Menstration plays such an important role in human reproduction yet you never hear about it in other animals.
Do any other animals menstruate?
If it evolved via natural selection in humans, then why don’t our primate cousins show a similar trait?
Is there a missing menstrual link or is this a rare and truly unique human characteristic?
Any takers?
I believe that other primates, ie chimps and great apes, do have something analogous to menstruation, but just not on a monthly cycle. Other animals have certain times of the year when they can reproduce, and unlike humans aren’t fertile most of the time. Female humans are fertile ALMOST anytime except during actual menstruation.
So I believe that the fundamental mechanism is very similar in other primates… but because the cycles are very different it might not appear so.
Are there any Zoologists/Reproductive Physiologists out there?
I’m totally pulling this out of my . . . ahem . . . anyway, human females mestruate more heavily than other species. This might have something to do with the fact that, as dolphinboy pointed out, we mate more frequently than other animals, so human females are at a greater risk for sperm-borne pathogens. Sloughing off the uterine lining when one fails to get pregnant helps remove pathogens from the body. This hypothesis is highly speculative, of course.
Female humans are actually only fertile for about one day a month; the day that they ovulate. The egg can only survive for about 24 hours or so without being fertilized.
You’re effectively fertile for a little longer than that, because sperm can survive up to four or five days in the body, but certainly not for the entire month.
Unspayed female Dogs who are not mated on their heat do menstruate. I’m not a dog owner, I don’t know exactly how often their heat is, but I once worked at a place where we had an “in-house dog” (a very lovely and friendly female weimeraner) and on at least one occaision she had to wear these amusing “doggy depends” to avoid any mess. They had been planning to breed her but missed the heat.
I seem to recall reading an article on menstruation/ovulation in Discover magazine within the last few years. On a search through the Discover website, I only found this (from Sept 1993), which may be what I was thinking of:
“Sex and the Female Agenda” by Jared Diamond.
This is the same Jared Diamond of Guns, Germs, and Steel fame.
MsWhatsit… can you provide a cite for you statistic? If females were only “fertile” one or two days a month then I would think there would be a lot less people around. The egg is released from the ovary and then travels over some period of time down the Fallopian (sp?) tubes towards the uterus… my understanding is that at any point during it’s travel it can be fertilzed. I would think perhaps two fertile weeks a month would be more accurate but I’m still trying to find a cite…
From here
The figures might be a bit skewed by the fact that outside the human race, most female mammals are almost constantly pregnant or nursing, or both.
Lance’s stat was about what I figured, a week. My own private musing, though, is that menstration in females could be an abnormality that was somehow passed along…oh, I know it’s not true, but it makes it seem less like god(s) is out to get us that way.
A woman can be menstruating and fertile at the same time.
An unusually short menstrual cylce (20 or 21 days) is uncommon but not unheard of. In this case a woman could ovulate as little as two days after the last day of menses. Hearty sperm, deposited near the end of menses could still be hanging around when the egg shows up.
Robby,
Thanks for the Diamond article. I’d not seen it and it makes better sense than anything I’ve found so far.
Jois
The only hypothesis I’ve seen to explain the uniquely frequent and heavy menstruation in humans is posited in The Handicap Principle, Zahavi & Zahavi (Oxford U Press, 1997). In fact, the theory posited by the authors explains a lot of anomolous physical and behavioral attributes in animals.
Briefly and inelegantly: Zahavi & Zahavi propose that certain attributes are displayed as proof of the fitness of the animal. A peacock, for example, has a brightly colored and heavy tail which he displays to attract peahens. The message conveyed is, *“I am so healthy, powerful and smart that I can obtain enough food to grow this beautiful tail, and to drag it around and even fly with it, all while avoiding all the predators to which it makes me so visible and vulnerable. If you want your offspring to be healthy, powerful and smart too, you will mate with me.” *
As to humans and menstruation, Zahavi & Zahavi say:
"Body processes, too, can be signals; a case in point is menstruation. Women are unique among mammals in the amount of blood and body tissue they discharge every month in their menstrual flow. This is not required for fertility, for most mammals conceive without it. What, then, is its purpose?
