Reproduction of Dog

Life cycle
In domestic dogs, sexual maturity (puberty) begins to happen around age 6 to 12 months for both males and females, although this can be delayed until up to two years old for some large breeds. Adolescence for most domestic dogs is around 12 to 15 months, beyond which they are for the most part more adult than puppy. As with other domesticated species, domestication has selectively bred for higher libido and earlier and more frequent breeding cycles in dogs, than in their wild ancestors. Dogs remain reproductively active until old age.

Most female dogs have their first estrous cycle between 6 and 12 months, although some larger breeds delay until as late as 2 years. Females experience estrous cycles biannually, during which her body prepares for pregnancy, and at the peak she will come into estrus, during which time she will be mentally and physically receptive to copulation.

Dogs bear their litters roughly 56 to 72 days after fertilization, with an average of 63 days, although the length of gestation can vary. An average litter consists of about six puppies, though this number may vary widely based on the breed of dog. Toy dogs generally produce from one to four puppies in each litter, while much larger breeds may average as many as 12 pups in each litter.

Spaying and neutering
Neutering (spaying females and castrating males) refers to the sterilization of animals, usually by removal of the male's testicles or the female's ovaries and uterus, in order to eliminate the ability to procreate, and reduce sex drive. Neutering has also been known to reduce aggression in male dogs, but has been shown to occasionally increase aggression in female dogs.

Animal control agencies in the United States and the ASPCA advise that dogs not intended for further breeding should be neutered so that they do not have undesired puppies.

Because of the overpopulation of dogs in some countries, puppies born to strays or as the result of accidental breedings often end up being killed in animal shelters. Neutering can also decrease or eliminate the risk of hormone-driven diseases such as mammary cancer, as well as undesired hormone-driven behaviors. However, certain medical problems are more likely after neutering, such as urinary incontinence in females and prostate cancer in males. The hormonal changes involved with sterilization are likely to somewhat change the animal's personality, however, and some object to neutering as the sterilization could be carried out without the excision of organs.

It is not essential for a female dog to either experience a heat cycle or have puppies before spaying, and likewise, a male dog does not need the experience of mating before castration.

Female cats and dogs are seven times more likely to develop mammary tumors if they are not spayed before their first heat cycle. Dog food containing soybeans or soybean fractions have been found to contain phytoestrogens in levels that could have biological effects when ingested longterm.

Gender-preservative surgeries such as vasectomy and tubal ligation are possible, but do not appear to be popular due to the continuation of gender-specific behaviors and disease risks.

The Reproductive Anatomy of the Female Dog
- Female dogs (correctly called bitches) have 3, 4 or 5 pairs of breasts (mammary glands ) placed in two rows running up the sides of their abdomen from the groin to the chest for nursing puppies.
- The number varies and one side can have one or two more than the other side.
- The nipples on a female who has not yet had her first heat are quite small, but they swell with the first heat and never return to 'puppy size'.
- If the bitch has been bred and is pregnant, then about 35 days after the mating the nipples will turn a deep pink colour and will begin to enlarge.
- The mammary glands produce the milk to feed the puppies and amazingly enough they manage to manufacture the right supply to meet the demand.
- Bitches who are extremely maternal have been known to produce milk for a starving pup or another animal baby even though they are not pregnant or nursing their own litter.
- The mammary glands are prone to cancerous tumors because of the estrogen produced.
- Once the bitch has been "spayed", (fixed or altered) the source of the estrogen has been removed and the chance of breast cancer developing is greatly reduced.
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- The vagina opening is quite small on a puppy too, and enlarges with the onset of heat and never returns to "puppy size", but it does decrease in size after the heat is finished.
- The vagina inside the dog is NOT a straight tube.
- The first 1 - 2 inches of the vagina (depending on the size of the dog) is at a 45 degree upward angle and then it levels out and runs about another 3 - 4 inches ending at the 'cervix' - the bottom end of the uterus.
- Also, inside the vagina, partway up, is the opening to the bladder - the tiny urethral opening, which allows the urine to run out of the vagina.
- Inside the vagina, as well, is a very strong ring of muscles, placed at about where the angle of the vagina changes.
- These muscles have a purpose.
- Once the male has entered the vagina, the bitch tightens these muscles to hold the penis in place.
- This 'tightening action' creates what is commonly known as a 'tie'.
- Once the tie has occurred, you MUST WAIT until the female relaxes the muscle and breaks the tie to release the male.
- If you attempt to pull them apart, you will cause serious pain and damage to both the male and the female dog.
- Pulling them apart will not prevent a pregnancy.
- By the time the female has 'tied' the dog, he has already ejaculated and enough of the sperm are on their way to the fallopian tubes, (where her eggs wait to be impregnated) to make a pregnancy.
- The 'tie' only assures that the seminal fluid - the ejaculate - will not leak back out and the slower sperm have a chance to swim up to the fallopian tubes.
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- The uterus in the female dog is not like a human uterus which is pear shaped.
- The dog uterus is Y shaped.
- The base of the Y is the cervix, which is usually tightly closed to prevent infection from getting into the uterus.
- It opens for the birthing of puppies.
- The two arms of the Y is where the whelps (puppies) develop, each in its own individual compartment.
- The 2 fallopian tubes sort of dangle over from the top of each arm of the Y and are each attached to the 2 ovaries where the eggs are made and then released to the fallopian tubes when the egg is mature.
- The eggs are not released all at once by the ovary to the fallopian tube.
- Bitches are multiple ovulators, which means that they make several eggs, and release them over a period of several hours, or even a day or so.
- It is the bitch who determines the number of puppies to be born.
- The male - the dog - determines the sex of the puppy.
- The ovaries not only make the eggs, they also produce the estrogen and the progesterone hormones.
- Surgical removal of the ovaries (spaying) therefore prevents the egg production as well as the cancer causing estrogen.
- The uterus and fallopian tubes are removed at the same time as the ovaries.
- Spaying is an operation requiring an anaesthetic and stitches to close the abdominal imcision. The dog needs recovery time, but she recovers her activity level much faster than a human and do not seem to have the same pain level that humans have.

Overpopulation - United States
According to the Humane Society of the United States, 3–4 million dogs and cats are put down each year in the United States and many more are confined to cages in shelters because there are many more animals than there are homes. Spaying or castrating dogs helps keep overpopulation down. Local humane societies, SPCAs and other animal protection organizations urge people to neuter their pets and to adopt animals from shelters instead of purchasing them. Several notable public figures have spoken out against animal overpopulation, including Bob Barker. On his game show, The Price Is Right, Barker stressed the problem at the end of every episode, saying: "Help control the pet population. Have your pets spayed or neutered." The current host, Drew Carey, makes a similar plea at the conclusion of each episode.

Ref : http://en.wikipedia.org and http://www.petbitsforyou.com