The Phenomenon of Unstable Sexual Dimorphism in Rodent Populations: Does Sexual Dimorphism Increase in Pessimal Environmental Conditions?

In order to study the phenomenon of unstable sexual dimorphism (USD) in syntopic populations of sympatric rodent species (bank vole, pygmy wood mouse), the variability of the size and shape of the mandible of males and females from five localities of the floodplain forests of the Sakmara, Samara, and Ural rivers (Southern Urals) was compared. In age-homogeneous samples of underyearlings of both sexes, sexual dimorphism (SD) was compared by centroid size (SSD) and mandible shape (ShSD) using geometric morphometrics. The favorable habitat conditions were indirectly assessed by the share of species in the catch and their abundance per 100 catches per day and the fatness index (IF, %). SSD (size sexual dimorphism) was often, but not always, manifested in favorable conditions for the species (high abundance and proportion of species and IF) and was not expressed in pessimal conditions (low abundance and share of species and IF), whereas ShSD (shape sexual dimorphism), on the contrary, was expressed to the greatest extent in pessimal conditions. Negative regression relationships were found that are similar for both species, and for the entire sample set, significant negative coefficients of linear correlation were found between the values of the ShSD (according to Mahalanobis distances, D) and the shares of species in catches (r = –0.76), as well as their abundance (r = –0.78) and fatness indices (r = –0.85); this reflects the strengthening of the ShSD in pessimal conditions for development. Since the ShSD of mandibles in rodent species is regularly high in adverse conditions, it can be used for environmental monitoring purposes as an indirect indicator of a rapid morphogenetic response to pessimal conditions and as one of the signs of the onset of local biotic crisis phenomena after exposure to climatic, anthropogenic, and/or biotic factors.