Sexual selection and male-biased size dimorphism in a lineage of lungless salamander (Ampibia: Plethodontidae)

Abstract

Evolutionary biologists have long focused on the patterns and causes of sexual size dimorphism (SSD). While female-biased SSD is common among ectotherms, a few lineages predominately exhibit male-biased SSD. One example is the clade of desmognathans, a monophyletic group of two genera within the Plethodontinae of the lungless salamander family Plethodontidae. Members of these two genera have a unique pattern of SSD: males mature earlier and at smaller sizes than females but reach greater maximum sizes. We used comparative phylogenetic methods to test whether SSD in these salamanders is the result of sexual selection on males. Spatial evolutionary and ecological vicariance analysis indicated a significant divergence in SSD associated with the phylogenetic origin of the desmognathans. Phylogenetic least-squares regression across the two most speciose genera of the subfamily determined a significant relationship between SSD and adult sex ratio. While male desmognathans are not sexually dimorphic in head size, they have a unique head morphology that causes their heads to grow more rapidly as their body size increases as compared with the heads of other salamanders. This pattern of allometric growth combines with a powerful bite force and enlarged premaxillary teeth to create formidable weaponry that probably is more responsive to sexual selection.

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

“…Furthermore, these hypotheses are based on knowing the ancestral state of sexual dimorphism of ( Desmognathus + Phaeognathus ) + Aneides which needs further examination. Camp et al. (2019) suggest that the evolution of sexual dimorphism in Desmognathus was facilitated by sexual selection in males.…”

“…Investigating the dynamics of growth rates at a finer scale and under more controlled conditions would help identify the proximate causes of the dimorphic and derived monomorphic patterns. Furthermore, comparing the growth patterns to species of Desmognathus would be informative as well, as this genus is characterized by large, powerful heads and jaws (though without dimorphism) relative to juveniles ( Camp et al. 2019 ).…”

“…In a review of sexual size dimorphism among families of amphibians, selection on female body size was found to be the driver behind most sexual dimorphism ( De Lisle and Rowe 2013 ). Important exceptions to this general pattern include the plethodontid genera Phaeognathus and Desmognathus , which have male-biased sexual dimorphism in body length ( Bruce 1993 , 2000 ; Bakkegard and Guyer 2004 ; Camp et al. 2019 ).…”

Rapid Ossification Helps Explain Small Body Size in Urspelerpes brucei (Caudata: Plethodontidae)

“…A contributing factor could be that there appears to be little to no intraspecific male combat to further drive body growth in U. brucei . Males appear to lack the jaw musculature or bite scars seen in some larger bodied salamanders that are known to combat one another, such as guarding morph Eurycea (Pierson et al, 2019) or large Desmognathus (Camp et al, 2019).…”