Many times the success of a breeding facility is evaluated on the number of kittens produced. Of course, not every cat breeder is trying to maximize their breeding potential in numbers, but instead seeking that near-perfect cat. One can argue that with greater numbers of kittens, the odds of achieving the perfect cat are greater. Whether or not the breeder is seeking to have larger litters, litter size can be of concern. Singleton litters, or two-kitten litters, can have problems due to the size the kittens attain prior to birth. We must consider though whether or not the pregnancy started with only one or two embryos, or did embryo loss result in only one or two survivors?
It is recognized that many factors can influence litter size, and not all are manageable through husbandry. Among these factors are issues that relate specifically to the queen, the breeding management applied, the embryo itself, genetics and nutrition.
Queen
A logical place to start is the queen, herself. Of course the age at which she is bred will have some impact on pregnancy rate and litter size. Cats bred too young are harder to settle, and have smaller litters. This is largely due to the presence or absence of ovulation, and the number of ova released. Ideally the queen is fully mature before she begins her reproductive career.
Typically her first litter will average 3 kittens. The next couple litters will generally have 4 or 5 kittens.
Queens can have long productive breeding lives, and maintain good results up to the age of 9 or 10. As with other species, age does sway the ability to produce kittens. Part of this change is due to longer intervals between estrous cycles, another is fewer ova being ovulated, and the state of the uterus also affects reproduction. Spontaneous abortions are also seen to occur with greater frequency in older queens.
As queens age, especially those that are not bred routinely, the cyclical changes to the tissue lining the uterus, the endometrium, eventually causing scarring. These changes in the endometrium predispose areas to be unavailable for embryonic implantation. If the embryo can’t find an acceptable area into which it can establish itself and develop a placenta, it will die. The change in the uterus is called cystic endometrial hyperplasia. Basically, the stimulation of the glandular tissue within the uterine lining that occurs with every estrous cycle becomes less able to return to normal non-pregnant status as the queen ages. This is a normal progression, and is seen to exert more problems in queens not bred frequently. Routine pregnancies decrease the numbers of times this tissue generates and then degenerates through repeated heats.
Breeding Management
Because the reproductive physiology of the queen depends on the amount of daylight hours to determine her breeding activity, it is easy for the owner to manipulate this as a control point. Cats are referred to as "long day breeders" because they need at least 14 hours of daylight to trigger the regular cycling of heat periods.
Since catteries are largely indoor facilities, the length of "daylight" is controllable. And this works both ways. If the breeder has a queen that she doesn’t want to cycle, she can limit the amount of light that cat receives to 10 hours or less consistently. The down side to this is that interrupted breeding can negatively impact litter sizes when the queen is finally returned to normal environmental conditions.
Queens are induced ovulators. Cervical stimulation will trigger a release of luteinizing hormone, which in turn triggers ovulation. If the queen is bred late, the ova may not be released as with a normal ovulation because the follicular cells have atrophied. It would be possible to have one or two ova released, but most likely the queen will not settle.
Due to the breeding induction of ovulation, the best chance of optimal ovulation is to breed 3 to 4 times within a few hours. More than that does not seem to increase the response.
Artificial insemination is a breeding alternative, and is a little trickier in cats than dogs. As seen in dogs bred by artificial insemination, decreased litter sizes are experienced in queens bred this way. When frozen semen is used, the decrease is most pronounced.
Infectious
Every cattery eventually deals with abortion. There are many causes recognized including infectious, physiological, genetic, drug related, nutritional and traumatic. Sometimes it is very difficult to determine the exact cause.
Less well known, is the fact that the queen may experience the loss of one or more embryos, and still have surviving fetuses, which she delivers on schedule. The same causes that can abort entire litters, can kill partial litters. The most vulnerable aspect of the developing embryo/fetus is the placenta. Anything that disrupts the integrity or function of the placenta is likely to kill the embryo or fetus attached.
The fact that some of the embryos/fetuses are unaffected, while others are resorbed, makes it very difficult to even know when this has happened. Without an early pregnancy diagnosis, it is natural to assume the queen started with a small litter. The only way to determine that this is happening is to use pregnancy diagnostics early, and potentially at various times through the pregnancy. Ultrasound is a great tool early on, and radiographs are very useful after the fetuses have started to deposit mineral within their skeletal systems, about day 45.
Knowing that the average litter size for a first-time mom is 2 to 3 kittens, and that queens which have had previous litters will average 4 to 5 kittens, the breeder can see red flags if their cats are producing far below those levels. They must then determine if the problem lies in poor conception, or increased loss of embryos. This is very important if the problem is to be reversed.
Viruses, bacteria and parasites can all cause the death of an embryo or fetus during pregnancy. One may see stillbirths, and fading kittens also associated with these infectious agents. Of the viruses commonly associated with fetal loss, feline herpes virus, feline panleukopenia virus, feline leukemia virus and feline infectious peritonitis are typical. Bacterial pathogens include hemolytic strep (hemolytic means that red blood cells are destroyed), E.coli, Pastuerella species, and staph. Mycoplasm and Chlamydia are also implicated.
Toxoplasmosis and Bartonella are parasitic causes of fetal loss and abortion as well.
