Do Dogs Have Periods?
Questions like "Do dogs have belly buttons?" and "Do dogs have periods?" tend to arise from anthropocentrism, the notion that the way humans experience the world is the only way that matters. The first of these questions presumes that, because humans have belly buttons, surely it's unusual or remarkable that dogs do not.
In fact, humans are the outliers; it is we who are unusual in the grand scheme. "Do dogs have periods?" is a similar question. Among the species that constitute the animal kingdom, only female primates -- ranging from the lemur to the human to the gorilla -- have periods in the way we generally understand the term.
Menstrual cycle vs. estrous cycle
In short, no, female dogs do not have periods in the same way that human females do. While mammals of all sorts -- including dogs and humans -- share the same basic reproductive organs, the ways those organs function is not similar. Human women go through a menstrual cycle, a process of preparation for egg fertilization that lasts an average of 28 days. Female dogs, on the other hand, go through an estrous cycle, similar in purpose, but different in execution, which lasts an average of 180 days.
One reason we ask, "Do female dogs have periods?" is that bloody vaginal discharge is present in both the menstrual cycle and the estrous cycle. In humans, the uterus builds up nutrients for the anticipated growth of a fetus. When an egg goes unfertilized, that material is secreted from the body. In dogs, when an egg is unfertilized, that nutrient-rich material is absorbed by the body over an extended period of time. The bloody discharge that emerges from female dogs originates in the vagina, not the uterus, and serves a different function. Rather than marking the conclusion of a cycle, in dogs, it signals the onset of fertility.
The estrous cycle in dogs
Dog menstruation is a misnomer, since dogs do not menstruate. The estrous cycle in dogs consists of four major phases: proestrus, estrus, diestrus, and anestrus. A dog's vagina discharges blood and other fluids during the first two of these phases, most heavily during proestrus. When we say a female dog is in heat, this constitutes the first two phases, proestrus and estrus.
During proestrus, which can last from three to 17 days, a female dog's body produces large amounts of estrogen. In dogs, this is accompanied by the start of bloody vaginal discharge, which is dark red to begin with, and caused in part by excessive hormone and pheromone production. Dogs also urinate much more frequently during proestrus. The hormones and pheromones in the blood and urine attract potential mates over large distances.
Estrus is the shortest part of the estrous cycle, lasting from four to seven days. This is typically when a dog is primed for mating and fertilization. During estrus, bleeding tends to continue, though it may slow and take on a lighter tint. Discharge in estrus can range from a lighter shade of red to pink to straw-colored. In this phase, a dog may sleep more, be less inclined to play, and begin building a nest in anticipation of pregnancy.
At the start of the third phase, diestrus, the bloody discharge ceases, whether the dog's egg has been fertilized or not. Diestrus lasts approximately 65 days, about the same span of time that marks a dog's pregnancy. If the egg is not fertilized, diestrus is the phase during which the nutrients that have accumulated to nourish the anticipated litter of puppies are reabsorbed by the body. The final phase of a dog's estrous cycle is anestrus, and for this two-to-three month span, the dog is sexually and hormonally inactive.
Dogs do not experience menopause, either
Because the estrous cycle is so drawn out, female dogs tend to go into heat an average of two times per year. Depending on the size and breed of the dog, the capacity to breed can begin as early as six months of age, or as late as 24 months of age. Given that disparity, the length of an estrous cycle is also size- and breed-specific. Some small dog breeds may go through their reproductive cycle up to three times a year, while some very large dog breeds may complete a full cycle only once per year. Yet another difference between human and canine reproduction is that dogs remain capable of breeding well into seniority, and do not go through menopause.
Spay and neuter your dogs!
There is a bit of folk wisdom that is worth dispelling before we conclude. It is thought that a dog's long-term health is improved if she is allowed to go into heat once before being spayed. Much like, "Don't cross your eyes or they'll stay that way," this is simply untrue. Dogs can safely be spayed and neutered from six to eight weeks of age thanks to modern veterinary practices.
The benefits of spaying your female dogs are many. Spaying your female dog reduces the chances that she will develop breast, ovarian, and uterine cancers later in life. It prevents her from developing a deadly bacterial uterine infection called pyometra, which any unspayed female dog can contract at any age, but the risk increases the older she gets. Get your dogs spayed and neutered! It's better for everyone!
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