“Menstruation is a reliable indicator of a woman’s physical condition. When a woman is sick, either bodily or mentally, or when she expends great physical effort, as in sports competitions, her menstruation may cease or become distorted. Pregnancy stops menstruation. Menstruation thus informs a woman’s mate or potential mate that she is in good enough shape to bear children – as witnessed by the amount of blood and tissues she can afford to “waste” monthly on menstruation – and also that she is not pregnant at that time, which is very important to one who wants to ensure that his chosen mate’s child will be his own.”
Note that a certain level of body fat must be achieved for the onset of menstruation, and that in hunting and gathering cultures, that level is not achieved until a woman is in her upper teens and has obtained sufficient skills to gather more calories than she expends in the gathering.
You can find more info about The Handicap Principle here: http://www.oup-usa.org/publicity/pr_0195100352.html and here: http://www.mcn.net/~jimloy/handicap.html
I recommend the book. The theory answers a lot of puzzles in evolutionary biology, such as hive systems, which no other theories have ever satisfactorily answered.
Sparteye –
That sounds like an interesting book, and the main argument seems very plausible for things like prey signalling predators. (it’s not a particularly new idea for sexual selection like the peacock’s tail).
However, I’m not so sure about the menstruation as a signal hypothesis. If it was true, then human males should find menstruation sexy, right? And, while I of course wouldn’t dream of doing anything other than celebrating my partner’s monthly manifestation of powerful feminine energy, I don’t know any men (myself included) who find menstruation much of a sexual turn on. This isn’t hard data of course, but makes me want to look around for anther answer.
Also, I’m interested in how the Handicap principle applies to hive societies. What is ‘unexplained’ without it? (Yeah, I’ll try and get the book, but hey I’m an American: I want to know NOW, darnit!)
Huh? I missed that part of the class. “Sperm-borne pathogens?” I just thought that the heavier uterine lining was there to nourish a fertilized egg should it implant. If it didn’t, there was no use for the lining and it was expelled, and the process would begin again the next month. Simple and uncomplicated.
I would think that a sperm-borne pathogen would have plenty of time to invade the body in between menstrual cycles, thus it would be unaffected by menstruation.
Quercus - I don’t know anybody who thinks menstruation is sexy, but pregnancy isn’t generally regarded as sexy either, and it is the ultimate proof of a female’s fitness for reproduction. The Handicap Principal says that physical and behaviorial signals manifest fitness for mating, but those signals do not themselves have to stimulate sexual drive. Other steps - like nestbuilding - come first for many species.
The hive aspect is interesting. I’ve always been bothered by the prevailing suggestion that truely altruistic behavior could evolve; any being in the life-and-death competition of evolution which promotes others’ reproduction over his own is doomed to evolutionary death. Biologists have attempted to address the problem by hypothesizing that certain relationships - such as siblings or cousins - between animals could explain the sacrifice of one for the other, there being sufficient commonality of DNA to promote the sideways propogation of the sacrificer. But that hypothesis still left me questioning how much benefit to a worker bee is the possibility that she shares some DNA with the queen? Not enough, and I can’t imagine how the altruistic DNA would become so prevalent as to control the behavior of hundreds or thousands of worker bees, who expend their lives helping the queen reproduce. Zahavi & Zahavi say (and I cannot really do their writing justice): helpless offspring and the need for food storage create a situation in which a social system can arise, because cooperative effort promotes the survival and reproductivity of all the participants. Significantly, DNA studies show that many species of social insect have nonsterile workers, who actually do reproduce albeit at a comparatively low rate. The individuals in the social systems compete for prestige, which in turn promotes opportunity to reproduce. So, workers give more than they get in a social system, but still get more (reproductively) than they would outside the system, and that is why they work.
