For men, not having a proper erection can be experienced as a real tragedy. What happens to other animals? Do they also have erection problems?
We must already understand what an erection is from a physiological point of view. Under “normal” physiological conditions, a context conducive to sexual activity activates the autonomic nervous system, which causes a increased nitric oxide levels (a vasodilator) in the arteries and smooth muscles of the penis. This results in an influx of blood into the corpora cavernosa of the penis and, to a lesser extent, into the corpus spongiosum. The ischiocavernosus and bulbospongiosus muscles simultaneously compress the veins of the corpora cavernosa, limiting the flow and circulation of this blood outside the penis.
As a result of the opening of the blood entry gate and the closing of the exit gates, the corpora cavernosa fill with fluid due to a gradual increase in blood pressure, and the penis becomes erect. When parasympathetic activity decreases and the muscles relax, blood is drained through the veins and the penis becomes flaccid again. Due to certain health problemsboth physical (mainly cardiovascular) and psychological, this system sometimes stops working properly, making sexual intercourse impossible.
Are there alternative mechanisms in nature?
Surprisingly enough, the human penis and the way it becomes erect is quite exceptional. Indeed, most mammals have a “bony aid” to keep the penis erect. This is the baculum (or penile bone), a bone located along the penis, which allows the male to penetrate effectively at any time, but which especially favors the lengthening of the duration of copulation.
This bone is surprisingly varied. Indeed, “the most diverse of all bones” (as it is now called) not only takes multiple forms, but also presents very different sizes: from almost vestigial (very reduced size), in certain species of lemurs, to surprising dimensions, such as the 65 centimeters long that found in walruses.
On the other hand, marsupials, hyenas, rabbits, but also equines share this absence of penile bones with humans. This group of “discriminated males” also lacks a second advantage, since the baculum, when elongated, protects the urethra during prolonged copulation by limiting its constriction, keeping it open and facilitating the flow of sperm.
But why doesn’t Homo sapiens have a penis bone?
If the first primates, which appeared at the end of the Cretaceous period, had this bone and it was preserved in most groups of mammals that emerged, why did it disappear in the evolutionary lineage that gave rise to our species?
The explanation could be that it would promote reproductive strategies in populations with high levels of postcopulatory sexual selection. Indeed, polygamous primate species (where sexual competition is very intense) have longer penile bones than monogamous ones, which would allow them to prolong coitus. In other words, the female would be “busy” longer, preventing her from mating with other males and, therefore, increasing the likelihood that the lucky mate would pass on their genes to the next generation. This hypothesis was tested in polygamous primates, where sexual competition is very intense.
She was also tested in an original experiment with two groups of miceone of whom was forced into monogamy. Over twenty-seven generations, the penis bone size of the monogamous group decreased. It therefore seems that if a species becomes monogamous, the selection pressure in favor of maintaining the bone is reduced.
On the other hand, about two million years agothe piece of chromosome containing the coding DNA sequence for this bone has been lost. This genetic mutation (deletion) occurred when our lineage of bipedal primates (hominins) was already well advanced and separated, for 4 million years, from that at the origin of chimpanzees and bonobos (which are polygamous and have a bone). This would lead us to the interesting conclusion that hominins became monogamous at this time, thereby removing evolutionary pressures to maintain bone.
Who is really the loser in this story, the men or the women?
In the book The Unfair Sexrecently publishedI explain that things are not always what they seem when viewed through an evolutionary prism.
In the case of the penile bone, apparently, it seems clearly disadvantageous to have to “make an effort” for the erection of the penis, especially when many problems, physical or psychological, can generate more than one “uncomfortable” situation for men. However, analyzing this fact from an evolutionary point of view, things are not so clear. With the disappearance of high levels of postcopulatory sexual competition, male hominins’ sole focus during copulation would be limited exclusively to ejaculation.
If, in terms of biological effectiveness, there is no difference between “quick” or long sexual intercourse… Can we not think that it is the females who are really the losers?
A. Victoria de Andrés Fernández is a lecturer in the animal biology department of theUniversity of Málaga.
This article is republished from The Conversation sous licence Creative Commons. Lire l’article original.
Source: www.slate.fr
