Évolution des menstruations
Why do only some animals have periods?
Humans are not the only organisms that have periods — some animals do too, but scientists still aren't sure why.
The menstrual cycle plays an essential role in human reproduction. However, most other animals don't experience menstruation.
So, which other species have periods, and what's the evolutionary point of bleeding periodically?
According to Deena Emera, an evolutionary biologist at the Buck Institute for Research on Aging, scientists know of around 85 mammal species, or less than 2% of mammals, that have a menstrual cycle. Most of these are primates, including our closest living relatives chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). Scientists have also discovered menstrual cycles in a few species of bats, elephant shrews and most recently spiny mice (Acomys cahirinus).
Because these animals aren't all closely related, the trait likely evolved convergently, meaning there must be some evolutionary benefit to it, Emera told Live Science.
Beyond these creatures, there are other animals that periodically bleed through their reproductive organs. Owners of unspayed dogs may know the unfortunate experience of finding blood on their favorite couch and realizing their pet has gone into heat, also called estrus. However, the bleeding that dogs experience comes from a different source than in menstruating animals.
In animals that bleed while in estrus, an increase in the hormone estrogen while the animal is fertile causes the blood vessels inside the vagina to dilate. This results in small amounts of blood leaking out of the vessels and getting expelled.
In menstruating animals, periods happen because of estrogen and a second hormone called progesterone. Additional hormones are also involved in maturing and releasing an egg in the lead-up to menstruation.
Progesterone is a hormone needed to maintain a pregnancy, and in menstruating animals, it starts to increase before the animal is pregnant. And before that increase happens, a rise in estrogen causes the uterine lining to thicken and new blood vessels to develop. Then, once an egg is released, progesterone starts to rise as estrogen falls.
If pregnancy doesn't then occur, the female's progesterone levels drop, and the newly formed blood vessels and other new tissues slough off in the form of period blood and bits of tissue. In non-menstruating mammals, the uterus does not transform in response to progesterone levels until after the female becomes pregnant, Emera said.
To Emera, this difference is intriguing from an evolutionary perspective. "The question isn't really, 'Why do we menstruate?'" Emera said. "The question is, 'Why do we prepare our uterus for pregnancy before we're even pregnant?'"
Nobody is quite sure what the answer is. But according to Emera, it could have to do with the fact that menstruating animals all give birth to small litters. Humans, primates, bats and elephant shrews usually have just one offspring at a time, while spiny mice have just one to four pups — far fewer than most mouse species.
Menstruating animals also have longer pregnancies, or "gestation periods" than their non-menstruating counterparts. Spiny mice, for example, have a gestation period of nearly double that of other mice. Because these animals devote so much time and energy to so few offspring, it's important that their offspring survive.
Researchers have found that, when the uterine lining is transformed for pregnancy, it can detect chemical cues released by the embryo that raise or lower its chances of successfully implanting. These chemical signals reflect aspects of an embryo's viability. This quality-assurance step happens in all mammals, but in menstruating animals that pre-build their lining, it happens much earlier.
"When you have a situation where a female is investing a lot, you totally expect systems to evolve to screen as early as possible against those offspring that aren't going to make it," Emera explained.
Robert Martin, a retired evolutionary biologist and academic guest at the University of Zurich, thinks menstruation may also play a role in sperm storage. Bats, for example, can store sperm in their reproductive tract for up to 200 days before fertilization, and humans have been documented to store sperm for up to nine days in the female reproductive tract.
When sperm stick around for too long, however, they start to degrade, which could cause chromosomal issues should they fertilize an egg, Martin told Live Science. He hypothesizes that the shedding of the uterine lining enables animals to shed this old sperm and make space for newer, more-robust sperm.
There are other theories as to why menstruation happens, but there is no concrete proof for one theory over the others. Martin said that more research needs to be done on menstruation, both in humans and other animals.
"There's been very little research, but there are so many practical applications," he said.
Abstract
According to a recent hypothesis, menstruation evolved to protect the uterus and oviducts from sperm-borne pathogens by dislodging infected endometrial tissue and delivering immune cells to the uterine cavity. This hypothesis predicts the following: (1) uterine pathogens should be more prevalent before menses than after menses, (2) in the life histories of females, the timing of menstruation should track pathogen burden, and (3) in primates, the copiousness of menstruation should increase with the promiscuity of the breeding system. I tested these predictions and they were not upheld by the evidence. I propose the alternative hypothesis that the uterine endometrium is shed/resorbed whenever implantation fails because cyclical regression and renewal is energetically less costly than maintaining the endometrium in the metabolically active state required for implantation. In the regressed state, oxygen consumption (per mg protein/h) in human endometria declines nearly sevenfold. The cyclicity in endometrial oxygen consumption is one component of the whole body cyclicity in metabolic rate caused by the action of the ovarian steroids on both endometrial and nonendometrial tissue. Metabolic rate is at least 7% lower, on average, during the follicular phase than during the luteal phase in women, which signifies an estimated energy savings of 53 MJ over four cycles, or nearly six days worth of food. Thus the menstrual cycle revs up and revs down, economizing on the energy costs of reproduction. This economy is greatest during the nonbreeding season and other periods of amenorrhea when the endometrium remains in a regressed state and ovarian cycling is absent for a prolonged period of time. Twelve months of amenorrhea save an estimated 130 MJ, or the energy required by one woman for nearly half a month. By helping females to maintain body mass, energy economy will promote female fitness in any environment in which fecundity and survivorship is constrained by the food supply. Endometrial economy may be of ancient evolutionary origin because similar reproductive structures, such as the oviducts of lizards, also regress when a fertilized egg is unlikely to be present. Regression of the endometrium is usually accompanied by reabsorption, but in some species as much as one third of the endometrial and vascular tissue is shed as the menses. Rather than having an adaptive basis in ecology or behavior, variation in the degree of menstrual bleeding in primates shows a striking correlation with phylogeny. The endometrial microvasculature is designed to provide the blood supply to the endometrium and the placenta, and external bleeding appears to be a side effect of endometerial regression that arises when there is too much blood and other tissue for complete reabsorption. The copious bleeding of humans and chimps can be attributed to the large size of the uterus relative to adult female body size and to the design of the microvasculature in catarrhines.
