Ultimately, many variables work collectively to determine litter size. Understandably we expect the dam to exert an influence, and it’s commonly thought that the sire, too, can impact litter size. Perhaps not so recognized is the impact the embryos themselves can make. Breeding management plays several roles in the number of offspring we can expect. And it’s not too hard to accept that genetics, environment and nutrition all have a part to play as well.
Dam
Now if we start with the dam, we can identify specific factors that will bear on litter size. Every litter starts with the estrous cycle. Within each cycle are four distinct stages which must work in harmony for optimal breeding results. The first stage, proestrus, is the initial signs that the female is coming in heat including the enlarging vulva and the bloody discharge. What we can’t see are the pheromones that the bitch is secreting to attract the attention of suitors, but we generally can observe the results. When she gets to the point where she will accept mating, she has transitioned into estrus, the second phase of the complete cycle. She is ovulating, and the number of viable ova available for fertilization is obviously going to exert a very strong influence on the final litter size. We may well have fewer puppies born than ova that were ovulated, but we will never have more pups that ova. Additionally, it is possible for the bitch to exhibit every sign of a fertile cycle, without actually ovulating. Unfortunately, unless we specifically look for evidence of ovulation using hormone assays or other diagnostics, we have no way of knowing whether or not ovulation occurred. When the bitch loses interest in accepting the male, she is beginning the diestrus phase. During this time the uterus, the mammary tissue, and dam’s metabolism is preparing for pregnancy, whether or not she is actually pregnant. During this phase the follicular cells on the ovaries are secreting progesterone to maintain a pregnancy. This stage will persist for about 60 days. Finally, her body shifts into the final stage of her estrous cycle, which is anestrus. It is commonly thought that this is a sexually quiescent time, with nothing in particular happening. Actually, it is a tremendously important phase when the uterus returns to a normal, non-pregnant state. We know that this phase requires at least 90 days for that repair and rejuvenation to reach completion. If the bitch we want to breed is cycling too frequently, her uterus may not be prepared to accept embryos, and the embryos die. It can appear that she was not successfully bred for whatever reason, when in fact she got pregnant, but early embryonic death (EED) occurred.
The age of the bitch can also affect litter size. Not surprisingly, very young and very old dogs produce smaller litters. The age of the bitch when she has her first litter induces an impact. When a bitch is 4 years old or older when she has her first litter, her litters will be smaller than a younger dog. With subsequent litters we see an overall decrease in litter size after 5 years of age in the majority of breeds. We expect the first litter for any bitch to be smaller. Litter size will increase with each litter generally through the third, sometimes the fourth litter. Then the number of pups decreases steadily.
Another factor the bitch owns is her breed. As one would expect, smaller breeds tend to have smaller litters, and vice versa. An important point however, is the breeder’s expectation. Often times the assumed average litter size is based on nothing more than conjecture and possibly the reports of other breeders. The problem with that is the population involved is too small to really reflect a true average value.
AKC performed a three year study involving 15 breeds, in order to gather data on litter sizes, ages of dam, number of litters, etc. A total of 728,271 litters were included in this time period. The representative data gathered is truly reflective of the breeds involved. Just to illustrate the information on some of the breeds, we’ll look at 2 breeds from each large, midsize and small breed dogs. Labrador Retrievers had a typical range of 5 to 10 puppies, with 7.6 average puppies, looking at over 85,000 litters. German Shepherds were evaluated with just over 44,500 litters, showing a normal range of 4 to 9 pups, and an overall average number of 6.6. In the American Cocker Spaniel breed, 39,000 litters provided data showing an average litter size of 5, with a normal range of 3 to 7 pups. Shetland sheepdogs produced over 28,400 litters in this 3 yr period, revealing a typical litter size ranging from 2 to 6 pups, and the average litter size was 4.3. Yorkshire terriers were represented by just over 53,000 litters, ranging from 2 to 5 puppies per litter with an average of 3.3. Chihuahuas had identical results based on 55,500 litters. The point being that if one expects a German Shepherd to consistently produce 8 pups with each litter, thinking that an "average litter size", he might conclude that she is not performing up to her potential if she’s producing 4, 5 or 6 pups, which is well within the normal range for that breed. Likewise, while Chihuahuas and Yorkies have a typical range of 2 to 5 pups, 80% of all litters are 4 pups or less.
