Dogs and cats don't grow up with social restraints that discourage copulation between siblings, so when they reach puberty, littermates can and do spontaneously mate. Some animal breeders purposefully pair brothers and sisters to maintain bloodlines and develop desirable pedigree-associated characteristics. Conversely, inbreeding can also lead to birth defects, an increased likelihood of genetic diseases and chronic health problems.
Heeding the Call
When a female goes into heat (also known as estrus), males simply follow their instinct to pass on their genes without any concern for the genetic ramifications. Littermates living in the same home naturally respond when the time is right.
The age of animal puberty varies — it can sneak up on you, resulting in unplanned pregnancy and the shock of an additional five to 10 puppies or kittens in your care. Generally speaking, dogs and cats enter puberty at around 6 months of age, but it can happen for some at 4 months old. Dogs go into heat on average twice per year; with small dogs, it can happen three or four times per year, while giant breeds may experience estrus only once every two years. Cats, on the other hand, cycle into heat about every three weeks during a breeding season that generally occurs between January and October.
A cat in heat may suddenly become vocal or start exhibiting excessive affection. An indoor cat who wants to get outside can also indicate a heat cycle, while the estrus posture of butt in the air with tail tilted to one side may be the most telling sign. Female dogs may urinate excessively, lick their genitals and seem clingy and agitated. Bloody discharge and a swollen vulva also occur with a dog in heat. Signs of heat can also be signs of a problem, such as a urinary tract infection, so see your vet whenever you feel uncertain of the cause of a particular behavior.
The average canine pregnancy lasts 58 to 68 days and a feline gestation period takes 64 to 67 days. Statistically, most dogs could safely produce two litters per year but a cat could feasibly give birth five times. This would not, however, be good for your cat's health or longevity, and could result in overpopulation and the death of unwanted animals.
Pregnancy can easily become an issue when you adopt brother/sister littermates or keep litters of puppies and kittens past the age of 4 months. The onset of the first heat cycle, when a female can become pregnant, follows the same time frame as puberty (4 to 6 months) for cats and small dogs, though it may be delayed until 18 to 24 months in larger breeds of dogs.
You may want to consider choosing same-gender littermates or adopting puppies from different breeds or parents to avoid the potential pitfalls of inbreeding. For the only guaranteed way to prevent unplanned pregnancies, spay or neuter your pets. Animals get creative around a female in heat. Keeping males away when pets live in the same household may prove difficult to impossible.