Large and interactive pipefish females display ornaments for longer with many males around

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Why do animals display sexual ornaments – to attract mates, to compete for access to them, or both? In the broad-nosed pipefish (Syngnathus typhle), ornamented females commonly compete for access to males, whereas choosy males provide uniparental care. During courtship, females show a dynamic ornament, consisting of a row of dark B-shaped signs along the sides of their trunk. Here, we focus on the social context of female ornamentation and mating success (egg transfer) under experimentally female and male biased adult sex ratios. We found that the duration of female B-sign displays was not explained by adult sex ratios alone, but rather by sex ratios in combination with female body length and the number of female interactions. In detail, duration of B-sign display increased more steeply with the number of female-female interactions under the male biased adult sex ratio: that is, when more mating opportunities were available. Similarly, display duration increased with female body length, but only under the male biased adult sex ratio. In addition, successful egg transfer occurred more often in the male biased treatment and correlated significantly with female display duration in the same treatment. Taken together, these results suggest that in the context of the broad-nosed pipefish, the ornamental B-signs in females primarily serve as a sexually selected attractive signal. Our results highlight the importance of an interactive mating environment, male availability, and female size.