Do Sibling Cats Mate Together – And What to Do

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  • We are all taught that siblings shouldn’t breed, whether we believe it to be a moral issue or a purely scientific one. When it boils down to it, there are some issues that can crop up when siblings mate and produce kittens. To help us understand why sibling cats mate together, why they shouldn’t, and how to prevent them from doing so, we turned to Janet Cutler, Ph.D., one of our Certified Cat Behaviorists at for answers.

    “Sibling cats will mate and have kittens together,” she told our interviewers. “If sibling cats are kept intact (not neutered) it is very likely that they will mate.”

    Here’s everything you need to know.

    Why siblings cats mate together

    Cats have a natural instinct to mate that doesn’t transcend genetic relationship. If a female cat is in heat and a male cat is around, they are bound to mate, related or no. According to Dr. Cutler, “Female cats are polyoestrus, meaning they come into heat and can get pregnant many times throughout the year.” That means there are lots of opportunities for siblings to mate together if left unchecked.

    At what age do cats begin mating?

    Cats will mate as soon as they reach sexual maturity. For female felines, sexual maturity is defined as the first onset of heat. Dr. Cutler says, “In general, female cats reach sexual maturity between 5-9 months of age and males between 9-12 months of age.”

    Unfortunately, most female cats are not physically mature enough to bear a litter at this young age, which makes it even more important to prevent sibling cats from mating. As soon as those kittens are around 9 months old, they will be mating if you don’t take action.

    Can siblings have kittens together?

    Siblings having kittens together is just as likely as any other two cats mating producing a litter. However, with inbreeding the kittens may not survive.

    “When siblings mate, there is increased risk of genetic abnormalities in the kittens as both parents will have very similar genes,” Dr. Cutler warns. “Kittens may or may not survive due to those abnormalities, and could have many lifelong problems.”

    Is it dangerous for a young cat to get pregnant?

    Even though your cat can get pregnant as soon as they have their first heat, they really shouldn’t bear a litter this young. A cat that is too young for motherhood may not survive the pregnancy or the birth. Even if they do survive, the survival rate for their kittens is dramatically decreased.

    Help, my sibling cats are trying to mate! Tips to prevent sibling cats from mating

    If you have sibling cats and you don’t want to have them fixed for whatever reason, you’ll need to take some steps to ensure that they don’t mate. Dr. Cutler suggests, “Unless you want to keep your sibling cats separate in your home, the best way to prevent this from happening is to neuter the sibling cats (or at very least one of them). Discussing this with your veterinarian to determine the appropriate time and solution for your cats is important.”

    At what age should I have my cat spayed or neutered?

    Generally speaking, you should have a cat spayed or neutered between 4 and 5 months of age. This is well before a male cat will try to mate with a sibling, and it is also just before or around the time of the female cat’s first heat. Getting them spayed or neutered at a young age prevents sibling inbreeding and other issues that can arise.

    Adopt siblings of the same sex

    If you are adopting siblings, you should get two of the same sex to prevent inbreeding. If your own cat had the litter and you are selecting a couple of kittens to keep for your own household, keep two of the same sex.

    Keep siblings apart when female cats are in heat

    The only sure fire way to prevent cats from mating when a female is in heat is to separate them entirely. You will need to have a separate litterbox, separate food and water dishes, and a separate space to be apart from the male cats in the home.

    Is it bad for sibling cats to mate together?

    Breeders traditionally mate closely related cats to create purebred lines. Line breeding and controlled inbreeding can produce healthy cats of a high pedigree, but these should be left to the professionals that understand the genetics of cat breeding. Sibling cats mating early also adds to the general cat population, which can be bad all around.

    What is inbreeding depression?

    Inbreeding depression is the fancy term for the phenomenon of reduced survival and fertility rates in the offspring of related animals. Inbred animals do not live as long, and they are likely to be depressed and listless.

    Essentially, when two siblings mate of any species, there is a reduced chance of survival of the litter. If they do survive, they could have a whole host of health problems. Often unsuccessful or accidental inbreeding leads to kittens that are themselves sterile.

    What are the potential defects for inbred kittens?

    There are a variety of physical defects and congenital health problems that inbred kittens could face. There is no way of knowing what if any defects will be present until the litter is born. The most common health defects in inbred kittens are:

    • Physical defects like crooked noses, misaligned jaws, abnormal eye set, and other asymmetry
    • Immune deficiencies, evidenced by frequent and severe infection
    • Congenital abnormalities like hip dysplasia
    • Temperament issues like aggressiveness or isolation behavior

    In addition, inbred kittens face untold horrors when they are dumped because they have these issues. Far too many inbred kittens wind up in shelters or dying in the elements because they were unfortunate enough to be born to siblings.

    Will kittens from siblings always have birth defects?

    In spite of the dangers of inbreeding cats, there are good chances that an inbred kitten will be healthy and normal. How likely it is that your sibling cats will have a litter of unhealthy kittens depends on their inbreeding coefficient. The fewer relatives they have, the less likely birth defects will occur.

    This is the strategy employed by cat breeders, but it isn’t a good idea to just breed sibling cats on your own. You don’t know the cats’ complete histories, and as such you have no idea how related they might be. However, siblings with different fathers are least likely to have birth defects among inbred kittens. Female siblings may have more than one mate, so you may not even know for sure that inbreeding took place until the kittens are born. Even then there could be doubt.



    • Nicky Thomas

      Nicky Thomas is an author and freelance writer specializing in writing features, general interest, and thought leadership articles. She also has a passion for interviewing people of all walks of life with an important message for the masses. Read more of Nicky’s work at

    • Janet Cutler, PhD, Cat Behaviorist

      Janet Higginson Cutler, PhD, CAAB, is a Certified Applied Animal Behaviorist. She earned her Phd at the University of Guelph, and runs her own cat and dog behavior consulting firm, Landmark Behaviour, in Canada.