Since all dogs are different, they will all react differently at the start of their estrous cycles. The earliest sign that a female dog may be in heat is a sudden change in personality, energy level or aggressive behavior. Many dogs will become more reserved or aggressive just before going into heat, but some will exhibit the opposite behavior, becoming more outgoing and friendly. Whatever the case may be, a sudden and otherwise unexplainable change in behavior is a sign that the estrous cycle may have begun or may be about to begin.
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The more telltale signs are physical rather than behavioral. The dog's vulva may become swollen or saggy and may change colors to a more vibrant pink or red. The nipples may also become swollen, more sensitive and deeper in color. Her level of thirst and rate of urination usually increase noticeably. Some dogs will also exhibit vaginal bleeding, although the volume of blood will typically be much less than that of a menstruating adult human. The amount of blood is also related to the size of the dog, with smaller dogs producing little or no blood at all.
The Rate of the Estrous Cycle
Most dogs go into heat for the very first time at around six months of age. Although just as it is with humans, there is no specific timetable for sexual maturity. Dogs that have not gone into heat by fourteen months should be examined by a veterinarian.
Estrous cycles tend to last roughly three weeks and usually reoccur every six to eight months. Unless a dog has been spayed, it should continue to go into heat at regular intervals well into old age.
The Effect of the Estrous Cycle on Male Dogs
Dogs in heat will leave telltale scents in their urine and vaginal discharges, and these scents can attract male dogs from far and wide. If they are allowed to get close enough to one another, virtually any healthy male dog will attempt to mount a dog in heat and virtually every dog in heat will allow this to happen. In the absence of all other indicators of the estrous cycle, a female dog that suddenly attracts strong interest from male dogs is likely to be in heat.
Because male dogs' attraction to these scents are so powerful, it is recommended that humans change clothes and wash thoroughly after direct contact with a dog while she is in heat. Otherwise, one might receive the unwanted advances of male dogs who pick up the scent.