Variations in females’ standing heat behavior may be due to hormonal changes in progesterone-estrogen ratios. Another equally important consideration that all breeders must acknowledge is female discretion. Studies have repeatedly shown that females’ breeding behavior can sometimes be changed simply by changing available mates (see Chapter 4 for more on this subject).
A vaginal discharge normally persists throughout the estrus period, but it changes from the usual bloody red of proestrus to a light pink or straw color as ovulation time nears. During estrus, most bitches will flirt with males by backing up to them, flagging their tails in the males’ faces, urinating frequently, and generally acting seductive. Females will sometimes mount available males and simulate copulation.
Estrogen hormone levels remain about the same, and progesterone levels begin to rise during estrus. Bitches become receptive to males as a result of interaction between estrogen and progesterone. Standing heat lasts several days, sometimes a week. After the first day on which a bitch will receive a male, estrogen levels drop and progesterone levels increase greatly.
Another measurable hormone should be mentioned. It is called lutenizing hormone (LH) and it originates from the pituitary gland. LH levels peak in the female’s bloodstream at the time she will first stand for a male. It has been determined that ovulation (release of eggs from ovarian follicles) occurs one to three days after the LH peak. In certain problem breeders, this information is of critical importance (see Chapter 4, Potential Breeding Problems).
Ovulation
Of further importance and interest is the time of ovulation and its relationship to fertilization of the ova (eggs). According to research published by Sokoloweski in 1977, there are about 700,000 ova present in a bitch’s ovaries prior to puberty (see References, page 178). By the time of her second or third heat, a quarter of a million remain. A rather large number of these eggs are expelled from ovarian follicles during each estrus. That expulsion of ova from her ovaries is called ovulation.
TIP
Be skeptical of literature or old wives’ tales about breeding on specific days of the estrous cycle. Forced breeding or artificial insemination of a bitch on an arbitrarily assigned day may be doomed to failure.
Ovulation is spontaneous in the bitch; all mature ovarian follicles rupture at the same time, releasing their ova into the fallopian tubes (oviducts) that lead from the ovaries to the uterus. After releasing its eggs, each ruptured follicle shrinks and is thereafter called a corpus luteum (yellow-body). Unruptured follicles return to inactivity until the next estrus period.
Ova have a fertile life span of one to four days. If a bitch is bred, the actively swimming sperm find their way from where they are deposited in the vagina through the cervix, into and through the length of the uterus, and into the oviducts. Canine sperm cells have a vital life of about seven days.
Actual fertilization, that is the union of sperm and ova, occurs about six days after the LH peak. It takes place in the oviducts, and one sperm cell unites with a single ovum. Normal, healthy males’ ejaculates contain enormous numbers of viable sperm. Ovaries may produce dozens of ova, of which usually less than 20 are fertilized and remain viable. The number of embryos that survive, implant in the uterus, and grow to become puppies is only a tiny fraction of the potential.
Embryonic Implantation
Puppies’ lives begin on the road as the microscopic embryos travel along the oviducts and into the horns of the uterus over a period of eight to 12 days after breeding. Embryos finally implant (attach) in the highly vascular wall of the uterus about 17 or 18 days after mating. The number of embryos attached in each horn is not necessarily equal. In cesarean sections I commonly found one uterine horn to contain only one fetus or sometimes none, and half a dozen or more fetuses in the other. This has no known significance, and is mentioned only as a point of interest. Embryos do not normally attach in the body of the uterus.
Breeding Time
Since ovulation takes place a day or two after the bitch will first accept a male, it seems likely that a breeding program will be highly effective if we breed a bitch as soon as she will receive a male, then repeat the breeding two days later. Generally, conception is more likely following the earliest possible normal breeding.
Conception success depends on a bitch’s attitude, her general health, and how well her hormones are working. Most of us who have kept intact females can attest to the fact that when “turned out,” most bitches do quite well on their own. Nature’s timing seems to be highly effective.
Whereas males are usually indiscriminate in their choice of mates, many times females will select their partners with discretion, allowing only a particular male or two the privilege of mating with her. Unfortunately, they do not always make the most prudent choices when left to their own devices.
Puppies of a single litter may be sired by more than one male. That usually occurs if the bitch is allowed to cruise the streets to choose her own mate(s), and is bred by several males during her receptive period (a practice to be discouraged at all costs).
Artificially Induced Estrus
Using female hormones, it is fairly simple to chemically induce outward signs of normal estrus in bitches. Artificially producing a fertile estrus is quite another project. A great many research trials conducted over several decades have yielded inconclusive results. Some trials reported relatively consistent success using oral estrogens daily. Some of that work is difficult or impossible to reproduce in practice.
Other researchers in the past had relatively good and reproducible success using a series of injections of estrogens, lutenizing hormone (LH), and follicle stimulating hormone (FSH). Unfortunately, LH hormone is no longer commercially available.
Breeding chart, using averages for the species.
Diestrus (Formerly Called Metestrus)
The third stage of canine reproductive cycles is known as diestrus. In literature from prior years, this phase was called metestrus, and had a slightly different academic definition. For practical purposes, metestrus and diestrus are synonymous. Diestrus lasts about 60 days. It is currently defined as the period extending from the last day a bitch will accept a male until the termination of pregnancy. If she is not mated, a female’s outward appearance during diestrus is indistinguishable from anestrus (see below).
Cavalier King Charles Spaniel pups mature quickly and are extremely energetic.
Because ovulation is spontaneous in the bitch, it is not signified by outward signs, receptivity, or male interest. Once ovulation occurs, the follicles from which eggs were discharged begin to recede. Diestrus is under the influence of the hormone progesterone, which originates from those corpora lutea (plural of corpus luteum). Progesterone is often called the hormone of pregnancy, and is responsible for the maintenance of the uterus and the fetuses implanted and growing therein.
Anestrus
For about 100 to 150 days, bitches’ reproductive systems are in a quiet stage of the estrous cycle known as anestrus. One reference lists the anestrus period as two to nine months. As in other phases of the cycle, breed, condition, nutrition, and size influence the duration of anestrus. During that period, bitches exhibit no signs of heat. Their external genitalia (vulvas) remain normal in size and color, and there is no vaginal discharge. Their temperaments and