The climb skill mentions that you must be using arms to climb, or at least,
You need both hands free to climb
which implies it heavily. But as we all know, snakes can climb.
What is unclear to me is that certain creatures seem to have an implied ability to climb without making climb checks. Almost always, these creatures have a climb speed listed separately on their stat blocks or character sheets.
What are the actual rules for climbing for non-human characters?
Do snakes and spiders actually have to roll a climb check every turn?
It seems silly to me that climbing creatures have the possibility to fall.
I'm specifically asking because I want to know if there's some necessary component to an Eidolon being able to climb that I'm missing, or if it can just get to a wall and sprint up it, even it it has no limbs.
Best Answer
Creatures without hands don't need to use hands to climb.
Almost all rules for skills (and pretty much everything else) are given in relation to the PCs, who are almost always humanoid. Thus, in most circumstances the creature using the climbing skill rules has 2 arms and 2 legs and will be climbing with them.
Non-humanoid characters can't follow these rules, just like you pointed out: spiders don't have hands but can climb, so there must exist an exception for them. Since they have a climb speed they are using something other than their hands to climb. It doesn't totally work with the rules, but that is why we have GMs and not computers.
From the SRD:
Note that with a Str score of 10 with no ranks in Climb, any creature with a climb speed can take 10 and get a total of at least 18* for a Climb check.
So according to the rules, yes, spiders and eidolons and other things with climb speeds should make checks to Climb, but in reality they would be redundant rolls because such creatures almost always have enough bonuses to their Climb skill to automatically succeed**. If circumstances forced enough penalties to their Climb, then actually rolling would be a good idea.
*For reference:
DC 15: Any surface with adequate handholds and footholds (natural or artificial), such as a very rough natural rock surface or a tree, or an unknotted rope, or pulling yourself up when dangling by your hands.
DC 20: An uneven surface with some narrow handholds and footholds, such as a typical wall in a dungeon.
DC 25: A rough surface, such as a natural rock wall or a brick wall
**Giant spiders get a bonus of +16 to climb, meaning that when they take 10 they get 26.