In the animal kingdom, men usually have to persuade, cajole or bribe their way into a lady’s favour, which means the males of many species cultivate gaudy displays to attract the females’ attention. Yet for one species of butterfly, the situation is sometimes reversed. Some female squinting bush brown butterflies (Bicyclus anynana) spend their time frantically displaying the bright eye-spot patterns on their wings to as many males as they can find in the hope of attracting a mate.
A female Bicyclus anynana showing off her eye-spots. Image: William H. Piel and Antonia Monteiro/Courtesy of Yale University
The role reversal only happens when females are exposed to cold temperatures as caterpillars. According to researchers from Yale University in the USA, the behaviour change helps the female butterflies survive in difficult times: a female who manages to persuade a male to mate with her also receives a parcel of nutrients from the male, increasing her life span. This is particularly important during the cold season, when it is harder for the butterflies to reproduce successfully.
In contrast, the males’ life span is reduced once they give up their nutrient package, so they will very carefully pick the best females. In this case, the ‘best’ is the one with the brightest eye-spot patterns on her wings.
The reason males of most species regularly have to compete for the affection of females is that, usually, mating is cheap for males. Sperm are small and easy to manufacture in their millions, so males can mate with any and every female they find. On the other hand, the female’s eggs need a lot of resources to produce and nurture, so she has to make sure they are fertilised by only the fittest males around. Effectively, the females are a valuable resource over which the males must compete for access.
Very occasionally, however, these roles are reversed. The best-known example of this is sea-horses, where the male becomes pregnant and expends most of the effort required to raise the young. He therefore takes much more care over who he mates with. In the case of the squinting bush brown butterflies, a female who can persuade a male to give up his nutrient-rich gift will live longer. Suddenly the males become the valuable resource, and a prize worth fighting for.
Reference: Prudic K.L., Jeon, C., Cao, H., & Monteiro, A. (2011) Developmental Plasticity in Sexual Roles of Butterfly Species Drives Mutual Sexual Ornamentation. Science. 331 (6013). pp. 73-75. DOI: 10.1126/science.1197114
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