Data from: Baltic pipefish females need twice as many males as they get

Sex role reversal in 2 pipefish species, Syngnathus typhle and Nerophis ophidion, is potentially explained by females reproducing twice as fast as males. Moreover, in oceanic populations from the Swedish west coast, females compete for males with males preferring to mate with larger females. However, in a brackish Baltic population of S. typhle, males do not prefer larger mates, whereas choosiness remains in the local N. ophidion population. We explore whether this absence of male choice in brackish S. typhle can be explained by males and females having more similar potential reproductive rates here, whereas the sex difference may remain in the local N. ophidion population. Contrary to our expectations, in both species, females out-reproduced males by a factor of more than 2, just as in the oceanic populations. We measured this experimentally as the number of males a female potentially could fill with eggs within the time span of 1 male pregnancy, in relation to males available in nature. Thus, we conclude that sexual selection on females is as strong in brackish as in oceanic populations of both species but that targets of selection via male choice are shifted to traits other than body size in S. typhle. Hence, costs and benefits of choice are probably more important than potential reproductive rates to understand mate choice. We suggest that it may be misleading to use targets of sexual selection, such as choice for large body size, as an indicator of the strength of sexual selection.

两种海龙鱼（pipefish）——粗吻海龙（Syngnathus typhle）与直吻海龙（Nerophis ophidion）——的性角色逆转现象，潜在可由雌性繁殖速率为雄性两倍的情况加以解释。此外，在瑞典西海岸的海洋种群中，雌性为争夺雄性展开竞争，雄性则更倾向于与体型更大的雌性交配。然而，在波罗的海咸淡水生境的粗吻海龙（S. typhle）种群中，雄性并不偏好体型更大的配偶；而当地的直吻海龙（N. ophidion）种群仍保留雄性择偶行为。本研究旨在探讨，咸淡水生境下粗吻海龙种群中雄性择偶行为缺失的现象，是否可归因于雌雄个体潜在繁殖速率（potential reproductive rates）更为相近；而当地直吻海龙种群的性别繁殖速率差异或许仍维持原状。与我们的预期相悖的是，两种海龙的雌性繁殖产出均为雄性的两倍以上，这与海洋种群中的情况一致。本研究通过实验量化了该指标：结合自然种群中可获得的雄性数量，以雌性在单次雄性妊娠周期内理论上可使雄性个体怀卵的数量作为衡量标准。据此我们得出结论：两种海龙的咸淡水种群与海洋种群中，施加于雌性的性选择强度并无显著差异；但粗吻海龙种群中，雄性择偶所针对的性状已从体型大小转向其他特征。因此，若要理解择偶行为的演化逻辑，择偶行为的成本与收益或许比潜在繁殖速率更为关键。本研究认为，将性选择的目标性状（如偏好大型体型的择偶行为）作为性选择强度的指示指标，可能会产生误导。