It's not uncommon for a neutered male cat to stalk, attack, mount, and hump another cat. This can result in stress for both you and the second cat. If this happens regularly in your home, there are a few ways you can find out what's going on and stop or manage the behavior.
Why Do Neutered Cats Hump?
Neutering may not automatically stop a male cat from mounting (grasping with his forepaws, gripping her neck with his teeth) and humping other cats. The behavior may be due to health issues, your cats' social hierarchy or other problems.
Humping behavior is normal for whole (sexually intact) male cats. Even after castration surgery, it takes time for the hormones to leave the body, and it's not unusual for mounting to continue for at least a few weeks if not longer.
Urinary tract infections also seem to lead to humping behavior in some cats.
It’s always best to first have your veterinarian rule out health issues. If it turns out that your cat is physically healthy and the humping behavior has been consistent, it's likely a behavioral issue.
Cat humping can be related to stress and anxiety. This is most likely when something has recently changed in the cat's environment like the addition of a new family member, a move, or even a neighborhood cat that can be seen from a window. Boredom is another cause of humping in cats.
However, cats also use mounting behavior as a way to reinforce social ranking. Cats reach social maturity between the ages of 2 and 4 years. Prior to that, they may get along famously, and then suddenly the cats’ social ranking starts to matter. Your male's stalking, mounting, and chasing your other cat away from important resources may reflect territorial issues or pushy behavior.
How to Stop the Humping
If a medical cause is found for your cat's humping, treatment for his condition will be your first step. Behavioral causes can be somewhat more difficult. These often require a good understanding of how the cat world works so you can make attempts to work with (rather than against) the natural instincts of your feline family members.
Reduce Stress, Anxiety, and Boredom
Make sure your cat is getting all the attention, mental stimulation, and exercise he needs. Play with your cat at least daily. If you can pinpoint a source of stress, do what you can to relieve it.
Offer Distractions or Deterrents
If you see your cat getting ready to hump, clap your hands loudly or drop a book on the floor. You might also offer a stuffed toy to a male that's intent on humping so he'll possibly leave your other cat (or you) alone.
Reward Good Behavior
Find ways to reward your male cat's good behavior. You can do this with treats, toys, or extra attention when he's being calm and interacting nicely with your other cats. This is much more effective than common discipline techniques, such as squirting him with water or yelling. And of course, you should never hit your cat.
Expand Their Territories
Giving your cats more space to increase their respective territories can help as well. Cats love to climb, so provide separate cat trees and window perches for each to claim. You can even have fun by installing elevated walkways for your cats to explore.
Sometimes, you also need to provide each cat with its own personal spaces for basic needs. Try to place food and water bowls in different locations so they're not fighting over resources. You should also follow the litter box rule of "two plus one," meaning that you need three boxes for two cats. Make sure these aren't within sight of each other to quell tensions and give all kitties their privacy.