Do Animal Companion advancement benefits stack?

[Abraham Z.]

If an AnC has a +2 natural Armor bonus and its level 4 advancement is listed as +3 natural armor, from level 4 does it have natural armor of +3 (they don't stack) or +5 (they do stack)?

[Jayder22]

I don't have a rules source for you, but I am sure that they stack. Just look at the Ankylosaurus. A heavily armored dino, has +9 to start, at advancement it gets another +2, it does not replace it's +9 with a +2.

[habibo]

the ac from stat block and the druid table stack if im reading your question correctly

[Darksol the Painbringer]

They stack, since in certain instances of animal companions, they get +2 at the start, and then another +1 at their advancement.

If they didn't stack, then their Natural Armor bonus wasn't an advancement at all. If anything, it was a devancement, since their natural armor bonus got weaker instead of stronger, which makes no sense both mechanically and logically.

[Cavall]

Man I can just imagine the anger I would feel with my hunter and his ankylosaurus if suddenly dropped a tonne of a.c. from leveling up.