Parallel Evolution and Ontogeny of Simple Penis among New World Cricetid Rodents

Abstract

Two penile types, simple and complex, seem to distinguish North and South American cricetids; their origins were investigated by study of dissected and stained glans of 101 adult and young specimens. A simple penis with only a proximal baculum was found in Abrothrix longipilis and A. sanborni from Chile. In the North American Peromyscus eremicus , juvenile males displayed three small and transient pieces of cartilaginous tissues exactly where those of the complex penis are located. It is suggested that simplification of the baculum was produced independently in the two New World groups. A. longipilis also showed elongation of the glans, probably inducing the formation of a ventral cleft, os bending, and regression of the distal baculum and lateral sacs; strikingly, its proximal baculum ossified late, when third molars erupted. Since ossification and distal bacular differentiation in muroids usually are induced by testosterone much earlier, and testosterone-dependent glands also have been modified in simple-penis species, such delay might be a mechanism inducing elongation, with simplification a secondary heterochronic effect.