientific validity. The project faced serious challenges: Audubon was rapidly aging, and the trip did not produce the in-depth research Bachman felt was necessary. John Woodhouse saved the project by traveling to Texas to collect and study southern mammals, and then to Europe to supplement missing information by drawing specimens from collections there.