Man as the Prayer: The Origin and Nature of Humankind by Yup Lee
Finally under severe stress and malnutrition, some females may have experienced physiological abnormality. It is very likely that the reproductive physiology of the primate species was affected in one way or another by malnutrition and physical stress.
There are some studies which indicate that the reproductive physiology of human females was influenced by malnutrition. For example, malnutrition has the potential to postpone the onset of menstruation, interrupt the normal female reproductive cycle and even impede ovulation.29 Furthermore, severe malnutrition is said to interfere with reproductive functions to the extent that the females concerned stop menstruating and become even barren.30 Menarche may be delayed among malnourished adolescent female humans.31
Physical stress is also found to influence the functioning of the reproductive physiology. Menarche is postponed among athletes32 and adolescents who practice strenuous physical activities such as ballet.33
Besides humans, physical or psychological stress among females of some primates and mammals is known to disrupt the normal production of various hormones which suppress ovulation, resulting in an abnormal lengthening of their estrous phase as well as in frequent occurrence of anovulatory estrous cycles.34
Based on such data, it is speculated that one day an unusual female may have shown up due to severe physical and psychological stress. Her body began to malfunction under severe strain, and her physiology went awry so that her private parts swelled up to become unusually big and reddish, visible from long distances whenever she was on heat. In the past, she may have displayed nearly invisible vulval swelling and/or pinkening during the time of estrus, just as with many prosimians and some New and Old World monkeys of today.35 But this swelling seems to have become unusually bigger and visible from distance.
To her surprise, males responded to the swelling and gathered around. Unexpectedly, the abnormal swelling became an asset highly valued by males as well as females. She became an instant Cinderella.
As a result of this, unusual genital swellings may have solved her sexual problem. Moreover, her exceptional physical feature would have been inherited by some of her descendants. With time, the sexual swellings became a rule rather than an exception among the lineage of the primate species involved. This means that sexual swellings were favored and selected among the lineage of the creatures involved because they increased their fitness by helping them to survive and reproduce.
In this way, the morphological changes of sexual swellings might have originated at least three times independently in the span of millions of years of primate evolution.
The sexual swellings seem to have developed further under the pressure of natural selection as secondary cues to make up for the time-honored primary olfactory signals. With the passage of time, however, the secondary ones became more and more emphasized among some descendants of the lineage until the primary cues became nearly obsolete. An example is the savannah baboons. Their females are known to display sexual swellings for 2 to 3 weeks, much longer than the time of ovulation.36 This trend of going far beyond the time of ovulation seems to have been set in motion by the competition among females for mates.37 Incidentally, the same tendency seems to have existed among ancestors of the great apes even though modern female orangutans do not display sexual swellings and female gorillas of today exhibit only small ones.
Female orangutans do not sport sexual swellings but their perinea swell up and turn whitish when they become pregnant. This kind of peculiar physical changes appears to be an ironical modification of the sexual swellings.
Among wild orangutans, males are nearly twice as heavy as females, and some males weigh around 100 kg (220 lbs.).38 Thus, they are the heaviest arboreal animals.39 Their great sexual dimorphism can be explained in terms of sexual selection, a special kind of natural selection proposed by Darwin.
According to the theory of sexual selection, advantageous characters like the enlarged canines develop over time among animal species because individual animals compete one another over mates within one sex or between two sexes to promote its own reproductive success.40
Based on this theory, it is reasoned that great body-size dimorphism evolved among orangutans due to fierce intermale competition over females in which only stronger, thus heavier males could have survived.41 If so, such an intermale competition might have been fueled by extended sexual swellings.
Early orangutans may have lived almost strictly in trees just as their present-day descendants do.42 If so, many males would have fallen down to death or injury in the course of melee, assuming that they fought one another over females high up in trees. Many estrous females and their young may have also become victims in the maelstrom of intermale aggression.
Under such circumstances, females with prolonged duration of sexual swellings should have been more easily embroiled and victimized just because they would have been exposed to danger for a longer period. On the other hand, females with a shorter period of sexual swellings than the others should have had a better chance of survival.
Consequently, being estrous became the most dangerous thing for females and their immature children. A shorter period of sexual swellings was a blessing. Thus, their sexual swellings seem to have been under the selective pressure to become shorter and shorter until they almost became obsolete. (more)
Copyright © 2000 Yup Lee. All rights reserved.