Meticulously, you inventory the primates’ other circadian rhythms. Primates sleep at night and are active during the day. Their metabolic needs vary slightly during this diurnal cycle, causing their temperature to fall one degree in the middle of the night and rise one degree in the late afternoon. Nocturnal animals (among which are a few species of primates) have the reverse circadian rhythms. Homo sapiens’ testosterone levels rise in the morning and dip in the evening. His levels are higher in the fall than in the spring. None of these slight variations translate into obvious observable changes in behavior. You pass over each of these primate circadian rhythms because they are too subtle to be of much use for teaching an animal about deeptime.
Running out of options, you repeatedly return to the idea of using a primate’s striking estrus cycle as the key internal timer, and you seek a way around the problem of its staggered estrus. During your initial survey, you had noted that a few primates featured a unique component of estrus missing from the majority of mammals and absent from all other sexually reproducing species. Your interest piqued, you eventually focus on the small cohort of thirty-one primate species that menstruate. “Hmmm,” you muse, “menses is a reliable circadian rhythm—a most unusual clock, but a clock nonetheless.”
The germ of an idea begins to take form within your mind. You note that menses marks the exact counterpoint of these few primates’ estrus cycles with an event—external bleeding—that just so happens to be both dramatic and episodic. The interval of approximately a month is also appealing to you, for its length fits in with your ambitious plan. The problem: This subset of female primates experiences so minor a menses (some barely bleed) that they hardly notice the event.
In a thunderbolt of inspiration, you hit upon the solution for how to teach this particular animal the meaning of deeptime: Abandon the idea of using estrus as the marker and substitute menses as the key element instead. Like Mary Shelley’s Dr. Frankenstein, you lock the door to your laboratory, roll up your sleeves, and begin to fashion a creature the likes of which no one—on earth or above it—has ever seen.
The Russian physiologist Ivan Pavlov performed the classic experiment psychologists have since named “Pavlovian conditioning.” Pavlov trained a dog to salivate at the sound of a ringing bell. Normally, a dog begins to salivate at the sight and smell of his owner bringing him food. Pavlov structured his experiments so that he consistently rang a bell whenever he fed the dog. Even though the sound of a bell had nothing to do with eating, the dog’s brain slowly built new neuronal connections that caused it to link ringing with the arrival of its dog dish. This conditioning required that Pavlov repeat the drill over and over again. Eventually, however, his dog learned to associate the arrival of food with the sound of the bell. One day, Pavlov clanged the bell’s clapper but did not deliver the food. The dog salivated, even though no food was forthcoming. Until the dog learned this new, disappointing lesson, it would continue to salivate at the ringing of the bell.
You decide to use a similar form of Pavlovian conditioning as the primary teaching tool to instruct Gyna sapiens how to recognize deeptime. Instead of a bell and food, you begin redesigning the estral and menstrual cycles of this singular bipedal primate. You set to work tinkering with three reproductive features in six interrelated steps to pull off what you are sure will be your most sensational feat of evolutionary showmanship.
Step One: You begin by synchronizing each Gyna sapiens’ estrus and menses with those of the other females whenever they congregate in tight communities over extended periods of time. This alignment means that both their signs of estrus and the subsequent sloughing of their uterine linings occur in unison. (Although we cannot know which of the six steps to be outlined occurred first—most likely they occurred nearly simultaneously—let us arbitrarily designate the harmonizing of women’s cycles as Step One in Mother Nature’s radical makeover.) Over thousands and thousands of years, the coincidence of women in close proximity bleeding together would begin to impinge upon the awareness of a few of the more intelligent and observant Gyna sapiens of the tribe.
Step Two: You begin to suppress all external signals of ovulation. You had briefly considered using these well-demarcated intervals as the key learning marker, but quickly discarded the idea. With hormones raging, animals in heat can think of nothing but sex. The mating instinct is so powerful that it would interfere with any attempt to teach an animal a new trick. Dog trainers are reluctant to introduce a new routine to a female in heat when nearby males are urgently barking. Conversely, the scent of an ovulatory female distracts a male dog, making him mad with desire. You reach the conclusion that you must select a less discombobulating periodic event than estrus.
Gradually, you suppress the smells, sights, calls, and gestures associated with ovulation. Your final goal is to make ovulation so opaque that even the female in which it is occurring would have difficulty recognizing it. Finally, external ovulatory signals that mark a most awesome moment in the life of virtually all other female animals disappear entirely. By eliminating a significant diversion from the newly emerging consciousness of Gyna sapiens, you refocus the female’s attention on the one remaining periodic event in her life—menses.
Step Three: In a stroke of genius, you sharply escalate the experimental primate’s menstrual symptoms. During the time that ovulatory signals were fading, Gyna sapiens’ menstrual bleeding became more copious, more odiferous, and an increasing nuisance. During her period, she experienced psychic and physical changes, many of which she came to dread, such as migraines, depression, anxiety, irritability, cramping, bloating, metallic tastes, and tender breasts.
