A dog's reproductive cycle, like any other animals, is controlled by the environment so that puppies will be born in the best possible conditions if the animals were in the wild. This means that typically in the late winter and into early spring as well as in the late summer or early fall if not bred in the first cycle. Different breeds will have different estrus cycles and the cycles will also vary as the female matures. During the breeding season females start to come into their estrus cycle, which is known as coming into heat. Females that are spayed and males that are neutered do not go through this cycle as the hormones that are needed to trigger the changes in the body are produced in the reproductive organs that are surgically removed during the spaying and neutering process.
There are actually three phases of the estrus cycle or heat of for the female. These are known by the major hormones released during each stage. A brief description of each stage of the cycle and typical behavior the female will show are listed below:
Follicular stage - at this stage mature eggs in the female's ovary start to produce a hormone known as estradiol 17B, which in essence triggers the next stage. The ovaries are full of individual, tiny eggs that are each covered by a tiny sac, known as a follicle. The brain and pituitary of the dog produces a hormone called FSH or follicle stimulating hormone that causes the eggs to mature. During this stage of the estrus cycle the owner will have what he or she considers the "normal" female. She will be happy, content and very active, not overly concerned about male dogs nor typically worried about other females in the area.
Luteal phase - during this phase the egg moves from the ovaries down the Fallopian tubes to the uterus. At this time the egg ruptures from the follicle and produces a yellow mass known as the corpus luteum. This occurs because the pituitary starts to release a hormone known as luteinizing hormone due to the presence of the estradiol 17B. The corpus leteum in turn produces progesterone, which triggers the physical behaviors seen in females in heat. This is when the female is receptive to being bred by the male. At this stage owners may start to wonder what has gotten into their normally well behaved female. It is not uncommon for females in this stage to whine, bark, pant excessively and even have trouble with vomiting and diarrhea. They may also urinate excessively, even in the house when fully housetrained. The vulva or exterior genital area will become noticeably swollen and a bloody discharge is not uncommon. This can be very heavy in some females and less noticeable in others. There are special diaper like pads that can be used to prevent this from becoming a problem if the female is kept in the house. Since there are multiple eggs released and the female will breed more than once if given the opportunity, it is possible for more than one dog to be the father of the litter. If the female does breed and conceive, the fertilized eggs attach to the walls of the uterus and continue to produce progesterone, preventing the dog from releasing any other eggs and allowing the body to prepare for pregnancy.
If the eggs are not fertilized they are naturally removed from the body and no progesterone is produced, telling the female's body it is not pregnant. This triggers the third phase or the quiescent phase when all is quiet until the next follicular stage is triggered by the hormones in the brain.
In Heat Behaviors
Different females will show different heat behaviors. Many become silly, very affectionate, and may tend to become more prone to paying attention to all other dogs. They will urinate frequently in the presence of other dogs, either male or female, and will often assume mating postures when they are ready to be bred. If the female is not interested in breeding and the male approaches, she can be quite aggressive in telling him to back off. Typically the younger females will have more exaggerated heat behaviors that more mature females.
Some females are reported to become very nurturing during this phase. They may adopt another dog or even a cat as a puppy, licking it and trying to get it to nest with them. Some will even develop a strong attachment to a stuffed toy and carry it around wherever they go. Many females will continue this behavior during their pregnancy once they are bred. Some may even develop a very strong attachment to a family member and become increasingly anxious if left alone or outside of the house, even for a short time.
Females can also go through a false pregnancy that can occur after an estrus cycle, regardless of whether the female actually mated or not. A false pregnancy can result in a discharge form the vulva, swelling of the mammary glands and all the normal "pregnant" behaviors. False pregnancies are more common in some breeds than others and typically if a female is prone to false pregnancies she should be spayed to prevent any complications.
Males will typically respond to females in heat by become more aggressive, dominant and non-compliant, especially in the presence of other intact males. This is relatively normal behavior and owners must continue to use appropriate training methods to control males even in the presence of in heat or receptive females. This really is the same if the male is a large or small breed, both can be very difficult to work with and even aggressive to owners when in the presence of a female in heat. Most dogs that are used for breeding purposes are very well trained at all times, largely due to the ability of the breeder to work with these dogs.
A key point to remember is that a neutered male can remain fertile for up to one month after neutering, so keeping him away from females in heat a during this time periods is very important to prevent unwanted or unplanned pregnancies.
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