Spanish researchers have for the first time documented sex reversal in a French bulldog.

The puppy, Tana, was brought to a veterinary clinic at age three months for vaccinations. Noticing a larger than normal clitoris on the puppy, several tests were performed and it was determined that the puppy was an XX male. In other words, the puppy had a female karyotype (XX chromosomes), but male gonads, in this case cryptorchid testicles. After testing negative for the sex-determining region Y (SRY) gene, Tana was determined to be an XX male.

"Canine SRY-negative XX sex reversal is a disorder of gonadal development where individuals who have a female karyotype develop testes or ovotestes," the researchers wrote. "To our knowledge, this case is the first XX male sex reversion described in a French bulldog."

According to the study, the phenomenon has been described in 18 other breeds, but never in the French bulldog.

The paper, "SRY-negative XX sex reversal in a French Bulldog," was published in the journal "Reproduction in Domestic Animals."