A Museum’s Butterfly Emerged Half Male, Half Female

The rarity is like a natural experiment that tells scientists how genes and hormones interact to produce different sexes

butterfly gynandromorph
The Academy's live Lexias pardais with gynandromorphism Isa Betancourt/ANSP

An unusual butterfly just crawled out of its chrysalis at the Academy of Natural Science of Drexel University. Its wings are slightly mismatched—the right dark brown wings carry creamy yellow speckles that are a tad bigger than the left velvet black wings with blue-violet iridescent edges.

When museum volunteer Chris Johnson saw the insect, he says, according to a press release, "I thought: 'Somebody’s fooling with me. It’s just too perfect. Then I got goosebumps." 

A butterfly specialist confirmed that the Lexias pardais was exactly what Johnson suspected: half male and half female, divided neatly down the middle. For the Washington Post Rachel Feltman writes that this is called gynandromorphism:

It usually happens early in development, when cells are just beginning to split to form an embryo. One of the early cells fails to split its sex chromosomes properly (for example, an XXYY might split into an X and an XYY instead of two XY cells). These cells continue to divide and proliferate, and they're signaling for the organism to grow into two different sexes.

Not all organisms with gynandromorphism are as striking as the butterfly. Some are more of a mosaic, where the different sexed cells are mixed throughout the body. Others are of a species with a less obvious differences between different sexes. 

Such individuals are more than just curiosities, as Ferris Jabr explains for Nautilus. By exploring instances where nature produces a strange, unusual version of a living thing, scientists learn how biology works and how creatures develop. Jabr illuminates how a gyandromorph zebra finch helped researchers understand that the "standard explanation of how a bird becomes male or female is wrong."

He writes:

Typical male zebra finch brains have a network of neural circuits dedicated to learning courtship songs, and the area containing these circuits is much larger than the corresponding region in the female brain. If sexual development was dependent primarily on hormones, then each half of the gynandromorph’s brain should be architecturally identical. After all, every organ in the bird’s body was bathed in the same androgynous cocktail of sex hormones released by its testis and ovary.

Instead, they found that the zebra finch’s singing region in the right half of the brain was 82 percent larger than in the left. Male zebra finches learn to sing, and females don’t. The hemisphere size difference in the bird was dictated not by hormones, but by the sex chromosomes in each cell of the brain.

Given the mismatched butterfly’s importance, the museum made the call to preserve the insect rather than letting it live for a brief two weeks inside the Butterfly Garden enclosure. Instead of risking that something would happen to the butterfly, it will now be on display for a "limited time" starting January 17, the statement reports. There the public can see and marvel about the complexity of genes made visible.

Get the latest stories in your inbox every weekday.