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…