External morphology measurements of several species of the plethodontid genus Aneides

While sexual dimorphism has long received special attention from
biologists, derived monomorphism, the condition in which both males and
females express similar derived features, has been less well
studied.  Historically, the appearance of “male-like” features in
females has been explained by the genetic correlation between the
sexes.  Recent work emphasizes the importance of studying the
independent selective forces on both females and males to understand
sexual dimorphism.  Sexual dimorphism and derived monomorphism in
the genus Aneides are examined in light of predictions of social
selection.  Aneides hardii shows the greatest degree of sexual
dimorphism in snout-vent length and head width, with the other species of
Aneides less sexually dimorphic. This reduced dimorphism, however, is not
a return to an ancestral monomorphic state, but rather exemplifies derived
monomorphism because females express traits that were limited in
expression to males of ancestral species.  Instead of calling
these “male-typical” traits in females, I suggest the term “derived
monomorphic” traits, as these traits are typical in these females and
“derived monomorphic” can apply to both sexes. Increased attention to
studying the patterns and ecological significance of derived monomorphism
will shed light on the underlying selective forces, including sexual
selection, on both females and males.