Split-Sex Animals Are Unusual, Yes, but Not as Rare as You’d Think

All serious butterfly collectors remember their first gynandromorph: a butterfly with a color and pattern that are distinctly male on one wing and female on the other.

Seeing one sparks wonder and curiosity. For the biologist Nipam H. Patel, the sighting offered a possible answer to a question he had been pondering for years: During embryonic and larval development, how do cells know where to stop and where to go?

He was sure that the delicate black outlines between male and female regions appearing on one wing — but not the other — identified a key facet of animal development.“It immediately struck me that this was telling me something interesting about how the wing was being made,” said Dr. Patel, a biologist who now heads the Marine Biological Laboratory, a research institute in Woods Hole, Mass., affiliated with the University of Chicago.

The patterning on the gynandromorph’s wing shows that the body uses signaling centers to control where cells go during development and what tissues they become in creatures as diverse as butterflies and people, Dr. Patel said.

Gynandromorph butterflies and other half-male, half-female creatures, particularly birds, have fascinated both scientists and amateurs for centuries. The latest sensation was a half-red, half-taupe cardinal that became a regular visitor in the backyard of Shirley and Jeffrey Caldwell in Erie, Pa. Although the bird would have to be tested to confirm that it is a gynandromorph, its color division strongly suggests that it is, scientists say.

Split-sex creatures are not as unusual as they may seem when one discovery goes viral, as the cardinal’s did. It extends beyond birds and butterflies to other insects and crustaceans, like lobsters and crabs.

Scientists say these instances of split-sex animals and insects could offer clues to why some human diseases strike one sex more than the other.

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Photo: Nipam Patel