Man as the Prayer: The Origin and Nature of Humankind  by Yup Lee                            


Ovulation occurs mostly on the last day of its second phase or on the first day of its third one while sperms in the Fallopian tubes remain effective for only about 48 hours.16

Adult pygmy chimpanzees, another close evolutionary relative of ours, exhibit sexual swellings in a similar fashion.17 But the duration of their full inflation is more than 20 days of their 46-day menstrual cycle.18 They mate usually during the maximal and submaximal phases of sexual swellings.19

Among gorillas, another one of man's closest living relatives, sexual swellings are very small in size and inconspicuous.20 Their maximal swellings are known to last for two to three days.21

In contrast, orangutans do not exhibit sexual swellings. Instead, their perineal region swells up and turns whitish when they become pregnant.22

How would this kind of peculiar morphological change have come into being among many species of monkeys and some great apes categorized as higher primates?

In the course of the struggle for survival, unusual things of all sorts happen among animals over a long period of time. Even some of the minor variations, if it is advantageous to the struggle for the existence of a species, are retained and inherited by offspring over time, bringing about certain changes in their physical traits.

As they are retained over time and become commonplace, the animals thus complicated become substantially different from their ancestors, forming a new species of animals. All kinds of animals are assumed to have made their appearance in this way on the face of the Earth. This process is termed natural selection by Darwin.23

The origins of primates and their evolutionary history are still fog-shrouded and remain a matter of speculation and imagination because we have few fossil material at hand.24

Because of this, it is still open to question whether the originators of primates lived on the ground or in the forest about 70 million years ago. But it is certain that they had adapted successfully to the arboreal habitat and evolved into many different species in the course of time; many kinds of prosimians, monkeys and apes arose one after another.

Continuous action of natural selection should have rendered long muzzles, the flat vision and black-and-white vision of primates, into short muzzles, in-depth vision, and colorful vision respectively. In the same way, the periodic morphological changes known as sexual swellings should have originated among distant ancestors of some primate species in the course of adaptation to the three-dimensional tree life.

It is speculated, based on the data of female morphological changes among extant primates, that sexual swellings might have evolved at least three times independently among Old World monkeys and great apes.25

Then, what kind of selective advantage did they get from sexual swellings, which warranted their retention and inheritance? It is a matter of speculation for the moment, but can be explained in terms of natural selection: natural selection seemed to have worked on the swellings to become bigger and more conspicuous because they promoted reproductive success of the females involved.

When females come into estrus, several changes are exhibited among many primate species. The color and/or shape of their genitalia as well as the odor and taste of their vaginal discharge alter under the influence of hormonal changes. Their behavior also changes markedly, for instance, in terms of presentation and tolerance of mounts. They may even emit some specific vocalizations to show their sexual receptivity.

However, it is not yet certain whether males respond automatically to only one of such changes like olfactory changes or if they take several different changes into consideration to make a decision. It is also open to question whether they take acquired factors like prior social experiences into consideration if different kinds of information are used to assess the situation for appropriate reproductive coordination. It can only be speculated, based on researches such as those done on the rhesus monkeys,26 that some primates may utilize not one but several different sources of informations such as olfactory, visual, auditory and behavioral cues in order to coordinate their reproductive behavior and that they may need some social learning and experience to develop appropriate behavior for successful mating.

It is speculated that sexual swellings initially evolved to complement other cues which used to signal female receptivity. If correct, this hints that females of such primates had experienced difficulties, sometime in the past, locating males even if they had specialized scent glands in the genital region just as many of today's prosimians and New World monkeys do.27

It is further speculated that they originally depended heavily on particular odors or chemical signals for successful mating. Females would have given out distinctive, special odors through excretion and secretion, especially vaginal secretion during estrus or the time of sexual receptivity, the way many living primate species do.28 Under such circumstances, males would have located females in heat mainly with the aid of such odors broadcast for mating.

But the olfactory cues may have lost their effectiveness if, for example, their feeding range became much wider under abrupt climatic changes which caused severe food shortage. Under such circumstances, females would have been forced to move to and fro over vast distances to acquire sufficient food. As a result, they would have experienced great difficulty in attracting appropriate males with their distinctive odor whenever they came into estrus. They may have been compelled to locate mates on their own because the latter had no idea of their reproductive status. No doubt, it would have been an arduous task for those who already had the big burden of pregnancy on their shoulders, including parturition and nursing of offspring. Sometimes, they may have wandered about vainly for mating opportunities, increasing exposure to prowling predators. Even if they were lucky enough to come across males, they could not make choice of mates with good genes. Thus, as time passed, their population may have dwindled and the fate of extinction came closer and closer. It was a serious crisis to overcome. (more)

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