Neutered male mounting female!!

Question
My male rabbit, Pan, who is 1 and a half was neutered in Feb last year. I have recently moved in with my girlfriend who also has 2 house rabbits both neutered. Pan is bonded with Hessie, a female who is 2 and a half. Over the past 3 weeks he has been trying to mount Hessie and this is now happening more frequently.. Is this normal behaviour? My girlfriend's buns don't exhibit his behaviour. Hessie is starting to get fed up of being mounted. What can I do?

Answer
Dear Terry,

If this is new behavior, and he hasn't been mounting Hessie before when they were together, he could have an adrenal tumor that's affecting his testosterone levels.

Please have him checked by a good rabbit vet to be sure all is well.  You can find one here:

www.rabbit.org/vets

If the behavior is new because the rabbits have not been together long, then that will calm down.  It's only when this type of behavior pops up suddenly when nothing has changed that I worry.  

It's also possible that Pan feels threatened by the third rabbit.  Are they together?  Or visible to each other?

Dana