Why Males and Females Are Different: The Science of Sexual Selection Book Review
Marlene Zuk, Leigh W. Simmons [authors]
Eiji Numata [Supervising Translator] Atsushi Endo [Translator]
Taishukan Shoten, Published November 20, 2023

Why Are Males and Females Different?: The Science of Sexual Selection is a fascinating science narrative that offers deep insights and intriguing examples regarding sexual selection. At the very beginning of the work, the authors mention the extreme difference in body size observed between female and male anglerfish. Using this as an introduction, the book systematically and clearly explains the sexual dimorphism, behaviors, and evolutionary mechanisms of males and females in the natural world. Let us first take a chapter-by-chapter overview of the entire book.

Chapter 1: Darwin's Other Big Idea

It focuses on the concept of sexual selection, which Darwin addressed in The Descent of Man, and Selection in Relation to Sex. It clarifies the differences from natural selection and explores controversies surrounding female choice, as well as sexual selection and the modern synthesis of evolution. It also delves into the importance of individual preferences, unraveling Darwin's groundbreaking ideas.

Chapter 2: Mating Systems: Number of Partners and Duration

We focus on the diversity of bonds between males and females during the breeding season. Touching upon red-winged blackbirds and marriage, competition for the monopoly of mating partners, and the existence of polygyny and polyandry, we explain how different mating systems evolved. We also discuss rules, exceptions, extrapair copulations, and applied aspects.

Chapter 3: Female Mate Choice

Focusing on what females choose, this provides a modern view of mate choice. It discusses sensory bias and the sexy son hypothesis, and explains with practical examples how the evolution of extreme secondary sexual characteristics and genetic compatibility contribute to evolution. Furthermore, it examines human mate choice.

Chapter 4: Gender Roles and Stereotypes

It challenges traditional ideas about sexual selection and explores the concept that males and females have natural "roles." It discusses the diversity of sexual behavior, such as coy females and eager males, sex-role reversal, and cases where individuals have both roles. Finally, it considers sexual roles in humans.

Chapter 5: Postcopulatory Sexual Selection

Focusing on postcopulatory sexual selection, we explain the battle between sperm and females. We reveal sperm size and number, the game of sperm competition, and the mechanisms by which sperm competition evolves testes. We also explore how females bias paternity by selectively using sperm from specific males.

Chapter 6: Sexual Conflict

We focus on the conflict of interest between males and females associated with mating. We examine how sexual conflict has influenced evolutionary processes, including chase-away sexual selection, sexual conflict over mating, and post-mating sexual conflict. We also delve into the evolution of male and female genitalia.

Chapter 7: The Survival of Species and Sexual Selection

We will focus on the impact of sexual selection on biodiversity. We will examine speciation, the relationship between diet and sex, the lock-and-key relationship, assortative mating in animals, and the effect of sexual selection on the evolutionary rate of diversity. Furthermore, we will explore whether sexual selection prevents extinction.

Chapter 8: Conclusion and Future Prospects

This book summarizes the main points and discusses prospects for future research. In addition, a list of illustrations, further reading references, and an index are provided.

This book is of great interest to readers with an interest in biology, exploring the theme of sexual selection in detail. Each chapter is filled with abundant examples and fascinating insights, which will help in understanding the astonishing diversity and complexity of the natural world.

The author expects a wide readership for this book. It is written particularly for the following readers: Students, whether majoring in biology or not, who have an interest in the subject: It is intended to serve as a textbook for university and high school students, providing a systematic and comprehensive understanding rather than fragmented knowledge about sexual selection. Therefore, it is written in an accessible style even for students not majoring in biology. General readers interested in animal behavior and evolution: It is also intended to be enjoyed as a readable book rather than a textbook by general readers interested in ecology and evolutionary biology. It will serve as an opportunity to understand the astonishing diversity and complexity of the natural world and to experience the fascinating world of ecology.

