Ann E. Caldwell

Sex Hormones and Physical Activity in Women: An Evolutionary Framework

This chapter presents an evolutionary framework for understanding why there are bidirectional relationships between sex hormones and exercise, and how these relationships are predicted to influence women’s behavior and health. An evolutionary perspective highlights the importance of the ways physiology and behavior have been shaped by natural selection to favor mechanisms and behaviors that increase the likelihood of reproductive success. Impaired reproductive function in response to low energy availability may increase the costs and serve as a barrier to regular exercise among sedentary women. This framework also leads to theory-driven hypotheses regarding factors—such as individual condition and cues of resource scarcity—that can predictably influence individual differences in the relationships between sex hormones and exercise.

Physical Activity and Reproductive Ecology in Adults

This chapter focuses on the energetic demands during adulthood that are predicted to trade-off against energy allocated to physical activity from a life history perspective. The primary energetic demands are reproductive effort and continued somatic maintenance. Research from reproductive ecology that helps to characterize the sex-specific costs of physical activity among women and men is summarized. Females face much higher energetic demands associated with reproduction, and a growing body of research suggests that female fecundity, or the biological capacity to reproduce, is sensitive to changes in energetic conditions (including increases in physical activity or exercise). Given the high metabolic costs of gestation and lactation for human females, researchers have hypothesized that this heightened responsiveness to energetic condition and energetic stress would be adaptive in order to decrease the likelihood of conception when it is questionable whether the energetic demands of pregnancy and/or lactation can be met. In men, however, reproductive function is much less sensitive to energetic stress, and physical activity and investment in mating effort are consistent with physical activity and increases in muscle mass. In environments where physical activity is necessary to provide for offspring, physical activity is expected to remain relatively high; however, in environments like or modern ones, where provisioning is primarily sedentary, the opportunity costs for men and women increase. Finally, a brief summary of the factors predicted to influence physical activity among post-reproductive adults is provided.

Physical Activity and Energy Expenditure in Humans

This chapter examines physical activity and energy expenditure in humans. It provides a summary of the difficulties in measuring and comparing physical activity and energy expenditure across studies. Next, what is known about energy expenditure and physical activity in humans and other primates is briefly summarized. Overall, humans exhibit a wide range of physical activity levels (PALs) across different ecologies and modes of resource acquisition. This diversity supports the prediction that a flexible system has evolved that responds to the energetic demands of varying environments. A clear, predictable pattern does not emerge when comparing PAL across these dimensions and populations, even within similar modes of subsistence. Men are sometimes, but not always more active than women. Many agricultural populations exhibit seasonal variation in physical activity levels that unsurprisingly correlate with workload. Differences in physical activity are even observed within the same mode of food production. This variation suggests that there is not simply one level of physical activity that humans have evolved to perform. Instead, it suggests that humans flexibly respond to competing energetic demands across environments, across seasons, and across the life span to selectively exert energy when the adaptive benefits to doing so outweigh the costs, and otherwise conserve and/or redirect energy.

Summary, Conclusions, and Applications

This book advocates the basic thesis that the physiological and psychological mechanisms that influence physical activity have been shaped by natural selection to flexibly respond to energetic demands and resource availability in order to optimize survival and reproduction within a given ecology. For the vast majority of human evolution, this system was constrained by resource scarcity, rather than abundance. Natural selection has therefore not had the opportunity to shape mechanisms to increase activity in response to the increased caloric availability and intake that characterize modern environments. Low levels of physical activity are associated with chronic and deadly conditions, suggesting that human physiology has evolved in many ways to require high levels of physical activity to function properly. A life history perspective can help to understand the typical patterns of energy allocation across the life span and predict the conditions and stages where allocating energy to physical activity will be more or less likely to occur. Understanding the ways an energy conserving system that flexibly responds to the costs and benefits of physical activity that varies across individuals, environments, and the life span has evolved, can inform efforts to increase physical activity and/or exercise. The chapter concludes with take-home messages and applications for public and private policy, as well as health psychology and intervention research.

Phylogeny and Life History Patterns

Phylogenic or comparative analysis allows us to examine how traits or behaviors evolved in humans and other animals, and to discover factors that lead to similarities and differences in traits and behaviors over evolutionary time. This chapter focuses on unique morphological, physiological, and metabolic adaptations that influence energy expenditure and physical activity in humans compared to other primates and mammals. Next, it examines the evolution of human life history patterns and how the human pattern compares to other primates and mammals. Finally, it compares physical activity patterns and energy expenditure in primates and humans. Throughout, the ways the unique aspects of human anatomy, energetics, and life history that are hypothesized to influence costs and benefits of physical activity, and developing and maintaining physical fitness and strength are highlighted. Humans have evolved in ways that favor sustained aerobic activity and require physical activity to build and maintain cardiovascular and muscular fitness and strength. In the absence of activity, however, human physiology appears to have evolved to favor decreases in muscle mass and increases in fat mass. These adaptations are thought to help compensate for the high energetic costs of large brains and to help care for multiple dependent offspring.

