The Evolution of Modern Human Mating Systems and Sexuality
Published by Greg January 25th, 2007 in Science Essays, Human Evolution 1001, Gender, Evolution
Professor Desmond Clark, the consummate British gentleman and Africanist archaeologist, was fond of telling his intro class “if we were chimpanzees instead of humans this class would be severely interrupted owing to the presence of at least one or two ovulating females in the room at this moment.” I never took a class from Desmond (different school) but this quip was passed on to me by Glynn Isaac, and Glynn has passed it on to all of his students, we to ours, and so on. So this little off-the-cuff joke is surely repeated sixty or seventy times per semester, worldwide.
But the point of this is not ribald humor. Among the great apes, we are odd ducks. Actually, we would not be so odd as ducks as we are as apes, because ducks are birds and our system of mating is far more bird-like than ape-like. Desmond was a contemporary and colleague of Louis Leakey, and part of the small group of well connected Africanists studying human evolution active in the mid 20th century. These scientists appreciated back in the 1950s and early 1960s that an understanding of ape behavior and ecology would be essential to understanding human evolution. This was an explicit effort to advance Darwin’s comparative methods. Louis Leakey was instrumental in setting up Birute Galdikas, Dianne Fossey and Jane Goodall for fieldwork in Southeast Asia and Central Africa. The fieldwork of these pioneers in ape studies has served, and continues to serve, this purpose: Placing human evolutionary biology in a firm comparative framework.
And it is from this place … the perspective of our nearest living relative, the chimpanzees … that humans are odd ducks in ways that demand an evolutionary explanation.
It is very rare to find mammals that live in groups in which there are multiple sexually mature males. If, in a social species, there are several males old enough to reproduce, the reproduction of all but one of them may be repressed. Modern males get a lot of flack for their bad behavior and are known for their “testosterone poison.” But any mammal socioecologist can tell you that human males are by and large very well behaved, considering … because we are one of those rare species with multiple sexually mature males who’s testicular function, testosterone flow, and all that arises from that is determinately NOT repressed. If we view human social systems, especially as concerns mating, from a chimpanzee perspective, then what we see in humans would seem impossible. If alien scientists from another planet visiting Earth were chimps, they would be shocked and amazed at human mating systems.
Female primates generally advertise their ovulation with estrus. Estrus is a misunderstood and misused word, so I should define it. Estrus is a noun meaning a particular physiological or behavioral state of a female mammal. Estrous is an adjective. So a female in estrus is an estrous female. Many writers ignore the difference and just leave the “u” off or on. Sometimes it is spelled with an “o” (as in Oestrus). Estrus is often equated to “heat” or “rut,” An “anestrous” female is a female not “in heat.”
An ovulating female is not an Estrous female unless she is signaling her ovulation overtly. Estrous is the visible or behavioral display of ovulation. As with all definitions, this gets dicey. For example, cats have facultative ovulation, meaning that they can ovulate “when they want to” as opposed to on a periodic or seasonal cycle. In order for a cat to ovulate, she must have lots of sexual stimulation. But the way she gets sexually stimulated is to show her interest by going into heat. Thus, in cats, an estrous female is not necessarily ovulating (but with sufficient stimulation and if all else is operational she will be soon enough). It is also not true that others (in particular males) fail to perceive the ovulatory state of a female if she is not in estrus. It may be so important for males to assess the ovulatory state of females that there is strong selection for this ability. In many mammals males can titer a female’s urine or vaginal discharge and closely estimate her ovulatory state even if she is not giving overt signals. this is not considered to be estrus. (We can go semiotic here for a moment: A female bighorn sheep shows symptoms of ovulation that can be detected by the male. A female chimpanzee passes into an estrous state that is an overt signal impossible to miss by even the dullest of males.)
To further complicate the definition (but with good reason) it should not be assumed that any female in estrus is necessarily ovulating even in species other than the cats. In many primates it is probably the case that females are estrous for days before ovulation. This gets the males interested, incites male-male competition, and serves to provide the female a part in the process of sexual competition she might otherwise not have. A thought experiment: Imagine that ovulation and estrus is perfectly correlated and almost instantaneous. Ovulation happens, estrus begins, and any nearby male can’t miss the signal. Copulation within five minutes virtually guarantees fertilization, with earlier or later copulations being, well, a senseless waste of sperm. Also assume ovulation is very rare and that all the members of this fictitious species are normally busy with feeding, avoiding predators, etc., but in loosely knit groups.
Under these conditions, mating would likely be almost random. A female enters ovulation and suddenly becomes estrous. The nearest male, or the nearest large male, charges in, they mate, and that male is the father of the female’s offspring.
There are good reasons to believe that females would benefit from choosing the male that fathers her offspring. This is a general principle and should hold true for all social primates. Estrus may facilitate this by prolonging the period during which mating may (or may not) fertilize an ovum, and allowing the female opportunities to manipulate the sexual politics that will always emerge during such a period.
