Latitudinal patterns in a reproductive trait driven by sexual selection

Many organisms with broad distributions show latitudinal variations in morphological phenotypes and life history traits, such as body size and phenology, in relation to environmental changes such as temperature along latitude. Such variations have usually been considered the result of natural selection, but sexual selection may also lead to these latitudinal patterns. Although a recent study has shown the latitudinal pattern in the strength of male‒male competition in medaka fish, such a latitudinal pattern related to sexual selection is rarely known in other organisms. Here, we show the latitudinal pattern of a reproductive trait driven by sexual selection in the Japanese black salamander (Hynobius nigrescens), where snout-vent length (SVL) in males predicts the outcome of male‒male competition over egg sacs. First, we conducted phylogenetic analyses to examine the phylogenetic pattern along latitude. From the constructed phylogenetic tree, this species was split into five lineages that were roughly divided along latitude. We also used field surveys to examine whether the operational sex ratio (OSR: an index of the strength of male‒male competition) varies across lineages with latitude. We found that the OSR was more biased toward males in a lineage distributed at lower latitudes due to its longer breeding period. We measured the SVLs of collected samples to determine if the latitudinal pattern also exists for SVL. Indeed, male SVLs were longer in lineages distributed at lower latitudes, whereas those in females did not differ among lineages. Our common garden experiment also showed that the individuals from a lineage distributed at lower latitudes had longer SVLs even when they grew under the same environmental conditions, suggesting that the latitudinal pattern in SVL is genetically determined. These results suggest that males at lower latitudes have evolved longer SVLs, driven by stronger male‒male competition. Our study provides the first example, to the best of our knowledge, of a latitudinal pattern driven by sexual selection and its evolutionary determinant in detail in the wild.