Environmentally determined juvenile growth rates dictate the degree of sexual size dimorphism in the Sardinian brook newt

Sexual size dimorphism (SSD) is widespread among amphibians, and it is attributed to fecundity selection, invoked for female-biased-SSD species, sexual selection, usually invoked for male-biased-SSD species, or other forms of natural selection. Further, SSD interplays with body size variability at population level. By using a male-biased-SSD newt (Euproctus platycephalus) as model species, we investigated body size and SSD variability across the whole species’ range, the island of Sardinia, looking for the main evolutionary force behind SSD. We found geographic variation of body size and SSD: newts from northern populations were generally larger than those from southern ones, with a larger degree of male biased dimorphism in the former. The mean age of newts varied among populations, but it did not follow any geographical pattern, and it did not differ between sexes. Southern populations reached sexual maturity earlier than northern ones, and maturity was positively correlated with temperature. Growth curves show that northern populations achieved male-biased SSD before reaching sexual maturity, whilst in the south males become larger than females after 6 years in age, following a non-asymptotic growth curve. The geographical pattern of the adult body size variation is attributable to delayed maturation of the larger newts in more northerly populations, and earlier sexual maturation due to warmer temperature in southern populations, which leads to smaller body size. SSD varies consistently with body size: it is evident in the north, where sexual bimaturation caused larger males, but it is weak in the south, where warmer temperature caused an earlier maturation of both sexes.