Social primates with multiple males show the strongest estrus signals. Social primates with a single breeding male show the weakest estrus signals. The estrus signal does not serve only to entice competition among males (or, a some primatologists have suggested, indicate that an individual is a female and signal where in the dark jungle she is … like these primates are stupid or something) but it also serves as a means of competition among females. Dominant males can choose among females based on the qualities of their estrus signals. (This was predicted from theory by Mark Pagel and demonstrated in the field at Gombe by Leah Gardner.)
But what does any of this have to do with human mating and sexuality? Everything! Some researchers and writers have noted that humans have “hidden estrus.” This is of course absurd and incorrect from a purely grammatical point of view. Estrus = visible display of ovulation, so “hidden estrus” = invisible visible display of ovulation.
No, human females either do not have estrus … do not display ovulatory state … or have a strategy like the chimps but extended, so they are always estrous. There may be no real functional difference between the two, but in any event the result is the same. The way primatologists put it is that “Females are in a continual state of sexual receptivity.”
At the same time, and this may be hard to believe, males are also different in humans than in chimps. According to primatologists like Richard Wrangham, male chimpanzees are generally uninterested in females sexually. Where human males are said to think about sex every nine seconds (probably a slight exaggeration) male chimps seemingly do not. They are only interested in female chimps who are in estrus. That makes a great deal of sense from an evolutionary perspective. Sex is costly. In a multi-male group bad things can happen to you if you approach a female sexually, when the other males gang up on you and go for the groin. I am told that in a famous macaque research colony … where the monkeys (in multimale-multifemale groups) wander around on a large island more or less “free” there is something like 1.6 testicles per male. The missing testicles … well, they represent the painful cost of inter male sexual competition.
So in humans, sex is not entirely for reproduction and both males and females, depending of various factors, are involved in the whole “sex thing” more or less continuously for a large part of their lives. The other big difference with humans, compared to chimps, is that there is a LOT more investment by adults in offspring, and there is a faily large amount of investment by adult males in offspring. Once you get males investing directly in offsprin, they get very touchy about exactly who the offspring are of. The greatest evolutionary cost a male can suffer, in a species where males invest in offspring, is unwittingly caring for an offspring fathered by another male.
This is the problem many birds have, which is why we see the courtship and nesting behaviors, and long term mating (by season or sometimes for life) in birds. This is where humans and birds converge. Humans have continuous courtship as to many birds, and sexual interaction is part of that courtship. This, plus occasionally remembering it’s your anniversary…
[there is a discussion of this post here.]
So… I’m getting the impression from reading several of your articles that the use of energy is at the heart of evolution.
IOW, is it something like, we have limited amount of energy, and somehow we figure out how to expend it to our advantage (and the advantage of our offspring)?
I am so NOT a scientist but am to fascinated with the topic of evoluion… so forgive me if I’m sounding really uninformed here!
In terms of human mating…. in males, would you say their limited energy is more wisely used for parental investment than just sperm donation to numerous females? IOW, is it in their best interest (and the interest of offspring) to use their limited energy to help offspring?
If so… what about men who do not want offspring?
It seems like in much of the animal world, having offspring isn’t consciously chosen but in humans of course it is…. (I’m thinking this is pretty new? Maybe a few million years or so)? So, what do they do with this “excess” energy that is not being used to procreate or raise offspring? And, why did this phenomenon (of men not wanting offspring) evolve in the first place? (Maybe they get fat - smile- but could it be energy that is used for war? Or something benefitting the world rather than themselves)?
Thanks Greg…
It is in the best interest of males to simply distribute their sperm everywhere, if a) females will let that happen and b) the offspring will be fine without male investment.
A good example is birds. Birds have eggs in nests that require constant protection from egg eaters (eggs are one of the best foods available) and possibly incubation. Imagine that males in a certain species of birds never ever invest in offspring. They simply mate and move on.
Many of the eggs would be eaten by snakes, etc.
Now a genetic mutation arises that makes a male “want” to stay around and act like a female… site on the eggs, protect the eggs, feed the chicks, etc. Most of that male’s offsping will survive, while the neighbors without this allele (mutation) will have very few offspring. This novel allele will spread quickly over just a few generations and the species will be transformed into one with male parental investment.
There are plenty of species where the males have no way to invest, and they don’t. Where investment works, they do.
Males who do not want to have children? They get selected against! (by both Darwinian processes and by females!)
As far as what people want… there need be no relationship at all between what people “want” and what they do. People don’t want to eat food that causes cancer but they do. They don’t want to eat food that makes them fat but they do. It simply is not the case that at some point in time humans became a species that “knows” what it wants and what to do and then goes and does that.
Free will is an illusion. It is something that can happen but it does not just happen all by itself. Most people will never experience it…. ( the glass is half empty on this one )
Hi Greg… thanks for your insights.
