Here is an example of the 'Choose Animals' screen that is displayed before the 'Choose Skills' screen and just before you start playing a level in the game. In this case, all animals have already been chosen, and there is only five animals available at this stage of the game.

A screen shot from the upgrade menu of Lumberwhack. Only four animals and three skills have been unlocked so far and are therefor available for upgrades. Acorns that has been collected or rewarded when a level is completed is used as currency for upgrading. Both the damage and health of the animals increases when being upgraded, but the skills attributes increases a bit different depending on there specific unique ability.