Here’s what asymmetrical butterflies and sweaty armpits can tell us about our sex

If you ever want to go down a Wikipedia rabbit warren like no other, start your journey at ‘sexual dimorphism’.

Male golden-orb weaver spiders are smaller than females to evade cannibalism. All Australian king parrots are born with green heads, but only males mature into redheads. Anglerfish females are up to half a million times heavier than the tiny, parasitic, sperm-producing males.

More often than not, nature divides up behaviour and appearance between the sexes. But what happens when nature divides wrong?


Very rarely, and not in mammals, individuals can display a fusion of both male and female dimorphic characteristics. We’ve seen this occur in songbirds, chickens, lobsters, crabs, stick insects, and notably, butterflies. Sexual dimorphism in butterflies can easily be seen in the vibrant wing patterns. I have a hard time believing that scissors and superglue weren’t involved in creating the specimens below.

Two bilateral gynanomorphs, left and right, and a mosaic, middle. Source: Wikimedia Commons, images have been cropped.

These mismatched individuals are known as gynandromorphs. They can either be a pure bilateral or a mosaic form. If you cut a bilateral gynandromorphic butterfly from head to tail (but please, please don’t), one wing looks perfectly like a female and the other like a male. Whereas mosaics have a smattering of male and female throughout their wings.

This can happen if something goes wrong in the very first steps of life. The inherited genetic material determining sex, the sex chromosomes, fail to copy and divide correctly between two cells. This sounds catastrophic, but in some cases the butterfly grows normally. Opposite to us, butterflies with XY chromosomes are female, and XX are male. If cell division goes wrong in a male, cells with three X’s remain male, but cells with one X can become female.

Looking at a gynandromorph helps visually track where the descendants of these first cells go. The first two cells become the left and right wings, the following division becomes the upper and lower wings. This continues, the cell patterning becoming more and more detailed. Simple.

But everything, of course, becomes more complex in humans.

Females: Human Mosaics

There are people out there that can only sweat from one armpit. Take that as you may, but what’s behind this oddity is similar to what’s behind gynandromorphic butterflies.

Humans, thankfully, can’t accidentally form gynandromorphs. But all females, inheriting one X chromosome from Dad and one from Mum, have an invisible cell patterning all over their body. This is best seen in people with Blaschko lines, who look like they’ve had a barista swirl a cappuccino over their skin. It’s quite beautiful, really.

Blaschko lines will form a ‘V’ shape down the spine of an affected female. Artistic representation with coffee swirls art. Source: Fabrizio Verrecchia and Nathan Dumlao on Unsplash. Images have been edited.

Different to the butterflies, with sections of both male and female, we have areas of paternal X chromosome and maternal X chromosome. When a baby is only fifteen days old, made of five hundred cells, it begins to randomly choose one X chromosome per cell. All the cells derived from these five hundred, including your current skin cells, will follow the same choice. Doing this means females can match the number of X chromosomes that males have.

A condition called Anhidrotic ectodermal dysplasia causes loss of sweat glands, among other symptoms. Females can be carriers for this on their X chromosome and display minor symptoms. The cell lines that turn off this defective X will become patches of skin that sweat normally. Those that turn off the normal X will have patches of skin that don’t sweat. Hence, only one sweaty armpit.

Perhaps I fall into these Wikipedia spirals to show myself how wonderfully interconnected in biology we all are; or maybe I just need to buy stronger deodorant.

2 Responses to “Here’s what asymmetrical butterflies and sweaty armpits can tell us about our sex”

  1. Ehlana Tompsett says:

    Please don’t cut butterflies in half.
    How on earth do you get into these spirals? Or maybe the better question is how do you find your way out of them? At what point do you think “it’s time I take my findings and step away from the see of cat videos and memes” before you end up on the kitten huffing page on wikipedia?

  2. scripps says:

    X-chromosome inactivation in females is such an interesting topic, it’s cool how you’ve related it to butterflies as well