You actually answered another question I would have come to ask you at some point… is free will a reality?
More and more I get the impression that it is nearly a fantasy. Seems reality is more like we are becoming aware of our instincts… or something along these lines. We can articulate what our instincts are telling us but is our behavior from a place of deep rooted hard wiring rather than a place of conscious choice?
Those probably aren’t the correct words to use in the scientific community, but it just seems much of behavior has absolutely nothing to do with conscious thinking even though we think we have choices and make rational decisions (or irrational at times).
It seems to me that other animals cannot act in disharmony with their instincts so can we as humans, or are we just aware of our instincts and trick ourselves into thinking we are choosing?
Does this make any sense?
I’m sure “free will” is an old construct that at the very least needs a new look. This all links to consciousness as well. You know what consciousness is when you see it, but that does not tell you where it comes from or what it is.
So much happens that is not in the consciousness, and “free will” is so linked to it, that insisting on the right to practice free will may inadvertently lead to giving up the right to dream.
Ah, and that other bugaboo of a word that is often used but rarely defined has come up, Jennifer: Instinct.
I class discussion I sometimes like to disallow my students from using the word. Some discussion are so tied up into, and based on, a presumption of instinct (and what it is for, what it is, etc.) that some of the student virtually choke and are totally thrown off!
Thanks Greg…
I always hesitate to use the word “instinct” because it isn’t really the word for which I am looking but I don’t know what the word is.
It seems there are some behaviors that hard “hard wired,” some that are more or less tendencies, others that seem to come from somewhere else… a combination of a long history of patterns or experiences that have “worked”.
I use the term instincts to describe those behaviors that are not in our consciousness and seem to be deeply imbedded in our brains, but I suppose this is inappropriate. Hmmm… I’m learning a lot here!
Thanks so much!
“having offspring isn’t consciously chosen but in humans of course it is”
No, actually, if you ask the single mothers in poverty and eslewhere as well as those who employ the tactic of deceipt ( the current in style reproductive strategy of child abusing females, fat chicks and the ugly)to gain progeny about their welfare/workfare/childsupportpaid children, you will often find that the women chose the children as a means to gain financial support from a male who was unaware of the grand plan to use them, and their life and resources–before he got that ass( or two or three males, as the single mother proponents are known to do).
But if you ask any one of those men who got tricked, you will find high numbers of them - if they are literate, and honest- will tell you they were trapped, and only looking for a sniff under the doe-tail.
So, in the human I suspect large numbers of false responses to the question of choice–after all, men still DO NOT have solid or equal reproductive choice, nor options–as in opting out of–post female choice to have kids.
p.s.: we can indeed spot the estrus of females, or at least sense their fertility,and receptivity because we have the innate senses of sight and touch, as well as the ’sixth sense’ of intuitive communication? Is it possible that the constant denial of human female sexual signals is a throwback to the Victorian denial of female sexuality? In particular, human female labia swell when they are aroused, the breasts become more firm when near a period, etc, and there are also at least three distinct odors of a womans vagina: pre/post period smell (Like death itself…) and then the middle of the cycle smell ( kinda fishy, but kinda sweet too).
Is it possible that we don’t discuss these things because we collectively don’t know, or collectively continue to pedastalize women as ‘mysterious’ and all of that other crap?
Granted, the topic is ‘yukki’ to most people, and we are very far removed from this way of sensing a human process, but is it not possible to acknowledge that women do have symptoms of estrus, no matter how subtle they may be, or how hard they are to detect under twenty or thirty years of clothing wearing behavior?
Or are we just out of touch with each other?
Maybe I misunderstood, but Gadfly, wasn’t your 2nd post about exactly what the author went to such lengths to explain?
If I understood correctly, Estrous females are females that are consciously (then again, do we have free will?) displaying that they are ovulating or are about to. Physiological responses to this occuring have nothing to do with it.
And as a female with an irregular menstrual cycle, I myself don’t often know when I am about to ovulate. Then again, I am an adolescent, so perhaps my hormones are so often out of wack that I have not yet had a stable hormonic state to compare the ovulating and period state to.
If you put aside the body’s natural responses to ovulating, you might state that women wearing bright clothing/wearing fancy jewelry/”gussying” themselves up could be estrous behavior. Frankly, most men I know don’t even notice if I’ve gotten a new haircut.
But once again, you could debate that this leads to extreme behavior, in which men need more and more extreme visual changes for them to recognize a mating prospect. An extreme trait becomes almost a necessity, and you see males overlooking the more physically fit females for the visually stimulating ones. Footbinding, Victorian practices, etc.
I believe that subject was discussed in another post, but am unsure.
But once again, I am merely an adolescent who may be misinterpreting the text due to my lack of knowledge about the subject.
Oops. I suppose my posting was useless. No one has posted about this article in two years……….