Energetic Trade-Offs and Physical Activity During Childhood and Adolescence

One unique aspect of the human life span is an extended childhood. Compared to other primates, human offspring grow more slowly and remain dependent on their parents to produce food for much longer post-weaning. All energy during childhood is focused on somatic growth and maintenance, including substantial continued investments in brain growth, learning, and immune development. This chapter examines the important environmental factors that influence energy allocation strategies: resource availability, exposure to infectious diseases and parasites, and mortality risks. Research that demonstrates trade-offs between immune function and growth during childhood is summarized. Putting energy toward physical activity is hypothesized to trade-off similarly against these competing demands. Once pubertal maturation begins, there are increases in energetic demands for the pubertal growth spurt and investment in reproductive maturation. Research from traditional and modern populations suggest that competing demands of somatic growth (including brains) and reproductive maturation influence physical activity during childhood and adolescence.

Physical Activity and Life History Theory

Life history theory (LHT) is a branch of evolutionary theory aimed at explaining differences in the age schedules of growth, reproduction, and mortality . LHT proposes that humans, like other species, face trade-offs in how to optimally allocate the limited resources of time and energy among growth, reproduction, and maintenance (i.e., immune function, survival, longevity). Optimal allocation between these demands is expected to depend on individual and ecological parameters that influence resource production, mortality, and other factors. There are inherent trade-offs between somatic versus reproductive effort , current versus future reproduction, mating versus parental effort, and quantity versus quality of offspring. The theory posits that there have been long-term evolutionary pressures to balance energetic effort, and channel resources toward behaviors that maximize expected reproductive success across the life span. Life history strategies of energy allocation interact with the environment to fundamentally determine rates of growth, age of reproductive maturation, the pace of reproduction, reproductive senescence (particularly in humans), and life span. Physical activity is particularly well suited to be examined from a life history perspective because it both influences and is influenced by life history parameters (such as age of reproductive maturation, survival, and longevity) and is central to both energy expenditure and traditionally, energy acquisition. This chapter examines the costs and benefits of allocating energy to physical activity in humans.

Advantages of Evolutionary Theory for Understanding Physical Activity

Examining physical activity from an evolutionary perspective can lead to novel hypotheses or ways of integrating evidence across different levels of analyses that would not naturally be integrated from existing theoretical perspectives. This approach is particularly informative for traits or behaviors such as physical activity that are influenced by numerous environmental, psychological, and physiological factors that have likely been shaped by natural selection over time. In this chapter, the four categories of analysis outlined by Nikolaas Tinburgen, proximate (or mechanistic), ultimate (or functional), phylogenetic, and developmental, are detailed. The chapter describes how the four categories can complementarily describe how the physiological and psychological mechanisms that influence physical activity have evolved across evolutionary time. It also details how this approach differs from traditional approaches in health psychology and public health to understand and increase physical activity and exercise.

Energy Costs and Benefits During Fetal Development and Infancy

During fetal development and infancy, energy is primarily allocated toward somatic (particularly brain) growth and development, accumulating fat, and maintaining tissue. This stage is important to consider because several lines of research, summarized here, suggest that experiencing energetic stress and poor nutrition in utero and early in life, indexed by low birth weight and growth early in life, is associated with increased risks of cardiovascular and metabolic diseases. In more recent studies in humans and animals, it is also directly related to physical activity. It is hypothesized that energetic stress during development may lead to a thrifty phenotype, or an energy allocation system that is better suited for environments where resources are scarce or unpredictable. A thrifty phenotype that favors energy conservation and fat storage is predicted to influence physical fitness and activity, with mechanisms to reduce activity and favor a more sedentary lifestyle.

Proximate Mechanisms: Psychology, Neuroendocrine System, and Central Nervous System

An evolutionary, life history framework can provide useful interpretation of previous findings in psychological and epidemiological research on the proximate mechanisms that influence physical activity and inactivity . It can also allow for innovative and unique hypotheses to be generated that can be tested in future research on factors that mediate or moderate the effects of these variables on physical activity. This chapter provides examples of health psychology research on physical activity that can benefit from an evolutionary perspective. It then presents novel hypotheses of physiological mechanisms that orchestrate energy allocation from a life history framework. The HPG and HPA axes of the neuroendocrine system are summarized to understand the ways they influence and are influenced by physical activity. This can lead to research aimed at testing whether physical activity reflects predicted adaptive trade-offs between competing energetic demands. The chapter concludes by proposing an integrative evolutionary model that aims to assimilate the phylogenetic, developmental, ultimate, and proximate predictors of physical activity based on the evolutionary life history perspective presented.