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The subject of intentional breeding or meat rabbits is prohibited. The answers provided on this board are for general guideline purposes only. The information is not intended to diagnose or treat your pet. It is your responsibility to assess the information being given and seek professional advice/second opinion from your veterinarian and/or qualified behaviorist.
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Nipping on the rear usually is an indication that one bunny wants the other one to move. Jack gives Rucy very gentle nips on her bum when he wants to be left alone in the litterbox, or when she's blocking his way to get out of the pen.
He may be wanting her to move away so that he can be given a turn to be pet.
If he does it in a very aggressive way, then it looks like they are still working on parts of their relationship.
They should be sure during this time to give them both equal amounts of affection, so he doesn't feel threatened that she is being considered the "top"bun.