Gradually, over numerous generations, you caused the problems surrounding menses to mount. The physical, psychological, and physiological events associated with menses increasingly intruded on Gyna sapiens’ attention. Finally, they reached a tipping point at which the females could no longer continue to ignore them. Momentous changes had overtaken the hominid female destined to become the ancestor of modern women. She began cycling in harmony with other women, she lost estrus signaling, and she gained a bothersome monthly period. But all the elements of your complicated plan were not yet in place. Three more steps would be required.
Step Four: Intently, you searched for a recurrent event in nature that marked off an appropriate interval with which you could begin to entrain your experimental subject’s menstrual cycle. You knew you could not start your Pavlovian conditioning experiment until the rapidly evolving sections of Gyna sapiens’ brain, grappling with the novel concept of linear time, had two events, one internal and one external, to guide it. Another requirement: The recurrent event had to possess some sort of a spectacular indicator. The choices were limited: a day, a month, a season, or a year. A day was too brief, seasons too variable and their transitions too subtle, and a year was too long. The 29.5-day interval between two new moons, however, was just right.
Your original decision to use a primate for your evolutionary experiment now proved extremely fortunate, for a primate is among the few animals that can actually see the moon. Excepting birds and several other mammals, the majority have poor eyesight. Many have an eye positioned on either side of the head. At any given moment, they are looking at two different scenes. If one eye could see the moon, the other would be peering at the ground. Getting an animal to pay attention to lunar phases, you realize, will be much easier if both eyes happen to be looking at it.
The vast majority of animals habitually align their vertebral column parallel to the earth.* Few ever direct their gaze into the sky, preferring instead to keep their noses pointed at the ground, since for the vast majority of large animals the sky is boringly uninteresting. No food comes from that direction; neither do threats. Many live under cover of an impenetrable forest canopy and could not track the moon even if they tried. Animals that live in the open during daylight often seek dense brush at night to hide from predators. Predators that hunt out in the open during the day often spend the night p
ursuing prey in dense foliage.
The moon is 248,000 miles distant. Vision is a primate’s supreme sense. The function of seeing co-opts one-third of all the neurons in a monkey’s cerebral cortex. All primates are blessed with two of the most forward-facing eyes in animalkind. Both eyes look at the same scene, but view it from an ever-so-slightly different angle. This quirk of the primate’s visual apparatus allows it to knit these two overlapping scenes magically together.
Animals with forward-facing eyes converge the information garnered from their two eyes and use it to construct the coordinate of depth somewhere in their brains. Close one eye, and look at the world, and it appears flat. Open the second, and instantly the landscape transforms from a flat, two-dimensional world into a rich, three-dimensional one. Stereoscopic vision, a primate’s forte, along with other visual clues, allows it to see very distant objects and instantly estimate their spatial relationship to nearer objects with a degree of precision unsurpassed by any other animal, including eagles.*
Another feature solidly connects an upward-glancing bipedal primate to the moon. A magical moment occurs in every child’s life when he or she realizes that the moon is the child’s personal companion! As we move through the nighttime landscape, approaching objects glide by and then recede into the distance behind us. Not so the distant moon, which always keeps pace right alongside us. We stop, and the moon stops. We move, and the moon moves. The darkness of night accentuates this observation, because the moon is usually the brightest object. During the day, the same phenomenon applies to the sun, but because there is so much more to see in daylight we rarely if ever notice it. Moreover, looking directly into the sun is extremely blinding and hazardous.
There is something vaguely comforting, especially to a small child who has a natural fear of the dark, in knowing that the moon is a reliable and faithful companion that will not only light the child’s way but also be a steadfast companion during nighttime excursions. This simple but gripping observation establishes an early personal lunar connection that predisposes many people to feel kindly toward the moon throughout the rest of their lives.
When night falls, there is no other object in the visual landscape so distinctive as the moon. The appearance and disappearance of this silvery orb passing overhead has always enchanted humans. She leads a stately procession of her retinue of stars and planets in a celestial pageant. To prevent monotony, the moonshow changes its routine nightly and repeats its run every month. Before the invention of electric lights, the moon’s performance was a most closely watched show that held ancestral people enthralled. To a significant number of the population today, the moon continues to exert a mystical attraction.
Step Five: Having assembled all the key components, you begin the penultimate, crucial maneuver. You must coordinate Gyna sapiens’ menstrual cycle so that its average length is exactly aligned with the moon’s periodicity. The thirty-one primate species that exhibit menses have cycles ranging from seven to forty-two days. For example, the lion-tailed macaque’s cycle recurs every forty days on average; the tiny tarsier averages twenty-four days. The length of a lunar month is 29.5 days.* Many nonhuman primates have cycles falling close on both sides of this magic number. But, for your purposes, Gyna sapiens’ menses has to hover consistently around 29.5 days. A half-day less or more would undermine your entire enterprise.
The Barbary macaque’s menstrual cycle is thirty-one days. If a Barbary macaque menstruated on the night of a full moon, in the next cycle she would bleed when the moon had been waning for one and a half days. In the following month, the moon would have been waning for three days, and by the fourth cycle menses would occur a day short of a quarter moon. Within ten cycles, the Barbary macaque’s menstrual cycle would be completely out of phase with the moon. The low-wattage brain of this monkey, attending to a seemingly insignificant lunar synchronization with her nearly inconsequential menses, would make it extremely unlikely that she would recognize a correlation between the moon’s cycles and her menstrual cycles.
