Dogs and cats have natural instincts that sometimes tell them how to act or respond to a situation and when it comes to mating, these instincts are in full force. Humans have rules in place and mating is not done with close family members but dogs and cats don't differentiate between family and others when a female is in heat.
Dogs and cats can and will mate with their siblings once they are sexually mature. Litter mates are not off limits to other dogs and cats, even though they are related. Once a male dog or cat reaches sexual maturity, mating instincts may take over and it will attempt to mate with its female litter mates if they are in heat. Pets have no concept of relatives so siblings and parents are just other animals to them. There is no emotional aspect for pets while mating and thoughts or concerns of reproductive consequences due to genetics is of course not a concept they understand.
When Do Dogs and Cats Reach Sexual Maturity?
Sexual maturity will typically occur between 6 and 9 months of age but can be later in larger or giant breed dogs. A male will not try to breed with a female unless she is in heat so any other humping or mounting behavior that occurs outside of this timeframe is most likely dominance play. The period of time that a female can become pregnant lasts about a week but the entire heat cycle lasts about 2 to 3 weeks. Heat cycles will then occur in the female every 6 to 9 months.
How Can You Prevent Pets from Breeding?
In order to keep your dogs and cats from breeding you can either keep them separated when the female is in heat or get them desexed. Trying to keep a male dog or cat from a female that is in heat is very difficult as the natural sex drive will cause the male to be very persistent in getting to the female. Most pet owners opt for desexing because of this.
Males can be neutered and females can be spayed in order to prevent heat cycles and pregnancy from occurring. This completely eliminates the possibility of litter mates breeding with each other and is a procedure that only needs to happen once. Typically it is done before the pet reaches sexual maturity but some dog breeds, like German Shepherds and Border Collies, may benefit from waiting until they are older. Spaying and neutering dogs at a young age can help prevent unwanted pregnancies but the chances of your dog developing joint disorders, urinary incontinence, and cancer may increase in some breeds. You should discuss what is best for your specific pet with your veterinarian.
What Should You Do if Litter Mates Breed?
If you have litter mates and they end up breeding, your female dog or cat may become pregnant. In order to determine if your pet is pregnant, your veterinarian can perform an ultrasound 3-4 weeks after the breeding has occurred or X-rays can be taken after 45 days. Birth will occur between 57 and 68 days from the date of breeding. Your veterinarian may offer to spay your pet while it is pregnant and abort the fetuses if there is concern for serious genetic abnormalities or your pet can be allowed to give birth.
The puppies or kittens should be examined by your veterinarian to ensure they have no obvious congenital abnormalities from inbreeding that may prohibit them from growing and thriving. You will need to ensure the offspring are eating and growing well and bottle feed them if necessary. They will need to stay with their mother for at least 8 weeks before going to a new home if you choose not to keep them but the new owner should be made aware that the parents were siblings so congenital issues may show up later in life. Deafness, heart issues, cleft palate, malformations, joint issues and other health concerns may be a result of the inbreeding.
Once the puppies or kittens are no longer nursing you can then get your female spayed. This will prevent an unwanted pregnancy from happening again in the future.
Hart BL, Hart LA, Thigpen AP, Willits NH. Assisting decision-making on age of neutering for 35 breeds of dogs: associated joint disorders, cancers, and urinary incontinence. Front Vet Sci. 2020;7. https://doi.org/10.3389/fvets.2020.00388