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The surprising difference between estrus and menstruation: Why don't some animals experience menstruation?
In nature, animals reproduce in different ways, and these differences are not just behavioral but also physiological. Many mammals have an estrous cycle, while humans and a few other species have a menstrual cycle. This article will take a closer look at the differences between the two, and why some animals don't experience menstruation.
The estrous cycle is a series of physiological changes triggered by reproductive hormones and is usually not accompanied by the shedding of the endometrium.
Basic concepts of estrous cycle
The definition of estrous cycle comes from the Latin "oestrus", which means "enthusiasm". This term originated from ancient Greek and originally described a mythical insect. After many mammals reach sexual maturity, they will begin to enter the estrous cycle. This cycle will be affected by a variety of factors, including species, health status, and environmental changes.
The difference between estrous cycle and menstrual cycle
The most fundamental difference between the estrous cycle and the menstrual cycle is the treatment of the endometrium. In the absence of pregnancy, animals with estrous cycles resorb the uterine lining rather than shedding and expelling it like humans do. This means that the endometrium is maintained only if pregnancy occurs and is reabsorbed if fertilization does not occur.
Humans, elephant shrews, and a few other species have menstrual cycles, while most animals have estrous cycles.
The four stages of estrus
The estrous cycle can be divided into four main stages: Proestrus, Estrus, Metestrus or Diestrus, and Anestrus. These four stages alternate with each other to form a complete reproductive cycle.
Proestrus
During this stage, some of the follicles in the ovary begin to grow. The duration of this process varies between species, usually between one day and three weeks. At this time, the endometrium begins to develop due to the influence of estrogen.
Estrus
When a female animal enters estrus, she will display obvious sexual attraction behaviors, such as changing physiological characteristics and pairing behavior. This stage is often called "hot" and is the best time for fertilization.
Postestrus (Metestrus or Diestrus)
During this stage, hormonal influences cause the endometrium to become less active and the corpus luteum begins to form. If pregnancy does not occur, the corpus luteum will gradually degenerate and enter the next cycle.
Anestrus
This is the quiescent phase of the reproductive cycle, often controlled by light length or environmental changes. Even health conditions or pregnancy factors can cause this stage to occur.
Diversity of animal estrus frequency
The length and frequency of estrous cycles also vary widely between species. For example, some small animals such as cats may experience estrus every three to seven weeks, while larger animals such as cows experience it approximately every 21 days.
Some species go into estrus during specific seasons of the year, such as sheep and deer in autumn and winter.
Species-specific reproductive behavior
For example, female cats' estrus usually lasts 14 to 21 days, and they are induced ovulators, meaning mating triggers ovulation. The estrus cycle of female dogs is usually twice a year, and there will be obvious bleeding during this process.
Conclusion: Why don’t some animals experience menstruation?
Generally speaking, all these differences are related to the survival strategies of organisms. Animals that have an estrous cycle often develop a process that no longer requires endometrial expulsion due to evolutionary adaptation to the environment. This allows for more efficient use of resources and increases reproductive success. This evolutionary selection ensures that they can better adapt to the environment in the competition for survival.
In this context, we may be able to reflect on whether this diversity of reproductive physiology is an ingenious arrangement of biological evolution, or is it some unsolved natural mystery?