Embryo
Spacing of the embryos within the two horns of the feline uterus generally occurs by day 12 or 13 after conception. While it is advantageous for the embryos to have enough space to grow, there is some loss of embryos during this event. The average litter size delivered in cats is 4 to 5 kittens. Typically, the queen will have ovulated more eggs than that, and the greater the number of eggs, the greater loss of embryos during spacing and implantation. Researchers found that with 5 ova released and fertilized, generally 90% were implanted. As the ovulations increased between 6 and 8, only 82% implanted. When ovulations reached 9 to 11, only 50% actually survived to implantation. The optimal litter size resulted when the queen had 8 ova released, averaging 6.6 kittens per litter.
Occasionally an embryo will hit a snag in its development and stop. The interrupted development is typically fatal. This can be genetic in nature, a response to outside trauma, or just a development defect.
Genetics
Differences exist between breeds in regards to litter size. A study performed in Australia investigating reproductive patterns of pedigreed cats reported that the most prolific breed was the Burmese averaging 5 kittens per litter. The next was Siamese with 4.5, then Persian with 3.9, followed by Abyssinian at 3.5 kittens per litter on average.
Between 2001 and 2003, three pedigreed breeds were researched in the US, Canada and with some input from UK breeders, and one of the characteristics examined was litter size. Devon Rex breeders that participated had a collective average of 3.7 kittens per litter. Ragdoll kittens averaged 4.5 per litter. The Sphinx breed averaged 4 kittens per litter.
Further impact is exerted depending on the degree of inbreeding of the parents. The smaller the genetic pool within breeds, the easier it is to breed too closely. Genetic diversity is lost when inbred and linebred animals are too limited genetically. One of the cons involved with intensive inbreeding is the loss of vigor and smaller litter sizes, among other things.
While inbreeding can increase the chances of homozygosity of defective genes, even outbred cats can experience genetic defects. Sometimes these defects are serious enough that the embryo doesn’t survive into the fetal stage.
Nutrition
Tremendous research has been performed in feline nutrition covering all aspects of life. It is an ongoing endeavor as each discovery leads to other questions. One of the aspects that has been very forthcoming is reproductive nutrition.
Cats are somewhat unique in that they are obligate carnivores. They have to have a significant portion of their daily intake of food from protein. The best array of essential amino acids is provided by using both meat protein and vegetable protein. The greater emphasis should be animal protein, but balanced nutrition comes from providing all the essential nutrients in the necessary amounts. In order to achieve this, some vegetable sources must be used. Cats also have increased needs for certain amino acids (the most basic units of protein). These amino acids are taurine, arginine, methionine and cysteine. Each of these play a critical role in reproduction, and in a deficiency situation, can prevent or interrupt a pregnancy.
The level of fat in a diet also contributes to reproductive performance in cats. Optimal fat content can help to increase litter size from 4.5 to 5.5 kittens, as well as decreasing mortality. The essential fatty acids found in that fat are crucial as well. Omega-3 and omega-6 fatty acids are equally important, but are also dependant on the relative proportion to each other to work well. Arachidonic acid is another essential fatty acid particularly important in cats, especially in breeding. Queens eating arachidonic acid deficient diets had poor fetal viability. Additionally, omega-3 fatty acids EPA and DHA helped maintain pregnancy term to the optimal delivery time.
It is more understandable how critical nutrition is to the pregnant queen when one realizes that the queen’s diet must not only provide for her own support and maintenance, but also for the changes that occur in her reproductive tissues including mammary development, as well as the growing and developing embryos she carries.
beta-carotene is a potent antioxidant and in many species is a precursor to Vitamin A. Not so in cats. Felines lack the necessary enzymes to cleave the beta-carotene molecule into two vitamin A molecules. So both nutrients must be provided intact in the diet. One of the many roles of beta-carotene in maintaining health and supporting reproduction is preparing the uterus to receive embryos during early pregnancy. It stimulates the glandular tissue in the uterine lining to create a proteinaceous fluid, which will bathe and nurture the embryos until they can establish implantation. Without this positive environment, the embryos die. beta-carotene also concentrates in the ovarian tissues, enhancing the hormonal maintenance of the pregnancy.
Micronutrients such as minerals and vitamins cannot be overlooked when one is studying reproductive performance. Three of the minerals that have been recognized for their role in healthy pregnancy, and delivery of neonates, are copper, zinc and manganese. Deficiencies in any of these minerals can easily cause problems. Research has further documented that by binding these mineral molecules to amino acids, the absorption rate increases from the intestinal tract, making the minerals more available for metabolic use.
Perhaps the best known vitamin in pregnancy and fetal development is folic acid, one of the B vitamins. Enhanced folic acid minimizes the risk of developmental defects, especially cleft palate and other neural tube defects. This helps the fetus to thrive and fully in preparation for neonatal life. Vitamins A, D and E are also essential. One of the benefits of Vitamin A is the maturation of lung tissue, so that a newborn kitten can breathe well upon birth. Some of the actions of all these micronutrients occur with the development of the placenta, and continue throughout the functional life of this tissue.
This is a rather generalized overview of various factors that interact to provide normal reproduction, and or possible problems, when that interaction does not happen. However, it should illustrate that in fact there is seldom no single factor that is to blame, or to be credited, when pregnancies don’t perform as expected. It is by no means a complete or exhaustive list that is presented here, either.
Some variables are more manageable, while others are virtually uncontrollable. We can and should expect some normal variation in the reproductive life of any animal. We can also use performance measurements to recognize when that performance is perhaps not what it should be.