Why is Sex Fun?

Introduction

Sex is a fundamental part of human nature, yet our sexual behaviors and practices are quite peculiar when compared to other animals. In "Why is Sex Fun?", Jared Diamond explores the fascinating world of human sexuality, examining why we have developed such unique sexual habits and how they have contributed to our evolution and survival as a species.

This book takes readers on a journey through the quirks and oddities of human sexual behavior, comparing our practices to those of other animals and exploring the evolutionary reasons behind our distinctive approach to sex. Diamond's engaging and accessible writing style makes complex scientific concepts easy to understand, offering readers a fresh perspective on a topic that is both familiar and mysterious.

The Weirdness of Human Sexuality

Humans vs. Other Animals

If other animals could comment on human sexual behavior, they would likely find it shocking and bizarre. Diamond presents an amusing scenario where a dog observes human sexual practices:

1. Timing: Dogs would be confused by humans having sex at any time of the month, even right after a woman's period.
2. Pregnancy: They'd wonder why humans continue to have sex even when the female is already pregnant.
3. Privacy: The concept of having sex behind closed doors would be utterly baffling to most animals.

These observations highlight just how different human sexual behavior is compared to the rest of the animal kingdom. Out of the approximately 30 million animal species on Earth, humans stand out as having particularly strange sexual habits.

Mammals and Great Apes

Even when we narrow our focus to mammals, of which there are about 4,300 species, human sexuality remains an outlier. Most mammals, including lions, wolves, and chimpanzees, engage in public copulation without forming long-term pair bonds or nuclear family units. Adult males and females of these species typically lead solitary lives, coming together only for mating purposes.

Humans differ significantly from our closest relatives, the great apes (chimpanzees and bonobos), in terms of sexual behavior. While our unique posture and brain size have contributed to our evolutionary success, our sexual habits seem puzzling at first glance. However, as Diamond explains, these peculiarities have played a crucial role in our survival and dominance as a species.

The Evolution of Human Monogamy

Parental Investment

One of the key factors that sets human sexuality apart is the level of parental investment, particularly from fathers. In most animal species, the female invests more in offspring due to the higher biological cost of producing eggs compared to sperm. Eggs contain the necessary metabolic components to support embryo development, while sperm only need enough energy to swim for a few days.

Theoretically, males could maximize their reproductive success by mating with as many females as possible and then moving on. Females, on the other hand, are biologically tied to their offspring during pregnancy and the early stages of care. So why do human fathers often stick around and invest in their children?

The Benefits of Monogamy

Diamond argues that human monogamy evolved as a strategy to protect both the mother and child. In the harsh environments of our ancestral past, a solitary mother would have faced significant challenges in protecting and providing for her offspring. By forming a pair bond, both parents could contribute to the survival of their genes.

This mutual investment in offspring led to the development of the human social system we see today, where both mothers and fathers often play active roles in child-rearing. Monogamy, therefore, emerged as an adaptive strategy that increased the chances of survival for human offspring.

The Mystery of Recreational Sex

Hidden Ovulation and Constant Sexual Availability

Another unique aspect of human sexuality is our tendency to engage in "recreational" sex – sexual activity that occurs regardless of the female's fertility status. Most animals only mate when fertilization is likely, but humans have sex throughout the menstrual cycle, during pregnancy, and even after menopause.

Diamond explains that this behavior is linked to the phenomenon of hidden ovulation in human females. Unlike other primates, such as baboons, where females display visible signs of fertility (e.g., swollen, red genitals), human males have no reliable way to determine when their partner is most fertile.

This lack of obvious fertility cues led to the evolution of constant sexual availability in humans. Since males couldn't pinpoint the optimal time for fertilization, the strategy of frequent mating throughout the cycle increased the chances of successful reproduction.

The Evolutionary Advantages of Recreational Sex

At first glance, this pattern of constant sexual activity seems inefficient and potentially risky:

1. Energy cost: Sperm production requires significant resources from males.
2. Time investment: Mating takes time away from other survival-related activities, like finding food.
3. Vulnerability: Copulating animals are more exposed to predators or rival mates.

However, Diamond argues that these seemingly inefficient mating patterns actually provide evolutionary advantages. Concealed ovulation and frequent mating likely evolved to promote monogamy by keeping males invested in their partners and offspring.

