Preface:
Yet another Quora question and answer.
In fact this is my pet answer. I had a great time investigating it. However, I have now a very low opinion about the role of Quora in divulgating highre than avergae-good nature related content. Maybe I'm wrong, but the changes introduced in the site, oriented to increase the flow of readers and public in general with the aim of monetizing it, have drowned good content in natural sciences. To my understanding there are good contributors in physics and other sciences, but biology has gone to the rubbish bin since then.
I really would like to be wrong, but I don't have the time of checking it.
So, by now, just a glimpse of what it was for me once.
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Understanding
menstruation as the discharge, through the vagina, of blood,
secretions, and tissue debris derived from the involution of the
endometrium at the end of an infertile reproductive cycle, there there
is well-documented evidence that the catarrhines (humans, apes, and Old
World monkeys) menstruate and that the strepsirrhines (e.g. lemurs and
galagos) do not. The data on New World monkeys are ambiguous. There is
clear evidence that at least one species in each of the genera Cebus, Ateles, Alouatta, and Lagothrix exhibits menstruation. For other New World species, the data are conflicting.
Some species of bats also present menstruation: Molossus ater, a molossid bat, and Glossophaga soricina, Carollia perspicillata, and Desmondus rotundus, all phyllostomid bats, and Rousettus leschenaulti.
Menstruation
has not been detected or studied in the majority of bat species; thus,
it is unclear if menstruation evolved twice within Chiroptera or is more
widespread in the clade. Finally, there is well-documented evidence of
menstruation in the elephant shrew (Elephantulus myurus), a species belonging to the clade Afrotheria.
Observation
of the vaginal bleeding of the fulvous fruit bat. The body weight of
this one is 92.50 g, the full length from head to tail is 10.15 cm, and
the forearm length is 8.30 cm. Magnified vaginal orifice during
menstruation is on the lower left quarter. Wild Fulvous Fruit Bats
So
far, the list of animals that show menstruations, or overt
menstruations as other authors call them (I don't like this nomenclature
I prefer to call them menstruations. As a side comment I want to point
that definitions and nomenclature regarding female reproductive cycles
and menstruations are very vague and messy. It is hard to find a good
description of the differences between menstrual and strous cycles, some
scientits condider the reasorbing of the endometrium that happens in
the majority of Eutherian species as "covert" menstruation, etc. The
ambiguity of the literature here is very notorious and unfortunate).
Bellow this line some reflexions about the pehnomenom of menstruation in chimps and bats.
- Chimpanzees:
Apart
from humans, the majority of reports of menstruations come from captive
chimpanzees. This is a very interesting fact, because in spite of the
many years of detailed and careful observations of wild chimpanzees the
event of menstruation was firstly discovered in captive ones. The common
argument to explain this (females frequently pregnant or lactating)
fails because there is a prolongued period of teen infertility in young
females:
In females first estrus is seen in females at 10 years of age and is characterized by
anogenital swelling.
Menarche
occurs a few months after the first swelling and continues on a cycle
of about 36 days (Goodall 1986). There is a period of adolescent
infertility in female chimpanzees that usually coincides with permanent
emigration from their natal groups (Goodall 1986; Nishida et al. 2003).
During the transition period, females still exhibit sexual swellings
that may serve as a passport to gain males' tolerance in their new
social communities (Boesch & Boesch-Achermann 2000). Once
established in their new communities, young females cease cycling for
two to four years but continue to attract adult males and mate
promiscuously. First
parturition occurs, on average, between 13 and 14 years and the
interbirth interval is between three and five years (Goodall 1986; Boesch & Boesch-Achermann 2000; Nishida et al. 2003).
Auntie Rose, about 60 here, grooms her 5-year-old son Mandela in Kibale, Uganda, from
Menopause in Chimps? .
An
interesting side analysis is that menopause is absent in wild chimps,
although they show a decrease of fertility with age. However in captive
females some authors point the onset of menopause about 35 years of age,
while others deny it. In my opinion the absence of menopause in wild
chimps is not due to their shorter lifespan, because they also undergo
earlier menarches and the expand of the fertile years in older chimps
mothers is bigger than in the average woman. Also the menopause reported
in captive females ocurred after 25-27 years of reproductive cycles,
while their (obviously alive) wild counterparts are still fertile at
these years.
For more reading see:
I have added studies that bear contradictory conclussions because I just don't want to cherrypick.
- Bats:
To
my understanding, bats represent the biggest surprises here (which is
to some extent logic, given that there are more than 1,000 species of
bats). The findings in chiropterans can challenge what we have asumed
referring to menstruations and menstrual cycles.
There
are some interesting studies that demonstrate the presence of a
conventional menstrual cycle, with its menstruation event included, in
some species of bats, a phenomenom that was earlier supposed to be
restricted to primates:
To our
knowledge, this is the most comprehensive study thus far of menstruation
in a nonprimate species. For the first time, we have observed two
consecutive vaginal bleedings in a bat colony in its natural habitat. In
addition, we have demonstrated that fulvous fruit bats (Rousettus
leschenaulti) exhibit all the characteristics, both morphologic and
hormonal, of a true menstruation during the 33-day cycle.
Reports of menstruation in Molossus ater:
As in menstruating catarrhine primates, the endometrium of Molossus ater
is vascularized by spiral arterioles and populated by distincitve
granulocytes containing large, acidophilic granules. Increased coiling
of these arterioles did not appear to be an essential element in the
mechanism of menstruation in this species. M. ater is a
monotocous, seasonal breeder, with a relatively long gestation period.
Although it has a bicornuate uterus, ovulation and implantation appear
to occur only on the right side of the tract. The ability to menstruate
probably affords this bat an efficient mechanism for eliminating a
highly differentiated endometrium from the usual implantation site in
the event of a reproductive failure. In the wild, this may provide M. ater with another chance to establish a pregnancy at a still opportune time during the same breeding season.
This
study is extremely interesting because not only points to an event of
menstruation in a seasonal breeders but also indicates that this
phenomenon occurs in females that have not beed fertilized as a possible
adaptation to provide another chance of stablishing a pregnancy during
the breeding season.
This is very contradictory with the asumptions
we had regarding menstruations, that they are only present in mammals
with menstrual cycles (menstrual cycles are continous, and the Molossus
are seasonal breeders with estrous cycles), and that ovulation happens
at the end of the cyle, because the observations of menstruation have
been reported after mating and not being fertilized. In this excerpt from (
Wild Fulvous Fruit Bats ( ) it is explained more explicitly:
Menstruation has been described only in microchiropteran bats [
12–
15]
and is thought to occur only after coitus. Captive short-tailed fruit
bats (C. perspicillata) menstruate between Days 1 and 5 post coitus (
p.c.; the first day when spermatoza were identified in vaginal smears was designated Day 1
p.c.) [
12]. Red-mastiff bats (M. ater) menstruate between Days 4 and 10
p.c. [
14]. Significant growth of the endometrium occurs between Days 5 and 16
p.c. in short-tailed fruit bats [
12]. It is thought that microchiropteran bats menstruate after coitus because of fertilization failure or early embryo loss [
15].
The occurrence of menstruation gives those microchiropteran female bats
another opportunity to establish pregnancy during the same breeding
season [
12].
Therefore,
it is thought that there are two main differences in menstruation
between bats and humans. 1) Endometrial mitotic activity in humans
occurs during the preovulatory phase and reduces soon after ovulation,
while most endometrial growth in microchiropteran bats is postovulatory [
12]. 2) Microchiropteran bats menstruate only after coitus [
12,
14], whereas human menstruation is controlled by pituitary and ovarian hormones and is not dependent on coitus [
1,
5].
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