Male fish seem to prefer females with larger pelvic fins because it may be correlated with reproductive success, and females have learned to flaunt that feature, according to a study conducted on aquarium fish.
Although there is ample knowledge on male characteristics that influence sexual selection in animals, not much is known about female characteristics that result attractive to males.
Now for the first time, scientists studying the relevance of fin size, other than for locomotion, in a cichlid fish discovered that males prefer females with "generous" pelvic fins.
Researchers at the University of Bonn, Germany, studied the effect of female ornamentation on reproductive success in the fish Pelvicachromis taeniatus, a species native to Africa commonly used as aquarium fish.
The study has been preliminary published in the Journal Evolutionary Biology. According to Sebastian Baldauf, who led the research, this is the first study to investigate the allometry, the relationship between body size and shape, of a female ornament, in this case the size of the pelvic fin.
The scientists found that those females which develop excessively large pelvic fins, which differ from the fins of males both in form and color, wave their large purple pelvic fins during courtship suggesting that the fin is actively used during mate choice.
“The size of the pelvic fin might be exaggerated by male choice because males prefer female stimuli that show a larger extension of the trait. Female pelvic fin size is correlated with individual condition, suggesting that males can assess direct and indirect benefits.” the scientists state.
Baldauf and his colleagues believe that male sexual preference may have influenced the evolution of the much larger pelvic fin in females.
"It is clear that the males prefer females with larger pelvic fins and the females grow those fins disproportionately to other fins. Our results show that the size of the pelvic fin in females is positively related to fitness." Thus, "Males may benefit by being so demanding," say the authors.
Larger pelvic fins reveal individual quality, the scientists said. Because fins are related to swimming capacity and this in turn facilitates feeding, females with larger fins should be in better physical condition and with lower risk of death by disease or starvation therefore being better suited for reproduction. Their daughters, in turn, would also become more attractive to males of the next generation, thereby improving the chances for the species survival.
A couple of dwarf cichlid fish Pelvicachromis taeniatus, var. NIgeria Red. The male is in the center, the female at the right side of the picture.