We’ll leave the main issues involving the dam with the concluding thoughts that her health, reproductively, and in general, will act upon her ability to produce puppies.
Sire
So does the sire sway the litter size? Our usual method of evaluating the efficacy of a potential sire is by determining semen quality. The parameters by which that quality is judged are sperm concentration, morphology and motility. Normally, the concentration of sperm will be around 10 million sperm per pound body weight. We want to see 80% normal morphology at least. Motility needs to be not only movement, but effective movement linearly and with speed. Ideally 75 to 80% of the sperm on the slide will be traveling quickly in a linear manner. Not surprisingly, age does affect semen quality. Like the decreasing production seen in bitches as they age past 5, males will also show a decrease in semen quality as they get to be 6 or so. The assumption of semen quality impacting litter size is not really pertinent. Rather, it is the ability of a male to get a female pregnant at all. What we see when semen quality decreases, is fewer pregnancies resulting when breeding the male in question. In a young healthy male with good semen quality, we can expect 9, or even 10, out of 10 breedings to result in pregnancy (assuming breeding timing is appropriate). With lower semen quality, we may only see 3 or 4 pregnancies from 10 matings. So basically, if the semen has enough sperm to get one ovum fertilized, there is enough to get the other ova fertilized, again assuming the timing of the breeding is optimal.
Embryo
It can be difficult, if not impossible, to fully appreciate the differences between the number of fertilized eggs, developing embryos, and the fetuses that survive to birth. Early embryonic death claims a significant number of the "litter", with infectious diseases being the most common factor. If the embryo has a developmental defect, it may result in death and resorption. Developmental defects may result from genetics, trauma, exposure to drugs or vaccines, or infectious agents. Occasionally an embryo will inherit defective genes from both parents that kills it. In these ways, the embryo itself can affect the number pups delivered.
Breeding Management
Breeding management is a key contributor to litter size, and luckily, one in which we can exercise a great degree of control. The canine species is very unique in that the ovulated eggs are not immediately capable of fertilization. An additional 48 hours is needed until the ova are mature enough for fertilization. If a bitch is bred too early, by the time the ova is mature and ready for fertilization, the sperm have died off or are too old to achieve fertilization. Likewise, if the bitch is bred too late, the sperm will be very viable, but the ova are too damaged, too old, to accept the sperm. So this tells us that our best bet to get larger litters is to know as accurately as possible when ovulation occurs. Tools that can aid in this include the bitch’s behavior, vaginal cytology, and hormone assays of progesterone and luteinizing hormone (LH). The use of progesterone and LH assays can also tell us if the bitch is having an anovulatory estrous cycle.
Artificial insemination is becoming more common in the dog industry for a number of reasons, one is to take advantage of another dog’s genetics that is not located close enough to allow natural breeding. Another consideration is breeding a bitch that has proven to be difficult to get bred by natural cover. However, whenever we use artificial insemination, we automatically loose potential litter size. Whether we are using fresh or chilled semen, we can plan on having a 15% reduction in litter size. With frozen semen, that rises to 25% reduction. While we can expect fresh semen to remain viable for 5 to 7 days, that range decreases to 3 days with chilled semen. Frozen semen has at the most, one day of viability. Because of this, the manner in which the semen is deposited, and the location of deposition, is critical to maximize viability and results. Transcervical deposition occurs when a bitch is naturally bred with a copulatory tie, or when pipette manipulation during artificial insemination successfully winds through the cervix. Vaginal deposition can result from AI, or from a natural breeding that did not achieve a coital tie. When the person performing insemination is not technically trained, vaginal deposition is the most common result. The problem with this is that the vaginal environment becomes more acidic towards the vulva, and can kill sperm. The last option is to surgically exteriorize a uterine horn and deposit semen directly into the uterine lumen. This is the only option when using frozen semen due to the very short time of viability.
The frequency of breedings predisposes litter size. Smaller litters result if the bitch is bred only once, regardless of the type of breeding employed. On multiple breedings, it does not matter how many days separate the first breeding and the last, as long as it occurs within the period of receptivity (standing heat). Peak fertility is from the LH surge (which triggers ovulation) through the 6 days following. Litters from matings on day 7 from LH surge are the smallest (eggs degenerate and cervix closes). Due to the nature of surgical AI, one breeding is all there is. Understandably, it is critical to have the LH surge timed as accurately as possible.