If a male could easily identify when a female was fertile, he might be more inclined to abandon her after mating to seek out new partners. By maintaining a consistent sexual relationship, human males are more likely to stick around and contribute to the care of their offspring, increasing the chances of survival for their genes.

The Role of Men in Family Life

Male Parental Investment

Unlike most male mammals, who contribute little beyond fertilization, human males often play an active role in family life. Anthropologists consider this male contribution to be a critical factor in human evolution, particularly given the extended period of dependency in human children compared to other primates.

In early human societies, men's contributions primarily involved hunting and providing meat for their families. This behavior set humans apart from most other mammals, with only a few exceptions like wolves and African hunting dogs. Men's generally greater muscle mass made them better suited for hunting large game.

The Complexity of Male Contributions

However, Diamond points out that the role of men in providing for their families is more nuanced than it might appear. He uses the example of the Northern Ache Indians of Paraguay to illustrate this complexity:

1. Division of labor: Ache men hunt large animals, while women gather plant foods and insects.
2. Consistency of food provision: Women provide a more consistent food supply, while men's hunting success is less predictable.
3. Sexual incentives: Ache women are more likely to choose skilled hunters as sexual partners, even in a society where multiple partners are common.

This example shows how men's contributions to family life may be driven by sexual incentives rather than purely altruistic motives. By demonstrating their hunting prowess, men increase their chances of being chosen as sexual partners and potential fathers, thus passing on their genes.

The Potential for Male Lactation

In a surprising twist, Diamond explores the biological possibility of male lactation. This discussion highlights how human sexual characteristics and roles are not as fixed as we might assume:

1. Lactation without pregnancy: Most mammals can produce milk through repeated mechanical stimulation of the nipple, independent of pregnancy.
2. Adoptive mothers: Women can induce lactation through regular use of breast pumps before adopting a child.
3. Male potential: Physiologically, men have the capacity to produce milk if given the right hormonal stimulation.

Diamond explains that during adolescence, hormones cause significant differences between male and female mammals to develop. However, by manipulating hormones like estrogen and progesterone, it's possible to induce breast growth and milk production in both females and males.

Why Male Lactation Didn't Evolve

Given this biological potential, why didn't male lactation become the norm in human evolution? Diamond argues that in species where male parental care is crucial (about 10% of mammal species, including humans), other forms of contribution proved more beneficial:

1. Protection: Male lions, for example, are more useful in defending their cubs from threats than they would be staying at home to nurse.
2. Resource gathering: Human males were likely more effective in protecting territory and gathering resources than they would have been as primary caregivers.

This exploration of male lactation serves to illustrate how evolution shapes sexual characteristics and parental roles based on what provides the greatest survival advantage for a species.

The Evolutionary Puzzle of Menopause

A Unique Human Trait

Menopause, the cessation of a woman's egg supply leading to sterility, is another peculiar feature of human sexuality. Diamond points out that menopause doesn't fit neatly into the animal world and seems to contradict the principles of natural selection:

1. Reproductive potential: Most animals remain fertile until death, including human males.
2. Comparison to apes: Menopause is a major trait separating humans from our closest relatives, the great apes.
3. Longevity: Humans typically outlive great apes, even in protected environments like zoos.

The Adaptive Value of Menopause

Despite its apparent contradiction of evolutionary principles, Diamond argues that menopause offers several adaptive advantages:

1. Health preservation: By limiting fertility, menopause allows women to maintain better health and strength for a longer period.
2. Risk reduction: The chances of health complications for both mother and child increase with maternal age. Menopause eliminates these risks.
3. Resource allocation: By having fewer children later in life, women can better allocate resources to care for themselves and their existing offspring.
4. Cultural transmission: In preliterate societies, older individuals served as valuable repositories of knowledge and experience. Menopause allowed women to live long enough to fulfill this crucial role.

Diamond explains that the balance between fertility and health varies among species and is closely tied to life expectancy. For example, mice prioritize rapid reproduction over body maintenance, producing many offspring but dying young. Humans, on the other hand, have evolved a strategy that favors longevity and fewer offspring.

Body Signals and Mate Selection

The Language of Attraction

Diamond explores how humans, like other animals, use various body signals to communicate important biological information and intentions to potential mates. These signals can convey details about an individual's sex, age, and even aggression levels.

