The Evolution of Derived Monomorphism From Sexual Dimorphism: A Case Study on Salamanders

Abstract

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.

An evaluation of sexual dimorphism in head size and shape of Red Salamanders (Pseudotriton ruber)

“…We found preliminary, but inconclusive, evidence for differences in head size and shape that were consistent with our hypothesis that adult males in breeding condition would exhibit enlarged jaw musculature. In particular, our linear model suggests that large P. ruber with cloacal papillae have disproportionately wide heads, similar to patterns described in some Aneides (Staub 2021). Although the results of our analysis of head shape were not statistically significant, they were directionally consistent with our hypothesis, suggesting that specimens with cloacal papillae may have more pointed snouts and wider jaws.…”

“…This musculature leads to gross morphological differences in head shape and size that are quantifiable without dissection. Plethodontine plethodontid salamanders in the genus Aneides have a superficially similar head morphology (Wake 1963), although the genus exhibits a variable degree of sexual dimorphism across species (Staub 2021). Furthermore, their head dimorphism is apparently associated with more general agonistic behavior rather than strictly with mate-guarding (Staub 1993).…”

Dental Dynamics: A Fast New Tool for Quantifying Tooth and Jaw Biomechanics in 3D Slicer

“…For example, Aneides salamanders vary in their degrees of sexual dimorphism (i.e., jaw muscle size and tooth count) and the extent to which they display aggressive behaviors ( Wake 1963 ; Wake et al. 1983 , Staub 1993 , 2021 ). Comparing the tooth function of male and female salamanders may help to better explain the evolution of the novel dentitions found in Aneides salamanders.…”

Sexual differences in morphology and winter diet of the Eurasian magpie (Pica pica) in Hungary

“…Monomorphism is assumed to be the ancestral state, where there is no obvious signature of selection differentiating the sexes. However sexual monomorphism, can also be a derived condition, evolving from sexual dimorphism (Staub, 2020). Results obtained by Owens and Hartley (1998) suggested that size dimorphism is associated with intrasexual competition described by the mating system, meanwhile plumage colour dimorphism is linked to the frequency of extrabond paternity.…”