No matter how well a bitch is maintained, pregnancy and lactation deplete her body stores of energy, protein, vitamins and minerals. With each successive litter, that depletion becomes more pronounced. The degree of depletion will vary with litter size. After a large litter, the next litter is generally smaller than average. Luckily, the converse is true, with a larger than average litter following a small litter. There is an end to the ability of the bitch to rebound after each litter, and for this reason it is recommended to allow a skipped cycle periodically.
Genetics
Beyond the size and breed of the dam and sire, another factor is the degree of inbreeding. This can be reflected by an inbreeding coefficient. It is a numerical value ranging from 0 (not inbred at all) to 1 (completely inbred). In many breeds it has been demonstrated that the more inbred the dam, the smaller litters she will have. Add to that the observation that some family lines within breeds are not prolific ovulators, and the degree of inbreeding is even more profound.
Likewise, some responsibility rests with breeders when choosing animals to mate. Breeding stock is selected for many reasons, performance, conformation and health, but rarely, if ever because of reproductive traits. This is a mistake. Traits such as litter size, milk producing capability, and mothering ability are all very important factors which have high degrees of heritability. Other issues including libido should also be evaluated.
Nutrition
If the bitch is at her ideal body condition at the time of breeding, it increases the odds of her fulfilling her potential to produce a good number of puppies. Either too thin, or too heavy, the associated stress decreases that potential. When we consider the role nutrition plays, we start with the basics of protein and energy, and then tighten the focus to evaluate specific nutrients. In every category, the goal is to find that optimal level that provides the necessary nutrients in the appropriate quantity allowing maximum performance with minimal negative affect on the dam (drawing on her body stores). It’s important to remember that some nutrients pose a risk if over supplied, such as Vitamins A and D.
Even though the energy and protein needs of a breeding female don’t change significantly until around week six of pregnancy, other nutrients are key to ensure that her body responds optimally during her estrous cycle, breeding and early pregnancy. The needs of embryologic development are specific, including the uterine environment into which the embryos are delivered.
The dietary protein provided during estrous, early pregnancy and through the first 6 weeks influences the dam’s weight gain, litter size and birth weight of the puppies. Not only does the quantity of protein matter, but the quality. Digestibility is a key consideration, as is the source (animal versus plant). Ideally, both will be included in the formula, with the predominance being animal-source protein.
Likewise, in the early weeks of pregnancy, the dam’s energy needs are not significantly different that her maintenance needs. It is very important that both fat and carbohydrates provide that energy. Not only does each of these energy sources bring vital nutrients, but they also spare the dietary protein for those specific protein-dependent needs.
Years of research have documented several nutrients that directly and indirectly impact reproduction. Each nutrient may impact 2 or 3 particular events . Some of these are vitamins (A, C, E, biotin and folic acid), amino acids (tyrosine, Arginine, and Taurine), minerals (copper, zinc and manganese) as well as essential fatty acids (EPA, DHA) and antioxidants (beta-carotene, lutein). Even dietary fiber plays a role.
As the pregnant bitch approaches the last 3 weeks of gestation, there is a huge demand for all nutrients. The puppies will gain 75% of their birth weight in this short time. This translates into compelling need for a nutrient dense, energy rich diet provided in appropriate quantities. Depending on the number of pups and the breed of the bitch, she will likely need two to three times the energy during this time frame, and into lactation, of what is needed for maintenance. With large litters, that need can increase.
This influences litter size through survivability of the pups. Low protein, low fat and or low carbohydrates, can result in increased stillbirths, smaller pups and elevated neonatal death loss. Granted, if we consider this strictly by the number of pups born regardless of viability, it may not be seen to have as serious an impact, however, most breeders will recognize the number of surviving pups as the profit/loss key point.
Environment
Not surprisingly, there is considerable difference on the potential impact of environment depending on whether or not the dog is housed indoors or out. Indoor dogs have a fairly consistent amount of lighting and temperature. Outdoor dogs are influenced by daylight hours, and natural climate fluctuations. Areas that experience distinct seasons, show that variables, including temperature and humidity, sway litter size. Generally speaking, the largest litters are born in the spring, and smallest in the summer.
All these categories of concerns give us much to think about when we focus on litter size. Trying to evaluate all these variables can be difficult, but it will reflect a more accurate understanding. Careful, objective review of these factors may well provide the answer, or at least narrow the possibilities, if one is questioning the advent of small litters.