It is a lot more complicated than that. Zahavi & Zahavi go on for pages, covering the effects of haplodiploid reproduction, pheromones and initial nest populations.
IIRC, the “Handicap Principle” has little respect among biologists. The reason is simple. It is true that if an animal has a handicap and thrives, it indicates that it’s other traits must be highly fit.
However, the offspring of this animal will inherit the handicap along with the general fitness. The handicap, by definition, reduces the fitness. Let me try and quantify this: let’s say an animal would have a fitness of 10 without a handicap, but the handicap reduces its fitness to 6. The animal has a fitness of 6; it does not matter how fit it would be without the handicap.
Anyway, I doubt that this is a valid possibility for the evolution of menstruation.
“If the egg is not fertilized, it survives for only 12 to 24 hours.”
"Fertilization occurs when a sperm penetrates the egg after its descent into the fallopian tube. Once the sperm is in the fallopian tube, it can survive for up to five days and fertilize the egg at any point during that time. Studies indicate, however, that conception is most likely when intercourse occurs two days before or on the day of ovulation. "
If that’s not enough proof for ya, I learned it that way in my anatomy class. So there. 
I’ll get on to menstruation in a second. First,
JasonFin- I think the handicap principle is indeed fairly accepted in biology (though perhaps not using that particular name or formulation).
Maybe a better way to explain it is that if an extravagant display has a big reproductive benefit, then even if the display has some other drawbacks, it still improves fitness overall. (e.g. for a peacock, having to lug a big tail around might be worth it if it means he mates more often and has more children. And his sons will also have large tails, therefore have more offspring.) Using your numerical example, carrying the tail decreases fitness by 1, but getting more mates increases fitness by 3, so overall it’s a benefit.
Of course, that only explains one side (in this case, the cocks’); the question is why do peahens choose big tails? The answer is indeed that they want males who are strong enough to grow big tails – remember both their daughters and their sons get the strength genes, but only their sons get the drawback of carrying a big tail (and, as long as other hens like big tails, it’s not really a drawback).
You can do close analysis to show that peahen choosiness can spread and is stable over the long run, but I’m not going to go into it here.
But… to get back to menstruation… the key part of successful sexual handicapping is that there has to be some reproductive benefit – potential partners have to make choices based on the handicap signal. Spending effort on a signal nobody is looking for doesn’t help. To make an analogy, Vinnie wasting $200,000 on a car he doesn’t need might help get him laid. But Vinnie wasting $200,000 on Star Trek memorabilia collection is just gonna make him $200,000 poorer, cause chicks don’t dig trading cars like they do Porches (**).
So a good signal by females that they are indeed well-fed, fertile, etc. has to be picked up on by males. For instance fat deposits in, uh, all the right places is good signal picked up on by males that here’s a healthy, reproductively ready, female. In other words, us guys dig them (**) But guys don’t dig menstruation – it’s not a very good signal. So I don’t think it could have evolved as a signal to males.
** of course in addition to our instincts, humans have brains that allow our interactions to be much more complex and wonderful, and based on more interesting criteria than size of fat deposits or size of domesticated steed.
Eh, I’m out of my depth here, biologically speaking. Just repeating what I read in OMNI years ago . . .
Is it necessarily true that a pathogen would invade the body, per se, or might it take up residence in the nutrient-rich uterine lining?
IIRC, the claim made by proponents of this theory was also that humans menstruate more heavily than other animals, as well–the uterine lining is thicker. This may be to better nourish the zygote, but if a thicker uterine lining a great strategy, why do only humans employ it?
Just to sprinkle in a few more caveats, I’m talking through my hat. I have no post-secondary training in biology. I’m not advocating for this hypothesis in a serious way. I just find it very attractive: Heavy periods are because of diseases delivered by semen. Menstrual cramping is due to heavy periods. Therefore, cramps are caused by MEN. Those bastards. One more reason not to shave my legs. ; )