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 ovulations and uterine lining sheddings 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! Do not 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.

土豆没有月经，也没有发情周期。然而它们确实有繁殖周期并且是季节性的。月经和发情这两个术语，以及它们的拼写，那是关于什么的呢？有什么不同？

好的部分是相似性。将土豆排除在比较之外，我们可以发现涉及月经和发情的女性生殖策略之间有许多相似之处。所有胎盘动物要么有月经，要么有发情周期（发情？）。这些过程的一个主要相似之处是为胚胎植入子宫做准备。对于英语来说幸运的是，所有胎盘动物都使用类似的激素控制来完成这项工作。例如，雌激素的持续升高为子宫内膜（子宫内膜）做好准备，以便胚胎侵入并占据其空间。雌激素当然来自于正在发育的卵母细胞中。颗粒细胞一路走好！通过下丘脑/垂体系统对体内雌激素负荷增加的反馈反应最终导致黄体生成激素的释放，刺激所有哺乳动物的排卵。然后，一个或多个卵子就可以在输卵管中受精。如果受精成功，胚胎通常会植入子宫，子宫内膜会随着雌激素和孕激素水平的增加而不断生长和分化。

那么……月经和发情期有什么区别呢？如果你愿意的话，有两个大人物。

第一：如果排卵后没有怀孕会怎样？在来月经的动物中，子宫内膜脱落并通过子宫颈和阴道排出。血液和细胞碎片的释放是人类月经周期中“月经”或月经（拉丁语为每月）的基础。发情期怎么办？最大的区别是发情动物不会月经（有趣的是这是如何运作的！）。相反，在没有怀孕的情况下，这些发情雌性通过吸收或重组子宫上皮内膜来替换子宫内膜，而不是通过将其脱落并排出内膜碎片和血液。然而，在这两种替换子宫内膜的情况下，下一个生殖周期都会对随后增加的雌激素水平做出反应，并且子宫开始生长新的内膜。新鲜的子宫内膜对于着床很重要。哺乳动物不会长时间保留子宫内膜，而是在每个周期中准备一个全新的表面。胚胎确实需要高度维护。

处理子宫内膜——好吧——检查一下。另一个大的区别是什么？行为。让我们解释一下。有月经的物种很少见。其中包括一些灵长类动物，包括人类、一些蝙蝠、鼩鼱和一种老鼠。它们表现出规律的月经，不一定像人类那样每月一次，而是每年有几个激素、排卵和子宫内膜脱落的周期。月经物种的雌性也全年都愿意交配。

另一方面，发情动物（如狗）的雌性每年可能只有一到两个周期，其中雌性激素增加，这会导致子宫内膜的生长和准备。这些荷尔蒙动态也会导致女性行为的显着变化。发情期一词源自拉丁语，意为“疯狂”或“发情”，指的是雌性行为发生变化、突然愿意交配的时期。虽然这种狂热在附近渴望交配并愿意争夺交配的雄性中可能更为明显，但发情雌性的这种交配接受能力是由荷尔蒙决定的，并且与发情期或排卵期一致。说得通！当有卵时进行交配。

但是所有这些错别字怎么办？发情？发情？发情？啊啊！！选择一个并坚持下去！！实际上，“estrus”是一个名词 (‐us)，独立存在，但“estrus”(‐ous) 是一个形容词，修饰名词。因此，“发情”动物可以“处于发情期”，并且可以有一个“发情”周期。不过，“Oestrus”只是“发情”的“其他”英语拼写，比如“oestrogen”（如果你必须这样做的话）！不要让它卡在你的食道里！

这里要强调的是，对于一些没有绝育的母狗的宠物主人来说，母狗在发情周期的早期可能会出现血性分泌物。然而，这不是月经。相反，这种分泌物来自阴道壁而不是子宫内膜。在一个发情周期内没有怀孕的发情动物将进入“无发情区”——不发情，直到下一轮循环。绝情期是雌性不接受交配的时期，子宫内膜太薄而无法接受胚胎植入。新鲜的最好！虽然有些动物物种全年有多个发情周期，例如小鼠，但其他动物，例如熊、狐狸和狼，通常每年只有一个发情周期。哎呀，每年一两次繁殖机会。你确实需要把这件事做好。考虑环境如何影响发情的时间或频率是很有趣的，甚至可能是可怕的。也许我们会一边吃着一篮子炸薯条来思考这个问题。