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Authors: James McBride Dabbs,Mary Godwin Dabbs

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whole group of species. Forests turn into grasslands or an ice age begins, forcing old species out and making room for new ones. New neighbors arrive on the scene, making old ways of life less tenable. The ancestors of modern people appeared from one to four million years ago, and the world since then has changed in many ways. In adapting continuously to the world, including the physical and social aspects they themselves created, people changed over time in size, strength, intelligence, longevity, brain structure, and, undoubtedly, testosterone levels.
Evolution goes mostly unnoticed over the short run, as species adapt to small environmental changes. There are clues, though, that changes are in progress. Wisdom teeth have become a liability, and there are people who have genes for less than four of them. Many generations will pass before wisdom teeth become extinct, if they are in fact beginning the slow process of becoming extinct. Over the long run, the effects of evolution are more dramatic, as species change or disappear. Some species split and branch into many subspecies, and some remain largely untouched. Some species are more flexible than others. Horses are well suited to running at high speeds and carrying human riders, but the fact that there are few other animals like horses suggests their unique characteristics are of limited evolutionary value. Horses as a species are almost alone, and if their environment changes drastically, they almost surely will disappear. Ants, on the other hand, have thousands of species, adapted to thousands of niches all over the world. Ants in this way are more resourceful, and they will likely be here long after horses are gone. We never are certain until after the fact whether or not evolution is working well for a given species. Much of the outcome depends on chance and luck.
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Evolution has brought remarkable adaptive features to people, including the upright posture, complex hands, and large brain that enable standing, carrying, organizing, and building for the future. These adaptations support human culture, which evolves in much the same way as biological organisms evolve.
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People everywhere have developed language, elaborate kinship reckonings, and the politics of marital exchanges among families. Universal human practices include cooking, mealtimes, family feasting, dancing, funeral rites, gift-giving, law, and religious rituals.
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Testosterone influences human culture in many of the same ways that it influences the social life of other animals. Humans evolved with testosterone levels similar to the levels of many other ani-
 
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mals, including our closest living relatives, the chimpanzees. The evolution of testosterone in human beings resulted in the muscles, energy, sexual interest, and combativeness needed for survival in a primitive world. Whether these levels are suitable for survival in the modern world, where a few people can hold enormous power and whole nations are balanced between success and disaster, remains to be seen.
Natural selection deals with changes in the species as a whole. Males and females evolve as members of the same species, reacting to the same physical environment. Every species must fit into an ecological niche, and males and females alike must meet the demands of the niche. If the niche changes or disappears, both males and females will be in trouble. Both need to avoid predators, resist parasites and disease, and find food and protection from the elements. Natural selection leads to characteristics that the sexes have in common, and in the world of our ancestors, everyone needed a certain degree of toughness to survive. Both sexes needed testosterone. Both sexes also needed variability in testosterone levels, to take advantage of strategies that called for more or less toughness.
Sexual Selection
Males and females each have their own specialized needs, as well, and evolution works so as to select members of each sex who are best suited to get what they need. Sexual selection is the evolution of sex-linked traits, and testosterone differences are one of the main outcomes of this type of evolution. It is testosterone in the fetus that brings out the traits that have been most useful and most adaptive to males; and at puberty, the sex hormones in both girls and boys determine secondary sexual characteristics and lead to sexual maturity.
Sex hormones interact with certain genes that lie on the twenty-two autosomes that males and females share. Without male levels of testosterone, some genes may have less effect or no effect at all. For instance, both male and female impalas have genes for horns, but only male impalas actually have horns. That is not true of gazelles, a species in which both males and females have horns. Similarly, in birds, bright feathers may or may not be a masculine trait. Cardinals have them; wrens do not. Testosterone does not always affect the same genes in one species as in another. Sexual selection, like natural selection, is flexible.
 
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Estrogen, too, is necessary for the expression of some genes. Boars have nipples but no breasts, which do not develop without female levels of estrogen. Nevertheless, farmers who raise hogs do not use boars for breeding unless they have the correct number of properly placed nipples, making sure they will pass on good genes to any female offspring. With a great deal of sorting out, sexual selection results in each sex having characteristics that promote reproductive success, and those characteristics vary among the species.
To achieve reproductive success, members of a species need to produce offspring that live, thrive, and produce further offspring. Among mammals, females jockey for position to gain resources for their offspring, and males jockey for position to mate with females and father offspring, sometimes showing as much interest in keeping other males away as in mating. Females need to carry the fetus, give birth, and nurse the young. The female invests heavily in her offspring and needs to protect and nurture them. The male's role is more variable. In some species, males lose interest in family matters immediately after mating, while in others, males help raise their young.
Sexual selection is especially relevant to our concern with testosterone. If we understand the origins and consequences of the large and consistent differences in the levels of testosterone between the two sexes, we will be in a better position to understand variability within each sex, and to contrast low- and high-testosterone males and low- and high-testosterone females.
Males and females live in the same environment and are affected the same way by the forces of natural selection, but other forces affect each sex differently. The characteristics of men and women originated in prehistoric times in response to these other forces. The details of the everyday lives of our prehistoric ancestors are mostly lost to us, but archaeological digs have yielded clues about how our ancestors lived. We have skeletons of ancient people, bones of the animals they hunted, and some of the tools and weapons they used, but we can only make educated guesses about their social structure. We know more about our forefathers than our foremothers because, until recently, research has focused on the way men lived. Now researchers have begun to pay attention to our female ancestors,
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probably because during recent years more women have become involved in archaeological research.
 
