Why doesn't a dog have heat? - briefly
Dogs do not experience menstrual cycles like humans because they do not ovulate unless mated. Unlike humans who undergo regular menstrual cycles regardless of mating, dogs only release eggs and subsequently experience a heat cycle when there is a potential mate present.
Why doesn't a dog have heat? - in detail
The absence of a menstrual cycle, or "heat," in female dogs is a notable characteristic that sets them apart from many other mammals, including humans and other primates. This condition, known as induced ovulation, is intricately tied to the reproductive system's unique physiology and hormonal regulation.
In most mammalian species, females experience a menstrual cycle wherein the uterine lining (endometrium) thickens in preparation for potential fertilization. If fertilization does not occur, this lining is shed during menstruation. However, dogs do not follow this pattern. Instead, they exhibit what is known as an induced ovulation cycle, which involves a prolonged proestrus phase followed by estrus and then diestrus.
During the proestrus phase, the female dog's body prepares for potential mating by releasing hormones that stimulate the growth of ovarian follicles. However, unlike in species with regular menstrual cycles, the dog's uterine lining does not thicken significantly during this phase. Instead, it remains relatively thin and unchanged. This lack of significant endometrial development is one of the primary reasons why female dogs do not experience a menstrual bleed.
Once mating occurs, the hormonal balance shifts to trigger ovulation. Specifically, the presence of luteinizing hormone (LH) surges in response to copulation, prompting the release of mature ova from the follicles. This process is known as induced ovulation because it requires the physical act of mating to stimulate the release of LH and subsequent ovulation. In species with regular menstrual cycles, ovulation typically occurs spontaneously without the need for external stimuli.
Following ovulation, the corpus luteum forms in the ovaries, which secretes progesterone to maintain the pregnancy if fertilization has taken place. If the dog does not become pregnant, the corpus luteum degenerates, and hormonal levels return to baseline. This phase is known as diestrus and lasts approximately 60-90 days before the cycle begins anew with proestrus.
The absence of a menstrual bleed in dogs is thus a result of their unique reproductive physiology, characterized by induced ovulation rather than spontaneous cycling. This adaptation allows for efficient reproductive strategies, ensuring that resources are only invested in pregnancy when mating has occurred and fertilization is likely.