The Baculum in the Chipmunks of Western North America

This 1953 monograph represents the kind of meticulous, narrowly-focused taxonomic work that built the foundation of modern mammalogy. White's study centers on the baculum the os penis bone found in male mammals and its remarkable variation across seventeen species of chipmunks from Western North America. While the subject might seem arcane, the baculum has long fascinated biologists precisely because it evolves rapidly enough to distinguish closely related species yet remains stable enough within populations to serve as a reliable identification tool. White provides detailed measurements, morphological descriptions, and scientific illustrations for each species, arguing that this small bone offers insights into evolutionary relationships that external features like skull shape or pelage cannot. The research sits squarely in the tradition of pre-molecular taxonomy, when scientists relied entirely on physical morphology to parse the tree of life. For readers curious about how scientists actually classified organisms before DNA sequencing, or anyone interested in the fauna of the American West, this is a window into a particular moment in zoological history when careful observation of the smallest anatomical details could reshape how we understand species boundaries.