Evolution, referred to in early Japanese media as transforming (変身 henshin) is the natural process in the Tamagotchi Life Cycle when a Tamagotchi progresses through its life, through aging or other conditions.
All Tamagotchis naturally evolve over time as they age, starting from the egg to the baby, child, teen, and adult stages. On some occasions, it can evolve into a Senior stage. The majority of Tamagotchi characters have a variety of different forms they can evolve into, but what they become depends predominantly on how they were cared for in their current stage. For example, on the original Tamagotchi, if Marutchi is well cared for, it will evolve into Tamatchi. However, if it is neglected, it will evolve into Kuchitamatchi. On some releases, certain teens can only evolve into certain adults. In the Tamagotchi 4U and its remake, the teen stage is omitted, so the child stage lasts two days rather than one, however it also introduced the Personality Stage.
In the past, babies could only evolve into one or two particular child characters in a first generation. Obtaining characters based on gender and generation are also common requirements.
During the vintage era, babies generally evolved after 65 minutes, but in some cases (such as the second generation onwards on the Mesutchi and Osutchi) it could last over 24 hours. In the Tamagotchi Ocean, however, the egg hatching process, although still five minutes long, is counted as part of the baby stage, which had its duration reduced to one hour. Due to an oversight, the process is an hour and one minute long, with the baby stage being 56 minutes long, rather than 55. Child and teen characters evolve after a period of time between 24 and 72 hours.
Other conditions may be involved in order to raise specific characters, including Skill Points and Bonding. Evolution can also be deliberately invoked by the user, provided specific conditions are met. Overfeeding, underfeeding, not responding to calls for attention, marriage, and specific items are all ways of trigger a Tamagotchi to evolve into another form. On some releases, it can evolve further into a Senior if it hasn't married by a certain age. It can also evolve into a special character under certain requirements.
Conditions for evolution, and what a specific Tamagotchi character can become, vary between every release. Evolutions are not reversible and cannot be cancelled on most releases - on some modern releases where an evolution has a chance of multiple yet random outcomes it is possible to reset the device before the evolution animation finishes and reload to the Tamagotchi's previous stage as the device will not save the progress until the animation completes. This is most noticeable on releases such as the Tamagotchi Connection V2 where every evolution from baby to adult has multiple possible outcomes due to the amount of characters and the tiering systems.
Some characters that evolve through severe neglect (such as Debutchi or Osumotchi) can be reverted to their previous form - Kuchipatchi can transform into or from Patchiman on the Tamagotchi Plus Color, similar to how Kurokotchi could transform into Wooltchi using the Wool item on the Keitai Kaitsuu Tamagotchi Plus. A similar mechanic was introduced on the Tamagotchi Connection Version 2, where an adult Tamagotchi could wear a Costume item to transform, although the costume would disappear when the Tamagotchi went to bed. On the Genjintch Tamagotchi, if the user gives the Tamagotchi severe neglect, it may regress to its previous form in the opposite direction of the growth chart. Similarly, on the TamaOtch, there are four characters that appear in multiple growth stages, which could effectively be seen as de-evolution, because they can be obtained on the same growth stage at least once. On the Mori de Hakken! Tamagotch, if the user has met the requirements to obtain Helmetchi the adult character will regress to being Mayutchi before becoming Helmetchi.
A Tamagotchi close to its time to evolve may fall ill, typically within 24 hours of its evolution (about 20 minutes for the baby stage). The evolution itself is indicated with a loud sound effect and generally a special animation. The evolution animation may be portrayed with a fade-to-black effect (vintage releases), a flickering effect (modern releases), or a glowing burst effect (color-screen releases). However, on the TamaOtch, a sound effect resembling that of a vintage Tamagotchi being reset is heard, and the character switches directly to its next form with no animation. It can also evolve in its sleep, but no sounds or animation will occur.
Evolving into its next form may trigger other events in the Tamagotchi's life, such as getting extra money or being accepted into a new school.
Evolution in the Anime
Evolution also exists in the anime, but is rarely seen. In most instances, the characters keep the same name after changing their form in order to avoid confusion. Fuwamokotchi is the only known character whose name changed after it evolved in the anime series, evolving into "Natsutsugetchi". While Chamametchi and Lovelitchi's baby forms have their own names on the virtual pets, these names weren't spoken in the anime.
In GO-GO Tamagotchi!, Lovesoratchi underwent two evolutions. He first evolved into his baby form and into a child. Later on, he evolves again into his teen form. He goes through more evolution stages than any other anime character.
Lovelitchi isn't shown evolving, but it was revealed that she was a Choribotchi when she was a baby.