Sexual dimorphism of the Pallas spadefoot toad (Pelobates vespertinus: Anura, Pelobatidae) metamorphs is reduced when tadpoles are developed in an overheated water body

Abstract

Our survey was carried out in five local populations of Pelobates vespertinus (Pallas, 1771) in the floodplain of the Medveditsa river (Russia, Saratov region, Lysogorsky district) in 2009–2018. We have found that the sexual dimorphism in body length (SDIL) and weight (SDIW) of males and females of the Pallas spadefoot toad is a widespread feature of populations of this anuran amphibian species, characteristic not only among sexually mature individuals, but also among metamorphs. In metamorph populations, sexual dimorphism averages 3% and 9% in body length and live weight, respectively. It is less pronounced in metamorphs than in sexually matured individuals (by 3 and 4.5 times, respectively). The sexual dimorphism development level may undergo a significant transformation by temperature conditions during the tadpole development period in the spawning lake. The threshold value of the average temperature for the period of development up to the stage of metamorphosis (MDT90), at which any differences between males and females in body length are lost in metamorphs, is 18.2ºС. Against the background of climate warming over the past decade (2011–2020), there has been a significant expansion of the range area of this species, which is exposed to abnormally high water temperatures. Nevertheless, size–weight sexual dimorphism, even at the stage of metamorphosis, should hardly be excluded from the list of diagnostic characters of the species.