After your Chao emerges from its first evolution cocoon, it is considered to be an adult. However, its shape and colour are not permanent and will continue to change as it grows older! This is called second evolution, and will continue throughout its adult life. Unlike the first evolution, the Chao does not enter a cocoon for the second evolution.
An adult Chao can change into a total of nine different possible second evolutions: Normal, Swim, Fly, Run, Power, Swim+Run, Swim+Power, Fly+Run, and Fly+Power. However, the second evolutions are completely different for each of the five first evolutions.
It works just like it does with a child Chao: the Chao will gradually change over time depending on the animals that it interacts with. At first, you will find it very difficult to tell what ability type(s) the Chao has for its second evolution, as there will be very little difference. However, the effect will become increasingly more pronounced as it grows older.
Contrary to popular belief, second evolution is not permanent, has nothing to do with stats, and does not take place at any specific age. You can always change the second evolution of a Chao by simply giving it different animals. For example, if you have a Fly/Run Chao, you can give it Power animals and it will change into a Fly/Power Chao.
The format used on this page to describe evolutions is First Evolution / Second Evolution. For example, "Run/Power" is a shorthand way of saying that a Chao has a Run type first evolution, and a Power type second evolution. All Chao in Sonic Advennture on the Dreamcast are Neutral alignment. Hero and Dark Chao were not introduced until the next game in the series, Sonic Adventure 2.
In reality, there is nothing special about second evolution. It is almost exactly the same as the gradual change that a child Chao goes through before it evolves, with only a few differences: it is a longer, slower process; and it ends with the Chao's death, not with a metamorphosis.
Here you can view images of all the second evolutions. There are quite a lot, so you may want to use the below options to filter them!
|Normal/Swim||None||This Chao is exactly like a Normal/Normal Chao, except the tip of its head goes up and starts to curl.|
|Normal/Fly||None||This Chao is exactly like a Normal/Normal Chao, except the tip of its head curls forward.|
|Normal/Run||None||This Chao is exactly like a Normal/Normal Chao, except the tip of its head goes straight back.|
|Normal/Power||None||This Chao is exactly like a Normal/Normal Chao, except the tip of its head goes up.|
|Swim/Normal||None||Slightly taller than the Swim Chao, and has larger ears.|
|Swim/Swim||None||Light blue highlights, and the head is more rounded with longer ears.|
|Swim/Fly||None||A quite thin Chao, with lots of colours.|
|Swim/Run||None||A small Chao with many coloured markings, mostly consisting of green and aqua.|
|Swim/Power||Sumo Chao||Very fat, has a very large stomach that sticks out.|
|Fly/Normal||None||The wings turn orange and the jesters straighten back behind the Chao's head.|
|Fly/Swim||None||Feet become flatter, and the head becomes more spread out, as if to protect it.|
|Fly/Fly||(Advanced) NiGHTS Chao||Longer jesters and larger, brighter wings. Resembles NiGHTS a lot more than the Fly.|
|Fly/Run||None||The jesters curl backwards and the Chao becomes a more mellow pink.|
|Fly/Power||None||The Chao's jesters grow upwards and curve like a scorpions tail, and the Chao becomes much redder.|
|Run/Normal||None||A brighter blue colour and is just like the Run Chao in shape, except with straighter spikes.|
|Run/Swim||None||The Chao's feet become long and pointy and turn an aqua colour. Spikes become more joined.|
|Run/Fly||Elf Chao||Turns a weaker purple and its feet and spikes curl up.|
|Run/Run||(Advanced) Sonic Chao||This Chao is a rich blue colour and resembles Sonic.|
|Run/Power||None||Has a cone shaped head with a red stripy top, and stripy hands.|
|Power/Normal||None||Just like the Power Chao, except this Chao's head is more rounded.|
|Power/Swim||None||Its ears and feet have purple markings and its stomach and feet are quite chubby.|
|Power/Fly||None||Head is like a volcano, and fists are similar to those of Knuckles. Stripes are on its feet and ears.|
|Power/Run||None||Ears look like antenna, and go straight up. Hands and feet have orangy yellow markings.|
|Power/Power||None||Big and red. Huge bulky body, hands, feet and head. Looks powerful.|