Lapping It Up

Ovulating all the way to the bank

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It is safe to say that the core component of evolution is reproduction (and here we’re talking sexual reproduction: XX + XY = baby X?). And it stands to reason that the best way to be successful in this endeavor is that it be obvious for the interested parties to know when conception is most likely to occur for any given mating event. Female mammals are not equally likely to conceive on any given day – there is this thing called the ovulatory cycle of hormonal changes leading up to release of an egg (aka ovulation). This cycle differs in length for different species. For young adult human females, this is generally 28 days or so, though cycle length can vary and be less stable early in reproductive life and beyond 40 years of age (after which cycle length tends to shorten and become less regular, until menopause). For chimpanzees, our nearest relative, it is around 35 days.

Many female animals (mammals, birds, fish…) signal their reproductive status using a variety of cues: visual, auditory, chemical, behavioral) so that males will know, hey, now is the time! Among many Old-World monkeys and apes (of which you are a member of the latter) common cues in females are sexual swelling and ‘presenting’ behavior. Sexual swelling results from engorgement of the tissues surrounding the genital and perianal region and has been widely thought to signal ovulation. Presenting behavior (aka proceptivity) can initiate copulation, as an ovulating female will, as you might guess, present herself to a male. She might, for example, approach or follow a male, sit next to him, make certain facial expressions or gestures (such as lip-smacking in the case of female baboons), or turn her backside toward a male as a clear signal that she is willing to accept copulation. Many animals, including some primates such as rhesus macaques, employ chemical signals known as pheromones to indicate reproductive status. Literally, love is in the air!

But humans, as you might have guessed, are different. For one thing human females will be sexually active throughout their ovulatory cycle, not just when conception might occur. And they have what is known as concealed ovulation, meaning that it is not obvious to men when a potential sexual partner may be most likely to become pregnant [see footnote 1]. This 'most likely' period is known as the fertility window, and spans 6 days leading up to the day of ovulation. Since ovulation typically occurs around day 14 of a 28-day cycle, the fertility window will include days 9 – 14 (as you also might have guessed there is a bit of variability here as well). This 'window' is the period that couples who are wanting to make a baby will really pay attention to! There is much debate around why females would have evolved concealed ovulation, and more than one reason may be true. Perhaps it was to reduce competition and aggression among ovulating females wanting to attract a mate, or to increase investment by males in a female throughout her reproductive cycle and not just when she is within her fertility window. (This notion led to a controversial hypothesis around pair-bonding and monogamy in our ancestors). Concealed ovulation may also have come about to confuse males regarding paternity and reduce the likelihood of infanticide. In primates in large multi-male and multi-female social groups, it is not unknown for a male to kill a female’s infant, which would end lactation suppression of ovulation and bring her back into reproductive status sooner, thus giving him a shot at passing along his genes!

But just how concealed is ovulation in human females? You don’t see much in the way of genital engorgement, or even lip-smacking for that matter! Can men tell at all if, and when, a potential mate might be sitting on the sill of her fertility window with a come hither look in her eyes? Geoffrey Miller and his colleagues from the Department of Psychology at the University of New Mexico sought to answer this question. They noted that while ovulation is supposedly hidden in human females, some research had previously shown that women around the time of ovulation are more attractive to men through pathways such as body odor, facial attractiveness, more narrow waists relative to hips and even though language (such as use of more suggestive and creative phasing). However, most of these studies were conducted under controlled laboratory conditions – Miller and his colleagues wanted to see if ovulation could be detected by men in the so-called real world. They devised a rather novel way to directly assess whether men could detect if a woman was ovulating or not (that is, not suppressing ovulation by using birth control pills). And if she was not using the pill, could men detect a woman in her fertility window at or just prior to ovulation when conception is more likely to occur?

The researchers recruited 18 lap dancers working in several gentlemen’s clubs in Albuquerque, New Mexico. That’s right, lap dancers! Lap dancing is a unique form of exotic dancing in which the dancer – wearing only a bikini bottom or underwear but otherwise nude – sits on a male client’s clothed lap and ‘dances’ using gyrational motions [see footnote 2]. Dancers earn most of their money from tips, since having a firm price for a lap dance (which typically lasts about 3 minutes per song) would constitute formally selling a sexual experience (i.e., prostitution). Thus, a dancer’s income is directly related to the number of dances performed in an evening. Clients in these clubs will ‘sample’ different dancers at the start of an evening before selecting one they would like to have perform multiple dances and, as Miller and colleagues note, are thus able “to assess the relative attractiveness of different women through intimate verbal, visual, tactile and olfactory interaction”.  From the dancer’s perspective, their attractiveness translates directly into cash in the bank.

Of the 18 dancers recruited, seven were ‘on the pill’ and 11 were not. Each day, over a period of 60 days, each dancer would use an online portal to record mood, hours and place of work, tips earned, and whether they had begun or ceased menstruation. Now, to fully appreciate the results of this study it is important to consider that a woman’s normal reproductive cycle consists of 3 phases: days 1 to 8 is called the follicular phase leading to the fertility window portion of the cycle (aka estrus), and the luteal phase following ovulation and leading to menstruation [but see footnote 3]. In total they recorded data on tip earnings over 296 works shifts and 5300 lap dances (!) and the results were striking. For an average 5-hour shift, dancers not using contraception reported earnings of $335 during their fertility window (estrus), $260 during the luteal phase and $185 during the menstrual phase. Dancers using contraception showed no earnings peak during estrus, though a modest increase in earnings was seen at the beginning of the luteal phase (up to about $250/shift). In other words, men will tip more when sharing a lap dance with a fertile and ovulating woman than otherwise, suggesting that they are in some way able to discern the reproductive status of a dancer when engaged in an overtly intimate experience. Tipping non-fertile women, or fertile dancers at any stage of their cycle, could simply reflect expectations of the situation. That’s what a gentleman does in a gentleman’s club when a nearly naked woman is 'dancing' on your lap! So it seems that concealed ovulation is not so concealed after all!

Humans are considered a serially monogamous species, in that we tend to have one mate at a time but may have many mates over a lifetime. At some point in our evolutionary history women may have evolved ‘concealed’ ovulation for all the reasons note earlier and at the same time evolved cues (odor etc.) that might signal to a “high-quality extra-pair partner” just before ovulation, "whatcha doing, sailor?". In other words, the evolution of what Miller et al. call “opportunistic infidelity” which could result in a more evolutionarily fit offspring. At the end of the day, that’s (not) amore!

Footnote 1: A species of chimpanzee known as the bonobos will also engage in sexual activity at any time during the female’s reproductive cycle and are the only other primate species other than humans to partake in sex just for the fun of it! And humans are not the only primate in which females show concealed ovulation; among great apes orangutan and gorilla females are included as well (though they may show slight swelling of the labia during ovulation).

Footnote 2: NM law requires that dancers cover their genitals. Miller et al. note that this would also prevent the client from knowing if the dancer was menstruating, as the bottom clothing would hide the fact of her using a tampon.

Footnote 3: Because a woman can be fertile before, and sometimes after ovulation this Miller and co. divided the data into a menstrual phase (days 1-5), a fertile phase (days 9 – 15) and a luteal phase (days 18-28).

Where to find the science:

Miller, G. et al. (2007) Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus? Evolution and Human Behavior 28: 375–381. https:// doi:10.1016/j.evolhumbehav.2007.06.002 

Of Further Interest

Haselton, MG and K Gildersleeve (2016) Human ovulation cues. Current Opinion in Psychology 7: 120-125 http://dx.doi.org/10.1016/j.copsyc.2015.08.020

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