XP, an abbreviation for Experience (stylized as EXP in some cases), is a measure of how much food and/or prey an animal has consumed. Once a player reaches a certain amount of experience, they can evolve into the next tier animal. Experience can be earned by eating food, biting predator's tails, eating another animal, or feeding on Healing Stones. Notably, the only animal that doesn't have XP requirements to evolve is the King Dragon, as it only needs to eat all apex tier animals.
In the Settings, there is a way to disable XP popups, meaning when you gain XP from an entity/food source, the amount given will not show as it normally does right above your water bar.
Since October 10, 2016, any XP above 1,000 now reads like 1K, but if there is 1,100 XP it reads 1.1K.
Since December 25, 2016, any XP above 1,000K doesn't have a decimal point.
Since February 19, 2017, any XP above 1,000K now reads 1M but if there is 1,010K XP it reads 1.01M. The same applies with billions.
Since April 18, 2017, any XP above 1,000 now has an extra number after the decimal, and the letter after is uppercased.
XP requirements for animals
These are the amounts of XP you need to reach for each animal, as of April 18, 2019: