The role of the novel female ornament in the wide-bodied pipefish, Stigmatapora nigra

Abstract

Sexual selection can drive the evolution of exaggerated ornaments and armaments, which serve no purpose to survival but are beneficial for mate choice or intrasexual competition. Sexual selection theory was based on elaborate sexual traits displayed by males, with the expectation that males invest less in gametes, as sperm is less costly than eggs, and thus freeing up males to invest in sexually selected traits. Despite several taxa expressing female ornaments, there have been underlying assumptions that ornaments will have drastic fitness costs for females as a result of trade-offs with fecundity. This raises the question of whether female ornaments can evolve through sexual selection, and act as honest signals during mate choice or evolving through correlated inheritance with male traits.

In sex-role reversal, sexual selection acts more strongly on the female sex, and females compete for mating opportunities, which can lead to the evolution of ornaments. Sex-role reversed species are therefore good model systems to investigate sexual selection on female ornaments, and whether sexual traits can exist without requiring trade-offs with reproductive investment.

Stigmatapora nigra, the wide-bodied pipefish, is a sex-role reversed species in which the female displays an elaborate ornament. Under sex-role reversal, I would expect the female to be the courter, hence initiating courtship, whilst the male would be the chooser, and thus selective in mate choice. Little is known about the role of this novel female ornament; whether it is used in mate choice and/or female-female competition, and whether it honestly signals female quality. This formed the basis of this research.

Through mesocosm experiments I observed the courtship displays of S. nigra and found that the female ornament was actively displayed during courtship. Most of the female displays observed were directed towards males but occasionally females were seen directing displays towards other females, which may be an indication of female-female competition. Surprisingly, males initiated more courtship events than females and there was no difference in the proportion of active courtship displayed by both sexes, which was contrary to my predictions.

To investigate potential trade-offs between female ornaments and fecundity, I dissected females from three sites around New Zealand. Larger, more ornamented females were predicted to be larger, heavier, and have more eggs. Females maintained honest ornaments across all populations, despite variation in ornamentation and body size existing between sites. Overall, the ornament of S. nigra was predominantly displayed during courtship and reflects the reproductive quality of the female, suggesting that it may be important in male mate choice and thus likely to be under sexual selection.

Sexual selection can therefore explain the evolution of some female ornaments, and previous assumptions regarding trade-offs do not stand for female S. nigra. Some of the courtship behaviours of S. nigra were surprising, and potentially related to the fact that sexual selection can be dynamic and mating behaviours can fluctuate depending on the social environment. As this field of research progresses there should be more emphasis on sexual selection in both sexes and the plasticity of these selection pressures across space and time.