Abstract

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 hypothesize that this pattern may derive from a reduction in direct benefits to the male. We further hypothesize that if larger females do not persistently secure a higher reproductive success, either through pre‐ or postcopulatory 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 population, variation in female quality is low. We were unable to find differences in abortion rates or protein concentration in oocytes produced by females of contrasting sizes. Direct benefits from mating with large partners seem, thus, reduced in the Baltic population. We also found no evidence of any postcopulatory mechanism that could favor larger mothers as embryo development was unrelated to female size. While female size can still be selected through intrasexual competition or fecundity selection, the pressure for large female body size seems to be lower in the Baltic. Accordingly, we found a noticeable decrease in sexual size dimorphism in the Baltic population. We conclude that, although far from negating the significance of other selective processes, sexual selection seems to have a decisive role in supporting pipefish sexual size asymmetries.

Highlights

  • Due to local or geographically widespread variation in environmen‐ tal conditions, one can assume that the price of securing the best possible mating partner, and the extension of the ensuing rewards, likely differs throughout the distribution of a species

  • If larger females do not persistently secure a higher reproductive success, either through pre‐ or postcopulatory sexual selection, we can predict a decrease in sexual size dimorphism in the Baltic population when contrasted with that from the west coast

  • We evaluated the contemporary degree of sexual size dimorphism in the broadnosed pipefish from both the Baltic and Sweden's west coast

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Summary

| INTRODUCTION

Due to local or geographically widespread variation in environmen‐ tal conditions, one can assume that the price of securing the best possible mating partner, and the extension of the ensuing rewards, likely differs throughout the distribution of a species. If embryo development is affected by simpler processes, such as mat‐ ing order (see Braga Gonçalves et al, 2015), female size can still be selected if larger females are recurrently able to secure the most re‐ warding time slot when mating (e.g., mating first) This later scenario seems less likely as the short time elapsed between sequential mat‐ ings has been considered insufficient, at least in the Swedish west coast population (Mobley, Kvarnemo, et al, 2011), to support devel‐ opmental asymmetries in the brood. If larger females do not persistently secure a higher reproductive success, either through pre‐ or postcopulatory sexual selection, we can predict a decrease in sexual size dimorphism in the Baltic population when contrasted with that from the west coast To address these questions, we analyzed embryo growth patterns in full‐ and half‐sib broods, looking for indi‐ cators that might suggest male cryptic choice or half‐sibling rivalry in pregnancies harboring offspring from different size females. We evaluated the contemporary degree of sexual size dimorphism in the broadnosed pipefish from both the Baltic and Sweden's west coast

| MATERIAL AND METHODS
Findings
| DISCUSSION

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