Reduced sexual size dimorphism in a pipefish population where males do not prefer larger females

Within a species’ distribution, populations are often exposed to diverse environments and may thus experience different sources of both natural and sexual selection. These differences are likely to impact the balance between costs and benefits to individuals seeking reproduction, thus entailing evolutionary repercussions. Here, we look into an unusual population (Baltic Sea) of the broadnosed pipefish, Syngnathus typhle, where males do not seem to select females based on size and hypothesise that this pattern may derive from a reduction of direct benefits to the male. We further hypothesise that if larger females do not persistently secure a higher reproductive success, either through pre- or post-copulatory sexual selection, a decrease in sexual size dimorphism in the Baltic population should be apparent, especially when contrasted with a well-studied population, inhabiting similar latitudes (Swedish west coast), where males prefer larger females.

We found that, in the Baltic popula...