In this lecture we will discuss basic reproduction of the dog.
Let's begin by going over the reproductive anatomy of dogs and pointing out some unique features.
Starting with the male. One unique feature of male dogs as compared to many other mammals is the presence of only one accessory sex gland. On this diagram, locate the prostate. The prostate gland is the only accessory sex gland of male dogs. The prostate produces a watery secretion that is ejaculated prior to the sperm-containing fraction of the ejaculate.
Now find the penis. The dog's penis itself has two unique features. One is the os penis, or penis bone. This functions to provide rigidity for intromission, or entering the female's vagina. Once inserted, another important feature comes into play…the bulbus glandis. This area of the penis will swell when the penis is inside the vagina, which forms a sort of locking mechanism to allow for prolonged contact during and after ejaculation. This increases the chance of successful fertilization.
The swelling of the bulbus glandis, once inside the female reproductive tract, causes what is known as the copulatory tie. This is sometimes also called the coital tie, or just tie.
After intromission, the male will usually step down from the mounting position so that the two animals are in the position you see here.
During the tie, the two animals are physically unable to separate. The copulatory tie usually lasts anywhere from five minutes to an hour, and after this time the animals will separate naturally. It is important that humans to allow this natural separation to occur. Major damage can be done to the reproductive tracts when humans try to physically separate the animals prematurely.
It's not unusual for dogs to mate and tie several times.
Here we see a picture of a portion of the reproductive tract of a female dog, or bitch.
The paired ovaries are the female gonads; they contain the oocytes and are responsible for much of the dog's reproductive hormone production.
The oviducts are small ducts that transport the oocytes from the ovary to the uterus. The uterine horns join and form the uterine body anterior to the cervix. (Remember that anterior means toward the head of the animal.)
The cervix is a structure consisting of dense connective tissue that connects the uterus to the vagina.
And the vagina is the female copulatory organ that opens at the external genitalia, or vulva.
The mesometrium is part of a larger support system throughout the abdominal cavity; the mesometrium is specifically the part that supports the uterus.
Other terminology that we need to make sure everyone's on the same page when we're talking about reproduction. Estrous cycle—similar in concept to the menstrual cycle of humans. Each species has its own estrous cycle. We'll discuss the estrous cycle of the dog during this lecture.
Next on the list is oocyte. An oocyte is a developing egg cell.
Follicles are fluid-filled blisterlike structures on the ovary. Within each follicle is an oocyte and its supportive cells. The follicles are also very important structures in hormone production.
And ovulation. This is what occurs when the follicles rupture, thus releasing their oocytes.
The estrous cycle in dogs varies tremendously depending on breed as well as size of the animal.
The estrous cycle may begin as early as six months old in some smaller breeds and as late as 18 months in some of the larger breeds.
The duration of the estrous cycle in one particular dog is fairly consistent over time; but again, there's a great deal of variability among individuals and among breeds. To cycle once every seven months is a very rough average among dogs. Some breeds will have a tendency to cycle once every four months (for example, the German Shepherd) while others, such as the Basenji will tend to cycle once every 12 months.
The estrous cycle is a true cycle that progresses through the four stages and then repeats. So let's discuss the individual stages of the dog's estrous cycle now.
Proestrus. Proestrus is the stage of getting ready. This is the time when a bitch's reproductive tract is preparing for the possibility of breeding with a male dog.
Proestrus is what is happening when we observe a bloody discharge from the dog's vulva. This is the result of the loss of red blood cells by leaking of the blood vessels in the uterus as it prepares for mating.
During proestrus a bitch may become attractive to males and she may play and interact with them, but she will not allow mounting or mating to occur.
Estrus. Estrus is the period of sexual receptivity. Note that the stage of estrus is a noun and is spelled differently than the adjective describing the estrous cycle.
Proestrus was getting us ready for the possibility of mating, and estrus is when the bitch is receptive to the male so that mating can occur.
