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Unusually dynamic sex roles in a fish


Sex roles are typically thought of as being fixed for a given species. In most animals males compete for females, whereas the females are more reluctant to mate. Therefore sexual selection usually acts most strongly on males1,2. This is explained by males having a higher potential reproductive rate than females, leading to more males being sexually active (a male-biased operational sex ratio)3,4. However, what determines sex roles and the strength of sexual selection is a controversial and much debated question3,5,6,7,8,9,10. In this large-scale field study, we show a striking temporal plasticity in the mating competition of a fish (two-spotted goby, Gobiusculus flavescens). Over the short breeding season fierce male–male competition and intensive courtship behaviour in males were replaced by female–female competition and actively courting females. Hence, sex role reversal occurred rapidly. This is the first time that a shift in sex roles has been shown in a vertebrate. The shift might be explained by a large decline in male abundance, strongly skewing the sex ratio towards females. Notably, the sex role reversal did not occur at an equal operational sex ratio, contrary to established sex role theory3,4.

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Figure 1: Abundance of two-spotted gobies over the breeding season.
Figure 2: Seasonal change in the OSR.
Figure 3: Change in male and female mating competition over the breeding season in the two-spotted goby.

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We thank A. Berglund, T. Birkhead, R. Härdling, A. Kazem, H. Kokko, C. Kvarnemo, Å. Lankinen, I. Owens and J. Wright for discussions and comments, and the technical staff at the Kristineberg Marine Research Station for practical help. Our research on the mating dynamics of two-spotted gobies has been supported by the Swedish Natural Sciences Research Council, the Research Council of Norway, the Royal Swedish Academy of Science and the EU Access to Large Scale Facilities program.

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Correspondence to Elisabet Forsgren.

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Forsgren, E., Amundsen, T., Borg, Å. et al. Unusually dynamic sex roles in a fish. Nature 429, 551–554 (2004).

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