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 gynandromorph, 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 (above), an animal that’s half him and half her, split at the midline. The phenomenon has been documented in birds, crustaceans – and butterflies. Evolutionary biologists Josh Jahner explains “what most scientists think happens” to form these outliers: Butterflies’ sex chromosome are the reverse of humans’ – males have two alike (ZZ), and females have two different (ZW). A female’s egg sometimes contain a double nuclei, a Z and a W. When they’re “double fertilized” by a male’s Z sperm, the resulting embryo is half each sex. That is to say, their chromosomal aberration is ZZZW. Out of 30,000 butterflies found and raised in a lab only five bilateral gynandromorphs emerged.
THE DRAW BACK: The bilateral gynandromorphs appear to be sterile. They apparently fail at laying eggs, likely because of an irregularity in their reproductive systems. So their breed sports a striking fusion of color, it’s a beauty they apparently can’t pass on.
To read more regarding gender identity check out the January 2017 Issue of National Geographic. It is a special issue published regarding the Gender Revolution.