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The generally accepted view among researchers is that prehistoric men and women lived different lives. Men traveled widely, hunted, and dealt with politics, authority, and quasi-military activity, while women stayed closer to the home base, gathered food, took care of children, and worked together to maintain their community.
The remains of a Neanderthal settlement of a hundred thousand years ago support this view.
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Neanderthal people had bodies similar to ours, and they undoubtedly acted in many ways as our direct ancestors did. The remains of bones, tools, and fireplaces in the settlement show differences between males and females. Females stayed in the local area near a home base. They tended slow-burning fires and used plant foods and materials from nearby. Males traveled and hunted widely, used tools made from stone from far away, built small, hot fires, and brought occasional food back to the cave.
Both sexes need children for evolutionary survival, but only females can give birth. Although they need mates to impregnate them, females bear their own offspring and must be able to provide them with food and safety. Thus through sexual selection females have evolved with resources males do not have. For example, human females evolved with extra body fat on their hips and thighs, which provided our maternal ancestors with the extra calories they needed to survive pregnancy and lactation, especially during hard times. In some environments food is easy to find, and mothers do not have to leave their young unprotected for long periods of time to hunt for food. In harsher environments and in species with prolonged infancy, mothers find it more difficult to support their young without help from relatives and mates. Over periods of many generations, evolution selected females who were tough enough to take care of themselves and their offspring, and clever enough to enlist the help of others when needed.
Males need mates to bear their children. Sex-linked traits enable males to compete with other males for mates and attract the attention of females. Males of different species use different courtship strategies. Many give gifts: male bower birds present females with gifts of colorful pebbles and shiny treasures, and hornbill males bring lizards to prove they are good hunters and therefore will be good providers. In other species, males win mates through force, as hamadryas baboons do; they bite females who try to get away.
 
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Human males approach potential mates using a variety of tactics, including love, support, seduction, and rape. An interest in sex and power, the hallmarks of testosterone, are still important to males in competition for mates and offspring. Generally males succeed by being dominant, which means having power and using it to get what they want. Fighting is a way to be dominant, but not the only way. Males also strut, and they use bluff and bluster to good advantage in scaring off other males and attracting females.
The men in primitive times most likely to have many children were those who had won positions of leadership in the male hierarchy by being good hunters and fighters. As with the hornbills, hunting was as much about getting mates as about getting food. There was usually enough vegetarian food for everyone to eat. But meat was a useful supplement, and men who brought it home had a better chance of attracting women, who knew that babies fathered by powerful hunters had the best chance to thrive. High levels of testosterone contributed to the physical and mental traits that enabled our male ancestors to hunt, fight, and reproduce. These traits included a proclivity for action, sometimes violent; strong physiques characterized by broad shoulders and heavy muscles in the arms and legs; fat stored around the stomach, providing ready calories on long trips;
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and the skills necessary to track game and make weapons.
The physical characteristics of males are not easy to maintain, and come at considerable cost. It is physically demanding to forage for the calories to support a male's extra size and strength. Furthermore, extra testosterone leads to earlier death, which will be explored in more detail later. Females live longer. Just as they are the basic models
in utero
, females are the basic adult models for the species.
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Females have the appropriate body size and other characteristics needed to survive in the species' ecological niche. Characteristics that go beyond the basic modellike big muscles and bright plumageare associated with male reproductive needs.
What we know about our female ancestors and their Neanderthal cousins suggests that women in prehistoric societies did not need heavy muscles and combative dispositions to attract mates. Women were sometimes aggressive, but less often than males, and their average testosterone levels were lower. Women evolved with temperaments
 
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that were conducive to long friendships with other women and a cooperative approach to other relationships. We see a similar pattern today. Men tend to drop what appear to be strong loyalties when they change work assignments, jobs, or sports teams.
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Women maintain lasting relationships around their home communities.
There is strong evidence that the female capacity for friendship is the result of sexual selection. For instance, researchers have noticed sex differences in friendships among other primates. Irwin Bernstein, of the Yerkes Primate Center, can disrupt the hierarchy in a group of male rhesus monkeys by moving the animals to a new setting. When I asked whether he could do this with female monkeys, he said, ''No. They would remember their friends."
Modern men live an average of seven years less than women. They would probably live longer if they spent more time appreciating their friends than fighting their rivals. Men pay a price for having high levels of testosterone and the heavy physical apparatus and aggressive temperament that go with it. The fact that these costly characteristics survived the evolutionary process indicates that they are important to reproductive success, or at least have been important until recent times. When traits cease to be important, they wither and disappear over time, like the eyes of fish who live beyond the reach of light in the ocean depths, or the pigmentation of insects who live in dark underground caves and pools. Unessential characteristics that cost nothing to maintain can remain indefinitely, but those that are expensive will begin to disappear when they no longer contribute to survival. Something as expensive as testosterone must have contributed a great deal recently in our evolution.
Large Tools and Heavy Equipment
The dissimilarities between men's and women's bodies in size, shape, and development, which have emerged from sexual selection, add up to sexual dimorphism, and sex hormones control the process. It is easy to see why sex differences were important in ancient times. Men and women faced disparate survival requirements, and their bodies and behavior evolved to meet those requirements. But do the differences matter today? Is the way men and women act still related to differences in
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