My character has a bite attack from his race. What happens if he gains another bite attack from class abilities or spells? Does the damage increase? Can he make more than one attack? Or is it simply that only the better version applies?
I would prefer answers from a Pathfinder context but 3.5e is also acceptable.
You can only make one attack with a given natural weapon using the same part of your body. This is covered on a FAQ item:
Half-Orc--Toothy: Does this alternate racial trait stack with the Razortusk feat (page 168) granting you two bite attacks?
This is one of those areas where we tried to get at the same idea multiple ways. In this case, the answer is no, unless you somehow manage to get an extra mouth. Generally speaking, natural weapons can only be used once per round each. This also applies to the Animal Fury barbarian rage power (Core Rulebook, page 32).
A second bite attack would simply mean that you can choose the best of the two. You would need a second head (like an hydra) or a second mean of delivering the attack (you gaining bite attacks on your hands) to be able to use two different bite attacks.
Sometimes, an ability will grant you a benefit that only works with the natural weapon granted, like the additional energy damage on bite attacks granted by the Dragon Disciple prestige class. So if you have a bite attack from another source, that bite attack will not gain the additional energy damage granted by the class.
Check this question about how multiple natural attacks work if you need more insight.
So, generally speaking, you get to attack once for every natural attack you have. Gain another natural weapon, gain another natural attack.
However, natural attacks are a muddled confusing mess, in both D&D 3.5 and Pathfinder. Most of the rules overlap (and many of those are confusing), but some of the rules differ (and those are usually just two different flavors of confusing). This question is one of the big ones in this regard.
The consensus, in both D&D 3.5 and Pathfinder, is that gaining a natural attack does not automatically mean you have the limb you need to take it. So if you gain two bite attacks, but only one mouth, you have to pick one to use in any given full-attack. You could pick the other one the next time you attack, if you wanted, but you can’t use both during the same full-attack. Likewise for claws (can’t use more claw attacks than you have hands), wing buffets (can’t use more of those than you have wings), and so on. Gore attacks are usually one per head, even, rather than one per horn or whatever. The one usual exception is tentacle attacks; those usually come with the tentacle you need to make the tentacle attack.
Where is this stated in the rules? I could have sworn it was in there, but I’m having an awfully hard time finding it. Like I said, the rules in this area are a muddled mess. But it has been suggested by developer commentary, and is kind of implicit in the way natural attacks are handed out (e.g. the girallon arms soulmeld has to explicitly give you an extra pair of ghostly arms so you can actually use all four of the claw attacks it offers). It’s also consistent with the rules for mixing manufactured and natural attacks, which are explicit about how a hand that attacks with a sword has been “used” and cannot also attack with a claw.
This is certainly how I have always played and always seen things played. Even with this limitation, it is entirely possible to get unreasonable numbers of natural attacks even as it is (try looking up one of the various King of Smack and derivative theoretical-optimization builds for this taken to truly ludicrous extremes). Without this limitation, natural attacks would very quickly and very easily get out of control.
Some abilities that grant natural attacks state that they increase damage if that form of natural attacks already exists.
Otherwise, choose the best of the two attacks.
Each natural weapon allows you to make one natural attack in a full attack action using natural weapons. When an ability causes you to gain a natural weapon, assuming it specifies nothing else, you gain that weapon, in addition to any natural weapons you might already possess, in the same way that an ability that grants you a manufactured weapon would grant you that weapon irrespective of other weapon-granting abilities.
Because the idea of a humanoid with multiple 'bite natural weapons' but only one mouth is upsetting to some people, they may houserule that your bite-granting abilities cannot stack so long as you only have one head. This is a fairly common houserule, but seems to only be applied to weapons a human would ordinarily have in real life (claws, bite, slam), leaving more esoteric weapons (gore, wing, tail, crush, stomp, tentacle, etc) untouched, as abilities granting these are generally fluffed by such groups as adding appendages rather than 'improving' them, except when they're granted as part of a race with the necessary appendages by default but no default way of using them for natural attacks. Mechanically, there is usually nothing to support the distinction in the rules text, though several abilities do refer to the modification of teeth or nails (rather than heads or hands) as the source of the new weapon(s).
Basically, RAW each weapon is separate unless it says it's not, but YMMV, especially with Bite or Claw attacks.
