only those animals that are excellent representatives of the breed.
Estrous Cycle
Please note that two very similar words are used frequently in this book. The word spelled with an “o” (estrous) refers to females’ reproductive cycles. It includes all reproductive phases of intact (unspayed) bitches’ lives. Another word that is pronounced identically (estrus) is spelled without an “o,” and refers to “standing heat” periods, which are the second specific phases of estrous cycles. It is important to understand the differentiation. I will endeavor to use estrous only in combination with the word “cycle.”
This bitch in heat is wearing a diaper and looking wistfully out the back door.
Phases of the Estrous Cycle
Canine heat cycles are divided into four separate phases. Each phase has specific outward signs associated with it, as well as attitude, hormonal, and other internal changes. The paragraphs discussing hormone changes may seem academic, but if you read them carefully, you will find the information fascinating. The hormone discussion allows one to more fully understand the attitude and appearance changes of the bitches as they progress through their estrous cycles.
Blood serum hormone levels can best be determined by laboratory analysis using a technique called radioimmunoassay (RIA). Other techniques are adapted for home use. The home ELISA (enzyme-linked immunoabsorbent assay) test for progesterone is less accurate, but often valuable if interpreted correctly.
Proestrus
The first estrous cycle phase is called proestrus. Its outward, physical signs are obvious. All the female’s reproductive tissues thicken, her vulva swells, and visible genitalia becomes reddened. Bloody vaginal discharges are produced, and the bitch usually spends an extraordinary amount of time licking her external genitalia. She may act nervous and her appetite is reduced. In early proestrus, some bitches are snappy toward other animals.
As the days of proestrus count down, she tries to escape from confinement, seeking a mate. The bloody discharge attracts males from near and far. If housed with intact males, they will begin paying court to the bitches. If females are allowed outside, male dog visitors should be expected.
Prior to puberty, intact bitches’ internal reproductive organs (except their ovaries) are quite undeveloped. The actual weight and dimensions of prepuberal (before the first heat) uteruses are significantly smaller than those of sexually mature animals. As puberty is reached and proestrus begins, the uterus enlarges in response to hormones secreted from the ovaries. Although that enlargement is unapparent to the owner, it can sometimes be palpated by veterinarians with practiced fingers, and it is easily demonstrated by ultrasound imaging.
Proestrus lasts from two to 27 days, more typically seven to ten days. The average length of proestrus is generally accepted to be nine days.
During this phase, estrogen hormone levels rise gradually in the bloodstream, and levels of the progesterone hormone remain relatively low.
This Cavalier King Charles Spaniel has a rather large litter of six fine puppies.
Another hormone called follicle stimulating hormone (FSH) originates from the pituitary gland. For two or three weeks prior to the onset of proestrus, serum levels of FSFI rise. Its effect is to stimulate production of follicles within the ovaries. These follicles contain microscopic eggs called ova.
Estrus
The next phase of the reproductive cycle is termed estrus. It corresponds to the stage often referred to as “standing heat,” when bitches will stand for breeding. It lasts from extremes of two to 21 days, more typically six to ten days, and averages nine days. It is during estrus that a bitch will mate with a male; spontaneous ovulation takes place during this phase.
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.