The opposite problem occurs for the Japanese macaque, whose menstrual rotation is twenty-eight days in duration. If she begins to bleed on the full moon in one cycle, she will bleed again during the waxing phase of the moon, one and a half days earlier, the next time. Thus, within ten cycles she, too, will be completely out of phase with the lunar cycle.
You realize with a sigh of relief that not every ancestral Gyna sapiens within any tribe will have to have a cycle that matches the exactitude of soldiers marching in lockstep. Many modern women’s cycles vary considerably from the ideal 29.5 days. And not every woman who experiences a 29.5-day cycle does so for every cycle throughout her life. You understand that such perfection will be impossible to achieve. But you appreciate that, if the majority of Gyna sapiens’ cycles average 29.5 days, then sufficient numbers of them will entrain with the moon to shake out a few of them who will recognize this critical pattern.† And with the parallel development of an exciting new form of communication, you gleefully anticipate that all it would take for your experiment to succeed would be for a handful of smart, observant Gyna sapiens to begin pointing out the connection to the others.
Diffusing out into the population, this wondrous discovery would rapidly become a crucial component of the culture’s lore, to be continually renewed down through the generations by the most inventive myths, stories, taboos, and superstitions. At the same time, women shamans would mark the connection by creating a wide assortment of notational calendars, amulets, talismans, and other abstract representations. So vital is the knowledge of the link between the moon and menses that it would become embedded in the very core of a wide variety of languages. Along with their mother’s milk, children would imbibe it with their native tongue.
Step Six: Having accomplished the main part of the experiment, you hasten to add one final exclamation point to the link between lunar and menstrual cycles. You delicately calibrate Gyna sapiens’ onset of bleeding to coincide with the moon’s ides. Within every lunar cycle, there is a dramatic three-day period when the earth interposes between the moon and the sun. To an observer looking to the sky, the moon mysteriously disappears. The effect is to plunge the side of the earth facing the hidden moon into near-total darkness. These three nights, in some cultures called the ides, seem longer and more ominous than others in the cycle.*
Moonless nights would have been a time of rest and introspection for ancestral humans. With only dim starshine to guide their way, few would dare venture forth. Because of the sudden withdrawal of a woman’s primary sex hormones, menstruation also tends to induce an introspective state of mind. Darkness, repose, dread, alertness, and introspection are a fortuitous mixture, out of which often emerges a deepened receptivity to new insights. If you could arrange to have Gyna sapiens begin to bleed during the dark of the moon, the increased premonitory mood experienced by a primate that depends on light to see would significantly heighten its alertness.
Distinctive mood changes, unmistakable physical symptoms, and the dramatic external sign of blood loss occurring coincident with a fear-inducing event happening in the sky at a time when all the other females in the band were similarly indisposed could not fail to attract the attention of at least one clever Gyna sapiens. You now feel confident enough to begin to exult. You are sure that it will not be long before you witness positive results from all your complex and careful time-related preparations.
To reinforce Gyna sapiens’ awareness of the intertwining of the moon’s cycle and her menstrual cycle still further, just for insurance, you add another, related lunar-arithmetical coincidence. At present, obstetricians predict the date of a woman’s delivery by counting the onset of her last menstrual period as Day One, reckoning a pregnancy’s length by this method to be nine calendar months, or 280 days. A calendar month, however, is a day or two longer than a lunar month. If the count begins from the date of the last full moon prior to a pregnant woman’s first missed menstrual period, then the length of a human gestation
can be more accurately calculated. The average length of a human pregnancy using these parameters is 265.80 days. Nine lunar months equal 265.77 days. The statistical odds against this correlation’s being just a coincidence are astronomically high.
More often than not, conception occurs on the night of a full moon, with most babies born nine lunar months later, on the night of another full moon.* We still tacitly acknowledge the moon-sex-birth connection when we refer to newly married couples about to embark on their culture-sanctioned sexual marathon as going on their “honeymoon.” Anthropologist Chris Knight remarked, “No other primate shows so close a correlation between menstrual cycle length and the lunar month, nor between any whole number multiple of the menstrual rhythm and the length of gestation.”7
Before Western culture converted to solar calendars, midwives used the lunar method to calculate delivery dates. It is a common practice even today in many less developed countries and rural areas of industrialized societies. Skilled midwives kept careful track of the moon’s orbital periodicity to guarantee the accuracy of their predictions. Full moons also gained significance, because for many women it was a reliable external clue that they were ovulating.
Numerous reports in the ethnographic literature confirm that women the world over understand the connection between the moon and the length of pregnancy. Among the Tiwi in Australia, as soon as a woman knows she is pregnant she starts to “follow the moon.” Anthropologist Franz Boas, reporting on the Kwakiutl women of North America, comments, “The child is generally born on the night of the full moon.” In Vietnam, first-time mothers are advised by midwives, “For your first pains wait until the moon is full.”8
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