The author hopes to provide these readers with a new perspective on sexual selection and let them know how diverse the natural world is. Through this book, the author also expects to achieve the following impacts: Deepening the understanding of sexual selection: Readers will deepen their understanding of sexual selection and gain new perspectives on male-female relationships and evolution. They will recognize that sexual selection is a fundamental element of biology and understand its complexity. Insights into the natural world: The author wants readers to feel the wonder of various phenomena and evolution occurring in the natural world. By providing insights into ecosystems and evolutionary processes, the author helps readers view the natural world more richly. Fostering discussion and questions: The author hopes to stimulate readers to deepen discussions on sexual selection and generate new questions. It is hoped that knowledge about sexual selection will become part of everyday discussions and research. As outlined above, from beginning to end, this book provides readers fascinated by ecology and evolutionary biology with deep insights and knowledge, offering an opportunity to experience the beauty and complexity of the natural world.

This book also offers a new perspective on the potential for broad social impact, extending beyond the contexts of biology and the life sciences. Finally, I would like to focus on the curiosity stemming from my own research background and my hope that this book will help deepen our understanding of social issues related to gender inequality. From here on is a bonus.

At my current workplace, I am engaged in research on the mechanisms of gametogenesis, fertilization, and embryonic development, using Xenopus eggs as my main research subject. The focus of my research is how eggs—which are, in a sense, specialized cells—are produced within the body and function in cooperation with sperm to create a new individual. From this background, I was greatly intrigued by this book's deep dive into “the differences between males and females and their evolutionary biological significance.” The perspective of what kind of structure and function an egg possesses also entails the perspective of what kind of entities the egg and its partner, the sperm, respectively are—that is, "why are eggs and sperm different, and *how* are eggs and sperm different?" Realizing that such a perspective exists made me aware that I was beginning to ask a new question: If we can learn about the mechanisms of sex differences and sexual selection (if such a thing exists) between gamete cells—which are the sex-segregated entities that animal individuals known as eggs and sperm possess—by understanding the mechanisms of sex differences and sexual selection at the level of male and female animal individuals, what would that be?

Meanwhile, significant inequality in various contexts still exists in modern society, and its elimination remains an important issue. Also in the Sustainable Development Goals (SDGs), which are international social goals, gender equality, for example, is regarded as one of the key themes. However, the current situation is that progress in Japan is delayed. This seems to be another point that makes this book noteworthy. The book explores biological sex differences through the theme of "differences between males and females." My realization, and a question I want to delve into, is whether it is important to ask how that biological understanding relates to the social goal and issue of gender equality. The book's argument is based on the fundamental idea that males and females are exposed to different evolutionary pressures, resulting in sex differences. Applying this concept may provide insights into why men and women have different life patterns and behavioral tendencies. Furthermore, the book touches not only on biological sex differences, but also on the impact that sexual selection has on the entire ecosystem. The health and sustainability of ecosystems are critical issues facing modern society. Understanding how sexual selection affects species diversity and ecosystem functioning will provide useful information for formulating strategies toward a sustainable future.

Thus, this book has the potential to transcend the framework of biology and provide new perspectives to social discussions. For readers interested in the aforementioned social issues, the book will offer fresh insights into the relationships between men and women and the structure of society through the evolutionary aspects of sex differences and sexual selection, thereby laying the groundwork for more inclusive discussions. I believe this book has the potential to make a significant contribution to exploring paths toward a sustainable future through a biological approach. While engaging with the science of sex differences and sexual selection, readers will be able to deepen their understanding of social issues related to gender inequality and gain knowledge to help build a fairer and more egalitarian society.

Come, pick up this book filled with evidence and insights for learning, "Why Males and Females Are Different: The Science of Sexual Selection," and let's thoroughly enjoy it together!

Kenichi Sato
Kyoto Sangyo University, Faculty of Life Sciences

Why Males and Females Are Different: The Science of Sexual Selection - Book Review (PDF file)