A Third of Female Azurill Used to Change Sex When They Evolved
Azurill is 75% female. Marill is 50%. For three generations the game recalculated on evolution, and the maths did the rest.
For three generations, evolving a female Azurill had roughly a one in three chance of producing a male Marill. This was not a glitch, an urban legend or a mistranslation. It fell straight out of two design decisions that were individually reasonable and did not survive being combined.
Two ratios that do not match
| Azurill | 75% female · 25% male |
|---|---|
| Marill | 50% female · 50% male |
| Azumarill | 50% female · 50% male |
Baby Pokémon often skew female — it is a small piece of flavour, and on its own it is harmless. Marill and Azumarill are an even split. So the line changes ratio partway through, and something has to give when an Azurill crosses that boundary.
Why the ratio mattered at evolution
In Generations III to V, a Pokémon's gender was not stored as its own value. It was derived on the fly from the personality value — a large number rolled when the Pokémon is created — compared against that species' gender ratio.
That is an elegant way to save space, and it works perfectly as long as the ratio never changes. Evolution does not re-roll the personality value; it just changes what species the game is comparing it against. So an Azurill whose number fell into the female range under a 75% threshold could easily fall into the male range under a 50% one.
Run the numbers and the affected slice is the band between the two thresholds — the Azurill that were only female because the baby ratio was generous. Roughly a third of female Azurill sat in it, and every one of them came out of evolution male.
How it was fixed
Generation VI stopped deriving gender from the personality value and started storing it as its own property, set once when the Pokémon is created and never recalculated. From X and Y onward a female Azurill evolves into a female Marill, every time, and the behaviour quietly disappeared.
No patch notes, no acknowledgement. The mechanic was replaced for other reasons and this went with it.
Why it is worth knowing
Because it is a clean example of the thing that makes these games interesting underneath the surface: the rules are not describing creatures, they are computing them. Gender here was never a property Azurill had. It was an answer the game worked out on demand, from a number that meant nothing on its own, against a threshold that happened to move.
Players in 2003 who noticed it had no way to know any of that. They just knew that sometimes the girl came back a boy, and that nobody believed them.
Sources
- Azurill and Marill gender ratios — official game data
- Personality value gender determination, Generations III–V
- Gender determination change, Generation VI onward
Quoted Pokédex entries are reproduced from the games named above for commentary. Every number in this article was checked against the species' official data before publishing.
