The axolotl, a unique salamander native to the lakes near Mexico City, possesses remarkable biological features. Known for its neoteny, this amphibian retains its juvenile, aquatic form with external gills throughout its life, never undergoing metamorphosis into a terrestrial adult. This unusual biology, coupled with its extraordinary regenerative capabilities, often leads to questions about the stability of its sex. The concept of sex change, which occurs in some other aquatic species, is a frequent point of inquiry regarding the “Mexican walking fish.”
Genetic and Developmental Sex Determination
Sex in axolotls is established early in development and is primarily determined by genetics. Unlike mammals, axolotls utilize a ZZ/ZW sex determination system, a pattern common in birds and some other amphibians. In this system, males possess two identical sex chromosomes (ZZ), while females are the heterogametic sex (ZW).
The specific region controlling sex differentiation, known as the sex-determining locus, has been identified on one of their chromosomes. Scientists have noted that the sex chromosomes are physically very similar, suggesting this genetic mechanism is relatively new in their evolutionary history. Once the embryonic gonads differentiate into either testes or ovaries based on this genetic blueprint, the sex of the axolotl is fixed and stable throughout its life.
The Myth of Natural Gender Change
Axolotls do not possess the biological mechanism to naturally change their sex once it has been genetically determined and physically established. The idea that they can change sex often stems from confusion with other amphibians or fish species, such as clownfish, that exhibit sequential hermaphroditism. Axolotls do not share this capability.
The persistence of this misconception also has roots in scientific research involving hormonal manipulation. In laboratory settings, young axolotls can have their sexual development artificially altered by administering specific hormones, like estradiol or testosterone, during a sensitive period. This external intervention can induce a form of sex reversal.
Crucially, this induced physical change is not a natural occurrence and does not alter the animal’s underlying ZZ or ZW genetic makeup. The physical manifestation of sex can be overridden by hormonal changes, but the genetic sex remains the same. Furthermore, the difficulty in visually identifying the sex of juvenile axolotls contributes to the belief that a sudden change has occurred when they finally display mature features.
Distinguishing Male and Female Axolotls
Determining the sex of an axolotl is not possible until it reaches sexual maturity, which typically occurs between six and twelve months of age. The most reliable visual indicator is the appearance of the cloaca, the single opening for waste and reproductive fluids located behind the rear legs.
A mature male axolotl develops a pronounced, swollen bump around the cloaca, often described as a cloacal bulge or papilla. This structure is noticeably enlarged and protrudes downward. Female axolotls, by contrast, have a flatter, smaller, and less prominent cloaca.
Secondary characteristics are also helpful for identification. Females tend to have a rounder, wider body shape, particularly when viewed from above, because their body cavity is adapted to hold a large mass of eggs. Males typically have a more slender, streamlined body profile with a longer tail.