Estrus is also the period in which ovulation occurs.
Estrus technically begins on the first day that the female will stand for mounting and ends when the female no longer accepts the male for mounting.
The period following estrus is called diestrus, or some sources refer to it as metestrus.
There are two outcomes for diestrus in the dog.
If mating occurred and fertilization was successful, the dog will be pregnant and will enter gestation as her diestrus.
If fertilization was not successful or if mating did not occur, the dog will enter a pseudopregnant state. The reproductive system undergoes very similar hormonal changes whether or not the dog got pregnant. This is a unique aspect of dog reproduction.
The reason this happens is that following estrus the reproductive tract becomes under the control of progesterone. This is the hormone that is required to maintain pregnancy.
Where does progesterone come from?
After an ovarian follicle ruptures and ovulates, it becomes a transient structure called a corpus luteum, which is Latin for yellow body. The plural is corpora lutea.
The corpus luteum produces progesterone. After ovulation, there are several corpora lutea present and they will produce progesterone, again whether or not the dog actually got pregnant.
Again, this is not typical. Dogs are unique. They're different because the corpora lutea will function and will secrete progesterone whether or not the dog is pregnant, therefore leading to the two different outcomes for diestrus—pregnant or not. If not pregnant, the dog spends her diestrus in what is known as a pseudopregnant state.
The effects of pseudopregnancy vary considerably among bitches - some show no outward signs at all, but others may show physical signs such as increased appetite, weight gain, a distended abdomen, or enlarged mammary glands…even lactation in extreme cases.
Some dogs may also exhibit behavioral effects such as being moody or withdrawn. One common behavioral effect is to display maternal or nesting behaviors, where a dog will nest and treat toys and other animals as if they were her puppies.
Again, there is much variation as to whether a dog displays the effects of pseudopregnancy.
Anestrus is the final stage and represents another unique aspect of reproduction in dogs in that the bitch has estrous cycles with long period of ovarian inactivity between cycles.
Again, the length of anestrus varies greatly among dogs with factors such as breed, age, overall health, and environment all playing roles.
This slide shows an example of timing in an estrous cycle of a dog. If we take a look at the averages we can see how it works out that a bitch will cycle approximately every seven months. But remember, this is just an average and there are tremendous variations among our dogs.
Now let's point out some details about ovulation, which we know occurs during estrus.
How many oocytes are ovulated? Here again we see tremendous variation, with smaller breeds releasing fewer oocytes and therefore having smaller litter sizes than larger breeds. Example ranges are shown here.
The fact that dogs ovulate multiple oocytes and bear multiple offspring, or litters, classifies them as polytocous.
Another unique aspect of dog reproduction is that unlike many species, the oocytes of a dog are not yet ready to be fertilized when they are ovulated. They will need 2-3 days to mature before fertilization can occur.
Gestation in dogs lasts approximately 63-65 days.
The size of the litter can affect the length of gestation, with larger litters having shorter gestations by about one to three days compared to smaller litters.
The first sign that a bitch is pregnant is observed around five weeks, in the form of enlargement of the abdomen and mammary glands.
The bitch will gradually gain weight over the course of gestation. Her mammary glands will further enlarge during the last few days in preparation for feeding the newborn pups.
Within 24 hours of whelping, or giving birth, the bitch's body temperature will start to drop slightly. Temperatures will drop from a regular temperature of approximately 101.5 degrees Fahrenheit to 100 degrees or lower. This is one indicator that is easily observed and so is frequently used by caretakers to estimate impending parturition.
The bitch will become increasingly restless as parturition approaches.
Parturition then begins with a series of uterine contractions that lead to expulsion of the first fetus. The placenta for each fetus will often pass shortly thereafter. The mother licks away the membranes to clean and stimulate the puppy, and she may also eat the placenta.
The process will continue until all of the puppies have been born. At that time the contractions will cease, and the mother will continue to clean and nurse her young.
