A 2001 study of a species of bird (Collared Flycatchers) revealed sperm competition as a source of natural selection. The researchers established the secondary sex characteristics of male Flycatchers, to see which males gathered the most female attention. They were able to determine that the most attractive males were the ones who had the largest white patches on their foreheads.
This is seen in most species of animal. In humans, analogous secondary sex characteristics might be facial masculinity (square jaw, prominent “adams apple”, cleft chin etc), body smell, masculine (deep) voice qualities, and behavioral displays of sexual competitiveness with other males. The biggest indicator of general attractiveness in human beings is the symmetry of ones features, especially facial features.
Let’s go back to the birds for a bit…
So – males with a large patch are more attractive to females. Let’s call them “superior males”, as opposed to the “inferior males” with the smaller forehead patches.
Like a lot of animals, female Flycatchers pair-bond to a mate of the opposite sex in order to raise her or their offspring. The female wants to choose the most genetically superior mate – the best-looking male. So she will do her best to attract a superior male.
If the female fails to attract a superior male, she will settle for an inferior male. However, when a female is mated to an inferior male, the study showed there was a high chance that in the two days before she started laying eggs she would stop her mate from having intercourse with her, and would instead seek sex with a superior male or males.
The only females in the study who stayed completely faithful to their mates were the ones with superior males as mates. 40% of the total population of Collared Flycatchers mated with at least one male who was not their mate.
A study of human females showed similar results. The 2005 study showed that women become much more attracted to men she perceives as superior, when she is ovulating or in the fertile phase of her cycle. This particular study showed that a woman’s attraction to her mate, no matter how superior he was, remained fairly constant through her cycle, though other studies have shown an increase in attraction to her mate (particularly to a superior mate) during the fertile phase of her cycle.
The benefits to a female of breeding with a superior male are obvious – she gets his superior genes to pass on to her offspring.
The downside for a female Collared Flycatcher is that her mate will help more with raising her offspring if he is more sure that they are his. This is offset by the fact that the inferior male Flycatchers were generally better at parenting than the superior males were in the first place.
The 2005 study points out a similar problem for human females – that even though she may get superior genes, if her partner is jealous of her having sex with another, superior partner, then that jealousy lasts throughout her whole cycle, and may lessen his willingness to support her in raising her children.
One factor that probably affects human females over birds is the comparatively long child-rearing phase that humans have. It is a bigger risk to a human female to lose the support of their partner than it is for a Flycatcher, whose offspring-rearing duties only extend for a few months. So sex with a superior partner is just as desirable for women, but less likely in practice.
The conclusion of the 2005 study is particularly interesting:
Imagine an experiment that will never be done, one that parallels Rice’s (1996) seminal work on sexually antagonistic coevolution. Suppose that women were allowed to evolve in response to men but men not allowed to adapt to women. After many generations, women would likely gain an edge in the conflicts between the sexes—possibly evolving better means of circumventing male vigilance, reducing the costs of obtaining genetic benefits through extra-pair mating and, accordingly, doing so more often. Alternatively, if men but not women were allowed to evolve, men might evolve better means of detecting women’s ovulation and avoiding cuckoldry, thereby reducing the frequency of women’s extra-pair sex. Of course, neither scenario has occurred; the sexes have coevolved and, most likely, both sex’s genetic interests are compromised by adaptations of the other sex. The mating strategies and tactics of both sexes have possibly undergone substantial revision through rounds of adaptation, counter-adaptation, counter–counter adaptation, etc.
I’ll freely admit I have no studies to back up my own conclusions here (please let me know if you find something for me!). All this makes me wonder about whether cuckolding fetishes are the next evolutionary leap…. backwards, in a way, for men – giving women the evolutionary advantage of being able to gain the genetic benefits AND keep the support of a partner. Polyamory or other forms of non-monogamy may also support women in this way.
Hmmm, I’ll have to give it some more thought. It sounds good to me!