How Flashy Wings, UV Light, and Seasons All Play a Role in One Butterfly Species’ Mating Rituals

Bicyclus anynana butterflies mating

In Bicyclus anynana butterflies that develop in the dry season, mating rituals are reversed: Females use their flashy wing spots to attract males, rather than vice versa. Here, a dry-season male (left) and female mate. (Photo credit: William H. Piel and Antónia Monteiro)

By Edward Ricciuti  

Males of an African butterfly, with the improbable nickname of “squinting bush brown,” are the Lepidopteran version of chick magnets—if their generation of caterpillars chills out while it grows up.

Ed Ricciuti

Caterpillars of this butterfly (Bicyclus anynana) that develop during the cool dry season flip traditional sex roles as adults when ready to mate in the warm wet season that follows, with females actively courting males instead of the other way around. However, larvae that develop in the warm wet season grow into adult butterflies with more conventional habits, with males showing off to draw females as they reproduce during the subsequent dry season. Role reversal seems to be a function of temperature.

In both cases, the suitors attract the opposite sex by flaunting their butterfly bling, in the form of large eye-shaped spots on dorsal side of the forewing. The more dazzle in the ornamentation, the better it works, say scientists from the National University of Singapore in research published last week in the Journal of Insect Science. Their experiments demonstrate that the dry-season females that receive the biggest share of male attention are those with the most flash. The brighter the reflection of ultraviolet (UV) light—invisible to the unaided human eye—from the female wing spots, the better.

During experiments by the Singapore scientists, males were very picky about associating and mating with females whose eye spots were obscured by black paint, blocking the scales that reflect UV. They were much more likely to mate with unaltered females, which flaunted their spots, and spend considerably more time at it, say the researchers. While mating, males transfer nutrients to the females—described somewhat poetically as a “nuptial gift” —along with a packet of sperm, the spermataphore.

“The nutrients … are set aside during development and are probably used up for the building of the first spermatophore,” says Antónia Monteiro, Ph.D., associate professor of evolutionary developmental biology at the University of Singapore and senior author of the study. The males’ energy investment in providing nutrients is not the typical way of nature, in which  females supply almost all the resources in reproduction, while males mate and then go about their merry way.

Presumably, longer time mating translates into more nutrients supplied by the male, upping the female’s survival quotient. As well, the additional effort expended by the male during reproduction seems likely to take a toll on its survival. Neither sex studied by the authors of the paper, however, showed any change in normal longevity, which presents something of a conundrum. The researchers speculate that the longer mating may be the male’s way of guarding the females against his competition or enables the transfer of more sperm, promoting the survival of his genetic line.

“It would be interesting to count sperm in the spermatophores that are transferred to attractive and unattractive females,” Monteiro says.

A low-flying woodland creature, B. anynana ranges from the southern Sudan to Swaziland in areas that have clear-cut wet and dry seasons. Wet season food is abundant, so the nourishment of the spermatphore is not as prized by females, which therefore can be choosy about males. The reverse is true during the dry season, when temperatures drop. The eyespots of both sexes serve as a distraction to predators amidst the lush vegetation of the wet season, affording protection for females and advertising for males. During the dry season, however, the fact that females do not flash enables them to use their  brown body coloration to hide from predators in the sparse, withered vegetation. It is the males who take the risk by flashing eye spots, which may endanger individuals among them, but, since females are safe to reproduce, the loss of some is not detrimental to the continuation of the species.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.

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