Female dark eye display in sand gobies covaries with male attention and nest building, regardless of sex ratio

Little is known about what factors promote the evolution of female ornaments under sexual selection. In the sand goby, Pomatoschistus minutus, a small marine fish, females are very cryptic, except for a temporary ‘dark eye’ display, a highly conspicuous black pigmentation near the eyes. Nest-building males show nuptial coloration and courtship to attract mates but no dark eyes. The function of this female trait remains unknown. This study aimed to investigate if the adult sex ratio affects the occurrence, duration or latency of dark eye displays in female sand gobies. Two adult sex ratio treatments that reflect natural conditions (male bias: 3 females, 4 males; female bias: 3 females, 2 males) were used. We predicted more frequent or longer displays under female-biased adult sex ratio if its main function relates to intrasexual competition. We found no significant effect of adult sex ratio on the occurrence, duration or latency of the display. However, relatively rounder females displayed for longer, implying it may signal spawning intent. Consistently, spawning was more likely to happen in replicates in which a dark eye display was observed. Furthermore, better quality nests were associated with a higher frequency of dark eye displays, suggesting that nest quality affects the display or that the display encourages more male investment in nest building. Dark-eyed females also received more attention from males than nondisplaying females. Together, our results suggest the function of the display is related more to communication between the sexes than to competition among females. However, further research is needed to explore to what extent it is used by males in mate choice, affecting whether it should be called an ornament.