Q: If humans evolved from the primates, how did the female lose the large, red or pink inflated sac that fills up when the chimpanzee females are in estrus? I think another term for this is called, "hidden ovulation."
A: First, let's define estrus.
Wikipedia: "The estrous cycle (also oestrous cycle; originally derived from Latin oestrus) comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian placental females. Humans undergo a menstrual cycle instead. Estrous cycles start after puberty in sexually mature females and are interrupted by anestrous (pregnancy) phases. Typically estrous cycles continue until death."
In common person street language of 2008, it means "horny,"
"horned up." The lust stage - the sexual "static electrical
charge" build up that needs to be released - It's a stage that all healthy,
mate-aged males and females go through. It's basically our genes and hormones
letting us know that it is time get up off the couch and look for a mate and
replicate ourselves. (How males and females do this are called "mating
strategies" and within those strategies one can find the "battle
of the sexes"). When the female chimapanzee sac fills up, it lets the
males in their group know that the females are receptive to copulation because
they are within their peak ovulation timeline. And by all accounts, some scientists
have observed a "frenzy" of activity amongst the primate males when
they observe females with full sacs. (Try to imagine you putting on a red
clown nose as dictated by your local culture to signal your reproductive ovulation
receptiveness, and further attempt to visualize potential male friends reacting
to this obvious signal when you try to start a conversation).
The argument has not been settled as to why the human female "lost" this particular physical trait, but one strong theory is that your primate female cousins got fed up with all the frenzy over their visible sexuality and chose (sexually selected) to readdress this hyper-sex drive by males for sexual access as a bargaining chip for greater commitment and redirect the direction of the copulation ritual dance. This female behavioral mechanism works for you even today - "Let's see how much we can get out of this guy in terms of resources and commitment if he is so horny." There is nothing to be ashamed of ladies - you've been doing this since the dawn of civilization and you're doing it for your happiness today and future progeny. And if the human male of today does not want to follow your lead, then he could end up a reproductive loser and find that his life-long could end up being his five fingered-grip on reality.
Now, since alpha chimpanzee males have the sexual etiquette of a slobbering dog and they chose to take their women sexually where and whenever they felt like it regardless if the female had a rotten day, it is believed that they forced themselves only upon your ancestral females who displayed the right color, shape, or smell that was associated with the brightly colored angeogenital sac that was the entry way to satisfy their sexual desires. In equivalent terms today, that would be like a slobbering alpha male favoring large, silicon-perked-up breasts found on female porno-stars with easy sexual access values over the smaller-breasted, yet, smarter, less-sex-minded girls in the office pool. As primate groups grew, competitive alpha groups remained in the inner circle, while forcing weaker, beta and charlie males and females to the outer edges where fewer resources were available. We use the term "smarter" in reference to the ability to survive with fewer resources found at the outer edges of the group's primary territory. One way of being "smarter" is forming partnerships with males in order to survive. One way of forming partnerships, is through communication - thus arises the evolutionary pressure and logic to create a "language" to communicate.
The "weaker," yet smarter beta and charlie
groups on the outer edges of the competitive inner groups split off to become
humans. Within this split, the females with less obvious, angeogential sacs
slowly disappeared with each passing generation as your female ancestors introduced
"cycle-wide" sexual access to one male for his heightened commitment
to stick around and help with child rearing. If the biological pressure to
attract males visibly is removed (the red or pink sac) by having constant
sex with one male, then it would make sense that the biological messages within
the female body would signal to the brain that "once-a-month" visible
sac presence to attract males is no longer necessary. The constant sexual
usage would also send similar chemical messages to the brain that constant
sex is now the norm as opposed to once a month sex. Constant penis entry friction
throughout the entire cycle would also tell the brain that a lubricated vagina
would also help throughout the cycle, instead of just once a month. I am also
positive that a large factor in the disappearance of the sex sacs has to be
that males may have sexually favored your female primate ancestors with smaller
and smaller sacs, and thus, additional evolutionary pressure for hidden ovulation.
Copyright, Evolution's Voyage, 1995 - 2011