Sex-reversed males are morphologically similar but have lower testosterone levels compared to sex-concordant males in the agile frog

Nadine Lehofer, Veronika Bókony, Andrea Kásler, Zsanett Mikó, Nikolett Ujhegyi, Emese Balogh, Virginie Canoine, Edina Nemesházi 

Environment-induced sex reversal – the mismatch between genetic and phenotypic sex caused by external conditions during early development – is increasingly documented in ectothermic vertebrates, yet its fitness-related consequences in natural populations remain poorly tested. To address this gap, we compared sex-reversed XX males and sex-concordant XY phenotypic males in agile frogs (Rana dalmatina) sampled at arrival to a breeding pond. We assessed morphometric traits that are relevant to male breeding success in anurans (snout–vent length, body mass, scaled mass index as a measure of body condition, forearm length and width, and nuptial pad size and coloration) as well as water-borne urinary concentrations of testosterone and corticosterone. Sex-reversed males were morphologically indistinguishable from sex-concordant males across all measured traits. Darker nuptial pad coloration was associated with larger body size and condition. However, sex-reversed males had significantly lower urinary testosterone concentrations than sex-concordant males. This difference persisted after controlling for body mass, sampling date, and corticosterone levels, which did not differ between groups. Our findings suggest that sex reversal in agile frogs produces a mixed phenotype, combining typical male morphological appearance with endocrine differences that may contribute to reduced paternity success previously reported in XX males. Such mismatches between various aspects of phenotype as well as with genotype deserve more attention in species susceptible to sex reversal, as ongoing climate change is expected to increase the incidence of sex reversal in natural populations.