Female orgasm is a reflexive response accompanied by an endocrine surge of hormones like prolactin and oxytocin. A 2016 research drew our attention to mammalian lineage of orgasm by spotlighting ovulation. Pavlicev et. al. suggested that the human female orgasm is a homolog of the reflex that ancestrally induced ovulation. In other words, where there is orgasm, there is an occurrence of ovulation. So, when one speaks of ovulation, there are 3 main factors that determine its onset or delay.
Often experimented in rodents (rats, hamsters, mice) and rabbits, the seasonal clock provides the much needed information about reproduction, hibernation and migration.
With winter being an extremely harsh environment for the young, the adult animals’ reproductive cycles can be tampered by just tweaking the number of hours of light that they are exposed to (also referred to as the photoperiod): the idea being that longer lengths (15 h light/day) of photoperiod is indicative of summer and shorter (10 h light/day), is indicative of winter.
One of the 4 biological rhythms in our body is called seasonal rhythms as the body adapts to seasonal variations around it. The crux being that many of the young would not survive the winter, thus nature deemed it important to delay puberty, thus delaying ovulation to ensure that copulation and rearing of the young does not take place till summer. So by subjecting the animal to shorter days (10 h light/day), indicative of winter, one can ensure that puberty or in other words, ovulation is delayed.
Here, the possibility of copulation with a male can trigger ovulation in females. In the case of rats, they are able to even terminate their pregnancy! Called as the Bruce effect, when a pregnant female rat is exposed to an unfamiliar male scent, her pheromonal system regulating the release of estrogens, causes a miscarriage, thus making the female available for sex again. Some speculate that this is the female’s method of choosing the best genes for her offspring. Natural selection of Polyandry.
In cats however, especially within the pride, this is a survival mechanism. During a lion takeover, the lioness has but 4 choices:
Defend the cubs: The new alpha lion would make it a point to go out of his way and kill the predecessor’s cubs so as to avoid another takeover. In such a situation, as the lion is larger than the lioness, the chances are that she may die defending her cubs.
Abandonment: Some lionesses opt for this option. Either they abandon their cubs or they abandon the pride and raise the cubs in the wild on their own. Again, the chances of survival here are limited.
Pseudo-estrous: Many lionesses perfect the art of mimicking an estrous cycle just so as to mate with the new alpha. This is specifically done when the lioness is pregnant. When the cub is birthed, the alpha (obviously unaware that he's been duped) automatically assumes his parentage.
Spontaneous abortion: Bertram in 1975 recorded a lowered frequency of births upon a takeover. The theory that emerged suggested that the pregnant lions aborted the cubs so as to ovulate, copulate with the new alpha and birth a cub into a safer climate.
As we move up on the evolutionary scale, we notice a cyclical nature of ovulation taking place. External cues here, may or may not influence its occurrence. Noticed in humans, spontaneous ovulation when impacted by environmental factors (seasonal clock), makes use of factors such as the number of hours of light exposure and temperature. Bats, horses, hedgehogs and sheep also show evidence of being impacted by an external cue, be it photoperiod or pheromones.
Which brings us to the possibility of a sexual encounter with a partner. Cats and rabbits are ideal examples of how pheromones induce their ovulation to ensure copulation.
In spontaneous ovulation, the mammalian ovarian cycle is governed by pituitary hormones, namely the follicle stimulating hormone (FSH) and luteinizing hormone (LH). This stimulus is necessary for inducing ovulation.
Question we now have to ask: Which came first? Spontaneous, Partner-Induced or Environmentally-Induced?
For this one has to look beyond mammalian evolution. There exists evidence pointing to ovulation being induced by the environment in fish and frogs. Thus, suggesting that the environmentally-induced ovulation did originate first. However, partner-induced ovulation is an adaptive response, where factors such as the availability of a male for copulation or preference of polyandry to select the right genes, played a major role.
Ovulation and Orgasm Associations
To record orgasms across the phylogenetic tree, one must concentrate on a physiological trait such as Neuroendocrine discharge: Prolactin and Oxytocin. So their release, defined as a neuroendocrine reflex, sent researchers on a crusade across species to pattern the associations between orgasms and ovulations.
Across species, across ovulation styles, during ovulation, Prolactin (PL) and Oxytocin (OXT) are released, thus indicating that the orgasm and ovulation share a bond. Furthermore, the prolactin thus released maintains the corpus luteum (CL). CL in turn, causes the release of Progesterone which is necessary for implantation. So essentially, if the female reproductive tract is not stimulated and orgasm is not achieved, PL is not released, therefore CL levels aren’t maintained and progesterone isn’t released: no pregnancy.
Furthermore, OXT release during orgasm causes the propulsion of the egg down the reproductive tract. OXT also impacts the size of the corpus luteum thus released.
Now that we have established associations of endocrine surges with orgasm, let’s look at the anatomical structure responsible for female orgasm.
Clitoral positioning, Orgasms and Types of Ovulation
A phylogenetic tree is like a family tree that looks at the evolution of traits across species, over time and having been exposed to mutations.
So in order to compare the evolution of positionality of the clitoris and the type of ovulation chosen by a species, one must compare the resultant phylogenetic trees.
We begin with animals (birds, reptiles) that have one hole for pooping, peeing and penetration (Urogenital tract). That being said, these animals have the clitoris being positioned inside the tract.
As we move on to male-induced ovulators, the genitals and urethral orifices separate in primates and rodents. The clitoris seems to have moved from inside to on or away from the vaginal opening. This is because a surge of gonadotropin release is necessary for implantation. By that we are also referring to a pheromonal stimuli inducing orgasm and thus ovulation. So orgasm is highly necessary.
And as Bonaparte’s research indicates, the distance of the clitoris from the vaginal opening has a lot to say when it comes to orgasm due to penetration. The further away the clitoris and the larger it is in its aroused state, the more likely is it for a female to orgasm during penetration.
In humans however, we still maintain a large CUMD. The shift here is that orgasm was no longer necessary for implantation, however, that does not mean it doesn’t play a role. In spontaneous ovulators, these ancestral neuroendocrine reflexes have lost their role in ensuring fertilization (the clitoral stimulation is no longer required for implantation).
When one compares the movement of the clitoris with types of ovulation, one can see an overlap within these phylogenetic trees.
This is a plausible explanation for the evolutionary origins of female orgasm which depends heavily upon the mating success of nonhuman mammals. However, when we moved away from partner-induced ovulation and began ovulating spontaneously, orgasms were no longer a necessity to achieve implantation in humans. It however designed a different trajectory for itself by providing the clitoris the necessary recognition as the only organ in the entire body whose sole purpose is to bring pleasure.
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