Reduction of Sexual Dimorphism of Pallas’ Spadefoot Toad (Pelobates vespertinus: Anura, Pelobatidae) Metamorphs during the Development of Tadpoles in an Overheated Reservoir

This study was carried out in five local populations of Pelobates vespertinus (Pallas, 1771) in the floodplain of the Medveditsa River (Russia, Saratov oblast, Lysogorskii district) in the years 2009–2018. It was demonstrated that sexual dimorphism in the body length (SDIL) and weight (SDIW) of males and females of Pallas’ spadefoot toad is a widespread peculiarity of P. vespertinus populations, typical not only for individuals that reached sexual maturity, but also for metamorphs. In metamorph populations, sexual dimorphism in the body length is, on average, 3%; in the live weight, 9%. In metamorphs, it is less pronounced than in individuals that reached sexual maturity (by 3 and 4.5 times, respectively). The level of development of sexual dimorphism can undergo significant transformations by the temperature conditions during the development of tadpoles in the spawning reservoir. The threshold value of the average water temperature during the period of development up to the stage of metamorphosis (MDT90), at which differences between males and females in the body length are lost in metamorphs, is 18.2°С. Against the background of global warming over the last decade (2011–2020), there is a significant expansion of the range of this species exposed to the effect of an abnormally high water temperature. Nevertheless, size–weight sexual dimorphism even at the stage of metamorphosis should hardly be excluded from the list of diagnostic traits of the species.