From Creatures Wikia
The apple is a staple in the diet of any Creatures 3 creature residing in the Norn Terrarium. They grow on the largest tree in the terrarium, and often fall to the ground below for hungry norns to pick up.
Apples are high in protein - if your creature is complaining from a lack of this, it would be wise to feed it one . . . unless it's a Toxic Norn, of course! Those are better suited to eating the leftover apple cores.
- I should note that I've experienced a bug in Creatures 3, where if I pick the apples off the tree (instead of letting them fall) the apples stop growing for some reason - it may be just me, but I thought I would throw the precaution out there. --Gryph