Michel Schittecatte
(45:30) Dans la conscience corporelle, il y a aussi les émotions et les cognitions. Uniquement sentir le corps et mobiliser le corps n'a pas d'intérêt. C'est cette conjonction du corps avec l'émotion avec les sensations avec les mouvements et avec les pensées qui donne un accès. (...) C'est cette capacité curieuse qu'a l'être humain qui doit gérer trois cerveaux en même temps qui sont apparus à des époques radicalement différentes. Le cerveau sensori-moteur date de 4 ou 500 millions d'années, le cerveau émotionnel de 80 millions d'années et le cerveau cognitif 100 000 ans. Il est tout récent. Ces trois cerveaux doivent fonctionner en même temps et ça c'est la grande difficulté. Mais ils peuvent fonctionner en même temps si on les laisse fonctionner en même temps et si on ne donne priorité à aucun d'eux. Être un être humain, ce n'est pas être un être cognitif, ce n'est pas non plus être un être émotionnel, c'est pas être un être instinctuel et sensori-moteur, c'est être les trois en même temps de manière harmonieuse. Et c'est un défi.
(47:50) Tout notre fonctionnement est lié au fonctionnement antagoniste des deux branches du système nerveux. (...) Quand tout va bien et que nous ne sommes pas en danger, notre organisme est géré par le système parasympathique qui s'occupe de toutes les fonctions qui sont importantes pour assurer notre survie quand nous ne sommes pas en danger: le sommeil, l'alimentation, la digestion, la reproduction et plus tard l'engagement social. Le système orthosympathique prend les commandes quand nous sommes dans un danger immédiat. Ce modèle-là n'explique pas deux choses: ni le figement, ni la négociation.
(49:00) Le modèle de Stephen Porges ajoute quelque chose qui permet de comprendre ce qu'est le figement et ce qu'est la négociation. (...) C'est un cardiologue, c'est pas du tout un psychiatre ou un psychologue. Il étudie les morts prématurées chez les nourrissons et il a une capacité de mesurer le système parasympathique chez les nouveaux-nés. Ce qu'il observe, c'est un paradoxe: les nouveaux-nés qui ont un tonus parasympathique élevé ont plus de chances de survie (...) ce qui est normal puisque le tonus parasympathique est protecteur généralement. Mais quand ils meurent, ils meurent d'un tonus parasympathique élevé. Donc là, il y a un paradoxe. Voilà un système qui peut à la fois protéger et tuer l'individu.
(50:10) Lui est venue l'idée qu'il y avait deux systèmes parasympathiques, ce qu'il appelle la branche dorsale et la branche ventrale, laquelle serait apparue chez les mammifères avec le système limbique et qui permettrait la relation. Si vous n'avez dans votre répertoire comportemental que la fuite, l'attaque et le figement, vous ne pouvez pas créer de relations. (...) Le système de Porges permet de comprendre à la fois le figement (c'est la branche dorsale du système parasympathique qui vient verrouiller le système orthosympathique et qui crée l'immobilisation) et le système parasympathique ventral (...) qui permet d'entrer en relation par le système d'engagement social (...) qui permet la communication et la relation. Ce n'est ni l'attaque, ni la fuite, ni le figement.
(51:30) En modifiant le tonus parasympathique, vous pouvez créer de la relation. Par exemple, si vous diminuez légèrement votre système parasympathique ventral, (...) vous créez une petite activation. Par exemple, vous tirez la queue d'un chat qui dort. Il n'est pas content, mais il ne va pas passer directement en système orthosympathique. (...) Il va vous envoyer un signal qu'il n'est pas content. (...) Si vous arrêtez de tirer sa queue, son système parasympathique ventral revient comme avant. (...) Par contre, si vous augmentez légèrement votre système parasympathique ventral, vous créez une espèce d'état de relaxation, voire d'extase. (...) En modulant votre système parasympathique ventral, vous pouvez, sans engager les deux autres systèmes, soit faire face à un danger sans passer à l'action, soit répondre de manière positive à une interaction.
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