Photograph by James K. Adams
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The typical eastern tiger swallowtail (Papilio glaucus) is either all dark or all yellow—but this bilateral gynandromorph has a yellow side that's male and a dark side that's female.

Photograph by James K. Adams

Going Through Life as Half She, Half He

What’s a bilateral gynandromorph? A bird, crustacean—or butterfly—that’s a 50-50 split of male and female traits.

This story appears in the January 2017 issue of National Geographic magazine.

The difference in appearance between a species’ males and females is called sexual dimorphism. The term implies that there’s a bisecting line between sexes, a clear divide. But in the animal kingdom, a lot of creatures straddle it.

The natural world is replete with hermaphrodites, animals that may outwardly appear male or female but have the reproductive organs of both. Their less common cousins are gynandromorphs, animals that are a mosaic of male and female traits—say, the size and coloring of one with the genitalia of the other.

Rarer yet is the bilateral gynandromorph, an animal that’s half him and half her, split at the midline. The phenomenon has been documented in birds, crustaceans—and butterflies.

Evolutionary biologist Josh Jahner explains “what most scientists think happens” to form these outliers: Butterflies’ sex chromosomes are the reverse of humans’—males have two alike (ZZ), females two different (ZW). A female’s egg sometimes has two nuclei, a Z and a W. When they’re “double fertilized” by a male’s Z sperm, Jahner says, the resulting embryo is half each sex.

How rare are these specimens? In a 1980s study, a research team that raised nearly 30,000 butterflies found only five bilateral gynandromorphs among them. Colleagues at the University of Nevada, Reno have been “really excited,” Jahner says, to find four since 2011.

Jahner says gynandromorphs in his lab have tried but failed to lay eggs, likely because of an irregularity in their reproductive systems. So though their breed sports striking fusions of color, it’s a beauty they apparently can’t pass on.