Animals use a variety of communication methods, including auditory and visual signals. Birds, for instance, use songs to attract mates and plumage to indicate their sex. In humans, these signals manifest as what we commonly refer to as "sex appeal."

The Handicap Theory

Interestingly, many of these sexual signals can be detrimental to an animal's survival. Diamond introduces the concept of the "handicap theory" to explain this phenomenon:

1. Peacock's tail: The elaborate tail of a male peacock makes it more vulnerable to predators and hinders its movement.
2. Human penis size: The energy required to develop and maintain a large penis comes at the expense of other bodily functions.

According to the handicap theory, these seemingly counterproductive traits actually serve as indicators of genetic fitness. An animal that can survive despite such handicaps must have superior genes, making it an attractive mate.

Human Body Signals

In humans, various physical attributes serve as body signals for mate selection:

1. Male musculature: Originally indicated an ability to gather resources and compete with rivals.
2. Facial beauty: Reflects resistance to disease, age, and injury.
3. Female body fat: Indicates health and the ability to carry and nourish a baby.

These signals have evolved over time to help humans select mates with desirable genetic traits, even though we can't directly observe an individual's genetic quality.

The Impact of Agriculture on Human Sexuality

While not explicitly mentioned in the provided summary, it's worth noting that the advent of agriculture had a significant impact on human sexual behavior and social structures. The transition from hunter-gatherer societies to agricultural ones led to changes in:

1. Property ownership: The concept of land ownership emerged, influencing mate selection and inheritance patterns.
2. Social hierarchies: Agricultural societies developed more complex social structures, affecting mating opportunities and preferences.
3. Division of labor: The roles of men and women in society became more defined, potentially reinforcing certain sexual behaviors and expectations.

These changes likely contributed to the reinforcement of monogamous relationships and the development of cultural norms surrounding sexuality.

The Role of Culture in Shaping Human Sexuality

Although Diamond's book focuses primarily on the biological and evolutionary aspects of human sexuality, it's important to recognize the significant role that culture plays in shaping our sexual behaviors and attitudes. Cultural factors influence:

1. Sexual norms and taboos
2. Relationship structures (e.g., monogamy, polygamy)
3. Gender roles and expectations
4. Rituals and practices surrounding sex and reproduction

The interplay between biology and culture continues to shape human sexuality, creating a complex and diverse landscape of sexual behaviors and attitudes across different societies.

Conclusion: The Complexity of Human Sexuality

In "Why is Sex Fun?", Jared Diamond takes readers on a fascinating journey through the peculiarities of human sexual behavior. By examining our unique traits – from hidden ovulation and recreational sex to menopause and complex parental investment – Diamond reveals how these seemingly odd characteristics have played crucial roles in our evolution and success as a species.

The book highlights several key insights:

1. Human sexuality is remarkably different from that of other animals, including our closest primate relatives.
2. Many of our sexual behaviors, while appearing inefficient or counterintuitive, have evolved to promote pair bonding and increase offspring survival.
3. The evolution of concealed ovulation and constant sexual receptivity in humans has contributed to the development of long-term pair bonds and paternal investment.
4. Menopause, unique among primates, offers adaptive advantages by allowing for better resource allocation and the transmission of cultural knowledge.
5. Body signals and mate selection processes in humans reflect complex evolutionary strategies for identifying genetically fit partners.

Diamond's exploration of human sexuality reminds us that our sexual behaviors are not simply a product of conscious choice or cultural norms. Instead, they are the result of millions of years of evolutionary pressures, shaped by the need to survive and reproduce in challenging environments.

By understanding the evolutionary roots of our sexuality, we gain valuable insights into why we behave the way we do and how our unique sexual characteristics have contributed to our success as a species. This knowledge can help us better navigate the complexities of modern relationships and sexual dynamics, recognizing that many of our instincts and behaviors have deep evolutionary origins.

Ultimately, "Why is Sex Fun?" encourages readers to view human sexuality through a broader lens, appreciating the intricate interplay between biology, evolution, and culture that has shaped our sexual behaviors and attitudes. As we continue to evolve and adapt to changing social and environmental conditions, our understanding of human sexuality will undoubtedly continue to grow and change, offering new insights into this fundamental aspect of human nature.