The bite is a secondary natural weapon, as it shows up in the full attack but not in the attack line of the stat block.
When a creature has more than one natural weapon, one of them (or
sometimes a pair or set of them) is the primary weapon. All the
creature’s remaining natural weapons are secondary.
The primary weapon is given in the creature’s Attack entry, and the
primary weapon or weapons is given first in the creature’s Full Attack
entry. A creature’s primary natural weapon is its most effective
natural attack, usually by virtue of the creature’s physiology,
training, or innate talent with the weapon. An attack with a primary
natural weapon uses the creature’s full attack bonus. Attacks with
secondary natural weapons are less effective and are made with a -5
penalty on the attack roll, no matter how many there are. (Creatures
with the Multiattack feat take only a -2 penalty on secondary
attacks.) This penalty applies even when the creature makes a single
attack with the secondary weapon as part of the attack action or as an
attack of opportunity.
Secondary natural weapons get half strength bonus:
A creature’s primary attack damage includes its full Strength modifier
(1½ times its Strength bonus if the attack is with the creature’s sole
natural weapon) and is given first. Secondary attacks add only ½ the
creature’s Strength bonus and are given second in the parentheses.