Sexual dimorphism: what it is, research and data

What is sexual dimorphism? Does it appear only in non-human animals or also in people? Broadly speaking, we can say that sexual dimorphism encompasses those variations between males and females of the same species. Namely, their sexual differences.

In this article we will solve these questions in a more exhaustive way and also, we will know some investigations developed around sexual dimorphism in humans. In addition, we will see how these variations mentioned go beyond the simple physical or morphological aspect.

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What is sexual dimorphism?

Sexual dimorphism is a concept in biology that has to do with differences between animals of different sex within the same species. Specifically, it consists of a set of characteristics that vary between males and females; These variations have to do with their physiology or also their external appearance (for example colors, sizes, shapes ...).

However, it has been proven that sometimes

these variations go even beyond the external appearance, and they extend to psychophysiological, cerebral and even epidemiological aspects (especially in the case of humans). That is to say, in two words and roughly, sexual dimorphism can be summarized as: "sexual differences".

Most, but not all, species are sexually dimorphic; on the other hand, not all the species that present it present it in the same degree or level. In other words, there are variations in this sense as well.

An example of sexual dimorphism is that females of specific species, such as birds, reptiles, amphibians, insects... tend to be larger than males. Thus, this would be a characteristic encompassed within sexual dimorphism. In other species, however, it is the males that are the largest in size (for example, in mammals).

We should not confuse sexual dimorphism with sexual polymorphism; sexual polymorphism, unlike the previous one, implies that members of the same sex (for example, females) show different appearance.

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How does it manifest itself in the human being?

Sexual dimorphism also appears in humans, like animals that we are. The main difference between men and women in terms of sexual dimorphism is the distribution of abdominal fat.

This distribution varies in both sexes, although not in the same way in all ages. Specifically, and according to chronological age, the differences are as follows:

1. Early childhood

When we are born and we are still very small, this difference in the distribution of abdominal fat is very slight. Namely, it is a minimal difference; thus, the bodies of babies and children (both male and female) are more similar in this regard.

2. Puberty

This characteristic of sexual dimorphism at puberty becomes more noticeable at this age. Its explanation lies in steroid sex hormones, which begin to act and also do so intensely, releasing large amounts of them.

How does this translate? Basically, the accumulation of fat in women, unlike men, accumulates more in the buttocks, hips and thighs (it is the so-called "gynoid" distribution).

3. Adulthood

The previous differences in relation to sexual dimorphism between men and women (in terms of the distribution of body fat), remain constant over time, until the stage of menopause.

In this stage, steroid sex hormone levels decrease, which changes the distribution of fat between men and women; This means that the fat in women, in this case, accumulates especially in the waist (“android” distribution). In men, however, these changes are less noticeable, although they increase slightly throughout life.

4. From old age

Later in adulthood, the differences are reduced and the form of fat distribution resembles both in men as in women, both of which have an android distribution (accumulation of fat in the waist). That is, at this stage there is practically no sexual dimorphism.

Investigations: beyond the physical aspect

Sexual dimorphism in humans goes beyond the physical appearance or the distribution of body fat that we have discussed. It also appears in the brain: in your organization and activity.

Thus, there is research that has determined that the brains of men and women vary in this sense as well; that is, your brain is (and works) differently.

Brain

Said investigations, carried out mainly by professor and researcher María Paz Viveros, have shown how brain development is different in both sexes (also in rats).

For example, the critical period of brain differentiation is known to vary from rats to humans; while in rats this period is perinatal, that is, it appears a few days before birth and is extends a few days later, in humans this period is prenatal (that is, it appears before born).

But what happens in this critical period? It happens that testosterone and estradiol from testosterone (both gonadal hormones), They “masculinize” the brain at a morphological and functional level. However, other studies have also shown that the effect of these hormones reaches even up to adolescence, which makes preadolescence also valued as a period critical.

Thus, these critical periods of "male" and "female" brain differentiation are probably the cause of sexual dimorphism in humans. However, there are other factors that influence in the appearance of this sexual dimorphism, such as: genetic, epigenetic factors (interaction between genetics and environment), hormonal and pharmacokinetic (interaction between drugs and the body), etc.

For example, at the brain level, one of the differences that we find between the brains of men and women is in the hypothalamic-pituitary-adrenal axis. This axis is responsible for regulating how we respond to stressful situations.

Neuropsychiatric diseases

Sexual dimorphism, as we have already advanced at the beginning of the article, can go beyond variations in physical appearance or morphology. Thus, in the case of humans this dimorphism is also evidenced in some neuropsychiatric diseases or disorders (or psychological) in its epidemiological sense.

For example is the case of the adictions, where sexual differences have been observed in their prevalence, proportion in certain areas and periods of time, etc. It also happens with depression or anxiety, where, for example, it is known that this type of disorder is twice as frequent - or even more - in women than in men.

On the other hand, women are also more vulnerable to suffering from depression, especially in certain periods of their reproductive cycle, in the postpartum stage or in the perimenopausal period.

Bibliographic references

• Academy of Natural Sciences, ed. (1991). Proceedings of The Academy of Natural Sciences 143. Philadelphia, United States.

• Cahill, L. (2005). Brain sexual dimorphism. Research and science, 346.

• Fernández, J. (2012). The study of sexual dimorphism, a key element in experimentation. Science, Complutense Tribune.