It's called "superfecundation" (kinda sounds like a Mary Poppins song!) and is a very common feline reproductive condition. Technically it means that during a single heat cycle two or more eggs become fertilized by different sires during separate matings. Superfecundation is not unique to cats as it can occur in litters of puppies and occurs with some frequency among human twins (talk about Jerry Springeresque!). However, special features of feline reproduction makes superfecundation almost inevitable.
Whereas canines generally go into heat only twice a year, felines are "seasonally polyestrus," meaning fertility is triggered by lengthening hours of daylight and fertile periods repeat multiple times during this season of longer days. Queens in the northern hemisphere generally only stop going into heat during late autumn and early winter. Depending on the season, genetics and environmental conditions, puberty in young cats can begin as early as 4 months of age. A female kitty might find herself "in trouble" even though she herself is barely weaned!
As every farmer knows, when cats do start going into heat, it is an endless cycle of mating, pregnancy and kittens. In fact, cats can go into heat, or estrus, every 14-21 days. Pregnancy itself lasts about 60 days. Queens can become re-impregnated even while nursing a litter of kittens because many begin cycling again about 10 days after delivery. A particularly successful queen can raise up to three litters a year. In short, cats breed like rabbits.But just because cats are ready to reproduce at the drop of a hat throughout most of the year is not what leads to kittens from different fathers (although it probably contributes). Cats are unusual among domestic animals in that they are "reflex ovulators." As opposed to most species (including humans) in which an egg or eggs are released during ovulation regardless of whether mating occurs, actual intercourse is almost always required to stimulate release of an egg from the feline ovary.
Feline ovulation relies upon a complex chain of reactions -- the vaginal-spinal-hypothalmic reflex -- triggered during copulation from sharp barbs on the tom's penis. During the three to eight day fertile period during a given estrus cycle, a queen might mate with a number of toms. Each mating might result in the fertilization of one or more eggs released in response to the copulation. A typical litter consists of four to five kittens. Theoretically, then, each might be only half-brother and half-sister to the others.
Because cats are so prolific, the supply of kittens is always greater than the demand for pets. It is a bittersweet feeling for a veterinarian to behold that basket of adorable, genetically fascinating feline offspring. While we love cuddling the cute little kittens as much as the next guy, we also wish for some kind of cheap and easy birth control for cats. In some countries outside the U.S. oral contraceptives are available, but because they have been shown to cause horrible uterine infections, breast cancer and diabetes in the queens they are considered inhumane and are banned here. So, at this time, we are left with ovariohysterectomy (a.k.a. spay) or removal of the female reproductive tract.
Trap, neuter, release (TNR) programs are available to control feral feline populations. By systematically surgically sterilizing all the cats in a certain locale, you not only prevent them from breeding but also reduce the introduction of unsterilized cats into the "fixed" cats' established territory. Owners of pet cats must also take steps to remove their individual feline from the over-population problem by having it spayed or neutered as soon as medically indicated. All too often, owners who haven't prepared for the relatively minor cost of sterilizing one cat find themselves seriously financially overwhelmed with litter after litter of kittens, all of whom also begin reproducing at a prodigious rate. And no, cats do not care if the sire of her kittens is her brother or her father, so don't count on "that's gross" being an effective contraceptive in a home full of unfixed felines!
Although domestic cats do make wonderful companions with their strikingly beautiful feline features, charming personalities and funny behaviors, now you know where the term "catting around" originated!
