Each grapple check takes the place of one of your itterative attacks
When you are grappling (regardless of who started the grapple), you can perform any of the following actions. Some of these actions take the place of an attack (rather than being a standard action or a move action). If your base attack bonus allows you multiple attacks, you can attempt one of these actions in place of each of your attacks, but at successively lower base attack bonuses.
Note that for monsters with large numbers of natural attacks but a low BAB that this can make grappling a poor choice since they can actually loose out on quite a few attacks per round since grapple checks run directly off your BAB.