Why Would an Older Cat Attack a Younger Cat for No Reason?
While it may seem like your older cat has lost his mind and is attacking your younger kitty for no reason, there are many triggers for attacks between cats that appear to come out of nowhere. Older felines tend to be dominant over their younger counterparts, which can lead to fights between the two. Attacks can cause serious injuries and you'll need to take steps to bring peace and harmony to your home.
The primary cause of attacks by one cat on another for seemingly no apparent reason is redirected aggression. When a cat is stimulated, frightened or upset by something he sees outside, such as a neighborhood cat or a prey animal, and he can't get to it, he may attack his feline housemate. While he generally won't seek her out, if she is near him or simply approaches him in this agitated state, he will attack her in a way that seems unprovoked. This isn't a malicious attack, but more of a reflex. If these attacks happen often, close your curtains to prevent Felix from being stimulated by something outside.
As your feline friends age, they develop medical issues which can make them feel uncomfortable or even affect their cognitive abilities. Starting at around 7 years old, your kitty may start to experience problems with his hearing, sight, memory and even sense of smell. He may develop joint issues, dental problems and arthritis, all of which can cause him pain. If Felix, your senior feline, is approached by Fluffy, your younger cat, he may become annoyed and attack her because he's feeling unwell and stressed-out. He may also become startled by Fluffy's presence if his senses and cognitive abilities are impaired, attacking her out of fear for seemingly no reason.
Territory and Resources
Cats are territorial -- especially socially mature felines who are between 2 and 5 years old. If either of your feline companions is also sexually mature, at least 6 months old, your older cat may attack your younger cat to defend his territory and mating rights. Having your cats neutered or spayed can help alleviate such animosity between them, as can having enough resources for them in your home. Provide each cat with his own litter box, toys and food and water dishes. You also want to give the two an additional litter box.
Introductions and Signs of Aggression
Slow and careful introductions are an important part of establishing your cats' relationship with each other and in preventing fights between them. Separate your new arrival from your existing older cat and slowly allow them to smell each other's scent on towels you rub over each cat. Let them see each other through a pet gate at first, giving each cat treats around the other. Once no signs of aggression occur, including hissing, stiffening of the body, crouching, flattened ears and growling, allow the two to interact, giving them more treats and praise for good behavior.