Nobody wants to see pets hurting each other, especially if they're cats that typically get along. If you've found one or more of your cats biting another one of your kitties on the back of the neck, this may indicate that there's a problem going on. Here are three possible causes for your cat's behavior, and some ideas on how you can curb it.
The most obvious reason for why your cat could be acting this way is that it's trying to mate. Male cats tend to bite female cats on the back of the neck to hold them steady while they're mating. If your male cat isn't neutered or was only recently neutered, they may still be acting out on this impulse. Of course, if you have an unspayed female cat in your home, that's all the reason you need for this behavior. Ideally, getting both cats fixed will typically solve this problem right away.
Another reason for cats biting each other on the back of the neck is an act of domination. While cats aren't as notorious as dogs and wolves for having a pack mentality, cats will still sometimes duke it out among each other to determine who the alpha cat of the area is. For you, this would be the cats within your home fighting each other.
If one of your cats is biting another one in the neck and both are fixed, there's a good chance that it's an effort by the biter to make the other cat acknowledge their superiority. This behavior may continue as long as the cat that's being bitten continues to challenge the biter. Try to introduce more playtime into both of your cats' daily routines in order to help them burn off excess energy and prevent these domination reactions.
Finally, stress can lead a cat to behave this way. One of your cats could be biting another one simply because they're anxious, scared, or stressed out. Typically, this behavior is not connected to cats trying to mate with one another, or fighting over territory. Instead, your cat may simply be looking to make itself feel better by dominating another cat.
In this case, you should do what you can do lower your cat's stress level. Try to keep noise to a minimum in your home, limit the amount of strangers that come inside for a while, and if necessary, use a pheromone product to help soothe and relax your cat during all the hours of the day and night.
If your cat continues to behave this way, or if they manage to injure the cat that they're trying to bite, seek help from a veterinarian immediately. Some medical conditions can cause cats to act this way too, so don't ignore the signs of they persist.