[...] Crossman turned to a third technique—a test for a set of genes called ZFX and ZFY that show up on the X and Y chromosomes. In Eau10b, Crossman found both genes, confirming that the animal had a Y chromosome. But the whale’s DNA also contained a double dose of ZFX, the gene carried on the X chromosome. The result revealed that Eau10b had a Y chromosome and two X chromosomes, meaning the animal was neither male nor female. Eau10b was an intersex whale—the first of its species known to scientists.

This combination of sex chromosomes occurs when a cell receives an extra copy of the X chromosome during cell division. A similar event can lead to female offspring with three X chromosomes, or males with one X chromosome and two Y chromosomes.

Crossman doesn’t know how many southern right whales with XXY sex chromosomes might be out there. Even Eau10b’s fate is unknown, since the researchers didn’t identify the whale when they took the DNA sample in 1989. But southern right whales can live up to 70 years, so Eau10b may be wintering off Valdés Peninsula to this day.

While intersex animals are often infertile and unable to produce offspring to help a population grow, Velocci says that in social species such as whales, intersex animals likely play important nonreproductive roles that benefit the population in other ways.

Studying intersex animals has helped scientists better understand how genes and hormones shape individuals as they develop. Through the process of domesticating livestock, people have known about intersex cows for thousands of years. On Vanuatu, in the South Pacific, islanders nurture a unique strain of intersex pigs prized for their delicate spiraling tusks. More recently, researchers have also documented intersex horses, dogs, moose, sheep, fish, and many different types of invertebrates. Intersex animals are rare across species, Crossman says, but they’re “more common than we historically thought.”