Do I Menstruate or “Estruate”? – is this just a potato, potatoe thing?

Potatoes do not menstruate, nor do they have estrous cycles. Yet they do have reproductive cycles and are seasonal. The terms menstruation and estrous though, and their spellings too, what is that about? What is the difference?

The good part is the similarity. Excluding potatoes from the comparison, one can identify many parallels between female reproductive strategies involving menstruation and estrus. All placental animals either menstruate or have estrous cycles (estruate?). A major similarity in these processes is in preparation of the uterus for embryo implantation. Fortunately for the English language, all placental animals use similar hormonal controls for this job. For example, a continuous rise in estrogen prepares the uterine lining (endometrium) for an embryo to invade and take over its space. The estrogen of course comes from the growing follicles harboring the developing oocytes. Way to go granulosa cells! Feedback response to this increasing estrogen load in the body through the hypothalamic/pituitary system eventually causes the release of a spike of luteinizing hormone to stimulate ovulation in all mammals. An egg, or many, are then available for fertilization in the fallopian tubes. If fertilization is successful, the embryos usually implant in the uterus, whose lining has been growing and differentiating in response to increasing estrogen and progesterone levels, respectfully.

So …what is the difference then between menstruation and estrus? Two biggies if you will.

Number one: What happens if a pregnancy does not occur following ovulation? In animals that menstruate, the uterine lining is sloughed off and discharged through the cervix and vagina. This release of blood and cellular debris is the basis of the human “period” or menses (Latin for monthly) in the menstrual cycle. What about in estrus? The big difference is that estrous animals do not menstruate (funny how that works!). Instead, in the absence of a pregnancy, these estrous females replace their endometrium by resorbing or reorganizing the uterine epithelial lining, not by sloughing it off and discharging the lining debris and blood. In both cases of replacing the endometrium though, the next reproductive cycle responds to subsequent increasing estrogen levels, and the uterus begins growing a new lining. Something about a fresh uterine lining is important for implantation. Mammals do not hold on to their endometrium for very long, and instead they prepare a brand-new surface with each cycle. Embryos must really be high maintenance.

Dealing with the uterine lining – ok – check. What’s the other big difference? Behavior. Let us explain. Species that menstruate are rare. These include some primates, including humans, a few bats, shrews, and one mouse species. They exhibit regular menses, not necessarily monthly like in humans, but several cycles of hormones and uterine lining shedding each year. The females of menstruating species are also receptive to mating throughout the year.

On the other hand, females of estrous animals, like dogs, may have only one or two cycles a year in which the female has an increase in estrogen, which leads to growth and preparation of the uterine lining. These hormonal dynamics also cause significant changes in female behavior. The term estrus, from the Latin for frenzy or being “in heat,” refers to a time when behavior in the female changes, and she is suddenly receptive to mating. While this frenzy may be more obvious in the nearby males who are anxious to mate and willing to fight over it, this mating receptivity in estrous females is determined hormonally and is in concert with the period of estrus, or ovulation. Makes sense! Mate when an egg is available.

But what about all these typos? Estrous? Estrus? Oestrus? Argh!! Pick one and stick with it!! Actually, “estrus” is a noun (-us) and stands on its own but “estrous” (-ous) is an adjective and modifies a noun. So, an “estrous” animal can be “in estrus”, and can have an “estrous” cycle. “Oestrus” though, is just that “other” English spelling for “estrous”, like “oestrogen” if you must! Don’t let that get stuck in your oesophagus!

We should emphasize here for some pet owners of female dogs that have not been neutered, the female dog may show a bloody discharge early in the estrous cycle. This is not a menstruation however. Instead, this discharge originates from the walls of the vagina and not from the uterine lining. Estrous animals that do not become pregnant in an estrous cycle will enter the “no-estrous zone” – anestrus, until the next round of cycling. Anestrus is a period when females are not receptive to mating, and the uterine lining is too thin to accept an embryo for implantation. Fresh is best! While some species of animals have many estrous cycles throughout the year, like mice, others, such as bears, foxes and wolves, usually have only one estrous cycle each year. Yikes, one or two chances each year to reproduce. You really do need to get this right. It is interesting, perhaps even scary though, to consider how the environment impacts the timing or frequency of estrus. Maybe we will ponder this over a basket of curly fries.