The Complete Book of Dog Breeding. Dan Rice
normal, and they show no sexual interest in males. It is the period of uterine rest, repair, and stabilization.
The only reproductive hormone found in the bloodstream in appreciable quantities during anestrus is estrogen. Although some consider anestrus to be a period of reproductive inactivity, the animal’s pituitary gland and ovaries are actively preparing for the next proestrus phase to begin.
As you can see, proestrus accounts for about nine days, and estrus for another nine; diestrus accounts for two months, and anestrus four or more months. Most bitches (except Basenjis) complete more or less two estrus cycles each year. The age at which cycles commence depend less on climatic conditions and geographical variations, and are more influenced by nutrition, health, and physical size.
Interestrous Period
The time interval between periods of receptivity of canine females is termed the interestrous period. It varies greatly between breeds and between individuals within a breed. One lengthy study found the interestrous interval in German Shepherds to be as short as 149 days, and in Boston Terriers to be as long as 242 days. Bassets, Beagles, Cockers, Pekingese, and Poodles fell in between.
German Shepherds and Rottweilers sometimes have estrous cycles of four months or less. Bitches that demonstrate interestrous intervals of less than 130 days may not have ample organ repair and preparation time during anestrus. Infertility is occasionally seen in those animals, and hormone treatment can often be used to lengthen their interestrous periods.
Age
Canine estrous or heat cycles begin at puberty and occur throughout intact bitches’ lives until death. Spaying (removing the uterus and ovaries) ends the estrous cycles. Puberty is reached at varying ages—at five months in some toy breeds, or as late as 30 months in some giant breed individuals.
Many canine research studies use Beagles as models. In that breed, puberty is typically reached at about ten months, and it directly correlates with growth plane maturity. Since smaller breeds mature at a younger age than large breeds, it usually follows that tiny breeds experience puberty at an earlier age than giants.
We ordinarily speak of canine estrous cycles as six-month periods, but rarely do they take exactly six months to complete. The lengths of cycles not only vary between breeds, but considerable differences are common between females of the same breed. An individual bitch’s cycles are usually very predictable from year to year, but they may also vary due to health and condition influences. Some authorities speculate that the duration of lactation (length of time she nurses her puppies) may modify a particular bitch’s cycle as well.
TIP
If your bitch has not shown evidence of a heat period by two years of age, consult your veterinarian or a canine reproductive specialist.
A cute Havanese puppy looking for a playmate.
Most canine females’ estrous cycles occur more or less twice a year. Exceptions to that rule are found in the Basenji breed and in wolf hybrids. Those animals typically exhibit only one estrous cycle per year. It is interesting to note that the age a female reaches puberty relates to her body size or growth plane, but the length of estrous cycles is not correlated to body size.
Seasonality
Dogs’ fertility (the propensity for normal pregnancy) does not seem to differ in response to its climatic seasonal environment. An extensive study of a number of breeds and cross-bred dogs showed that estrus activity (periods of receptivity of females) were identical in January and July (11.4 percent in each month). A slight reduction of estrus activity was observed in December (10.9 percent), and the rest of the months of the year saw very similar figures (6.4 to 8.3 percent). Seasonal or climatic changes seem to influence the estrous cycles of feral canine populations.
Condition
Other than body size and breed peculiarities, several other factors influence puberty and estrous cycles. Among them are injuries, diseases, nutrition, and the presence of other intact females in the same home or kennel. They are discussed at length in the chapter on breeding problems (Chapter 4).
The general health and fitness of bitches play extremely important roles in their reproductive cycles. Animals exhibiting or recovering from injuries or illnesses may not cycle, or their cycles may be abbreviated or otherwise modified. That is another general rule to which exceptions are frequently noted.
I have seen live, reasonably healthy puppies born to weak, emadated bitches suffering from poor nutrition, complicated by infections, intestinal parasites, fleas, and ticks. High puppy mortality usually accompanies such litters, but dogs have an amazing ability to perpetuate their species in the face of adversity.
A German Shorthaired Pointer relaxing on the sand.
Example: I had occasion to treat a 30-pound mixed breed bitch about eight months old that a client picked up on the roadside of an Arizona Indian reservation. She was emaciated, anemic, weak, dehydrated, and nearly comatose. One femur was fractured with the bone ends overriding each other several inches. Her coat had once been long, but was now sparse due to an active mange infestation. She also harbored three types of intestinal parasites.
Together with general anesthesia and surgical bone repair, she was treated for her various parasite infestations and anemia. Her response to treatment and premium diet was exemplary. On a splint check-up visit two weeks later, I discovered she was about one month pregnant. Spaying her was considered, but her new owners elected to wait until after her other ailments were handled. Due to her stressed condition, I predicted she might not carry the litter to term. Surprise! She uneventfully delivered and raised five nondescript puppies.
Nutrition
Females with nutritional deficits due to starvation, poor-quality food, or unbalanced diets may cycle normally, but they may not conceive when bred. Some conceive but abort or resorb (see Fetal Resorption and Abortion, page 62) embryos early in gestation. The nutritional status of a bitch relates to the quantity and quality of food offered (see Pregnancy Nutrition, page 69) as well as her ability to normally absorb and metabolize the food she eats.
Parasites
Another important aspect of a dog’s general condition is associated with parasite infestation. Animals that are supporting large populations of internal or external parasites are being robbed of nutrition, regardless of the quality and quantity of food consumed (see Health and Immune Status of a Female, page 28).
Obesity
Obese bitches often have erratic estrus cycles as well. Obesity may relate to simple glutinous overeating, eating a high-calorie, unbalanced diet, lack of exercise, or occasionally, endocrine imbalances.
Induced Estrus (Dormitory Effect)
A unique and poorly explained canine estrous idiosyncrasy is often reported in scientific literature without interpretation. It is especially prevalent in kenneled dogs, but may also be seen when two adult, intact females live in the same house.
A bitch in heat influences other intact bitches in the immediate environment to begin proestrus. Sometimes referred to as the dormitory effect, it is studied by a few, denied by many doubters, and affirmed by most of us. Simply put, typical proestrus, followed by normal standing heat may be induced in one female by