Introduction

Sustainable, nature-based tourism should attempt to educate visitors about wildlife and the environment, while minimizing modification or degradation of the natural resources in the sites they visit. When possible, these activities should broadly benefit the social and natural environments by involving the participation of local communities (Ceballos-Lascuráin, 1996). And, if managed well, nature-based tourism should facilitate species conservation by raising the needed funds for wildlife and habitat conservation, while increasing public awareness of conservation issues. Unfortunately, rapid and unmonitored development of tourism projects in protected areas can produce deleterious effects on the very species we wish to conserve. Such risks may include habitat degradation caused by pollution and environmentally damaging development of infrastructure; animal crowding into restricted areas; and the introduction of invasive species.

These issues take on special importance when the species in question are (nonhuman) primates. Most wild populations of primates are relatively small and their reproductive cycles are protracted (with low reproductive rates relative to most mammals of similar size), so they are particularly vulnerable to population decreases, including those that result from human activities. Humans have contributed to significant population declines of wild primates through hunting and bushmeat consumption, habitat loss and fragmentation, and illegal capture of live primates for entertainment or other purposes. Tourism activities involving primates in their native habitat (including free-living rehabilitants as well as their wild counterparts) could benefit primate conservation but we must remain diligent about monitoring potential negative consequences, especially since primate tourism has been increasing in popularity over the past few decades. For example, the habituation to human presence essential to primate tourism may increase the likelihood that these animals will raid crops, invade garbage pits, and break into vehicles or lodgings for food. They may also become more vulnerable to poaching as a result of their loss of fear, their natural diet and ranging may become permanently altered, and their normal social behaviors may become altered (e.g. the appearance of a group of tourists may interrupt mating behaviors). Many of the chapters in this volume document these tourism impacts on the primates visited. Habituation could even lead to alterations in animal stress responses, possibly leading to immunosuppression with decreased reproductive success and increased susceptibility to infectious diseases (Muehlenbein, 2009; Muehlenbein et al., 2012).

INTRODUCTION

Women and men exhibit vast differences in reproductive physiologies, behaviors, and ecologies. The present chapter aims to illustrate these fundamental differences by utilizing recent theoretical and empirical developments in addition to clinical and anthropological data that help clarify the evolutionary bases for human male reproductive functions. We begin with a discussion of the various aspects of male reproductive effort, including the physiology and behaviors associated with seeking, attracting, and choosing mates, competing for and controlling mates, paternal behaviors, and the proximate determinants of spermatogenesis, libido, and erection. For the specific purpose of better understanding male reproductive ecology, we next provide readers with a description of male reproductive physiology, including major aspects of development, endocrinology, and senescence. Testosterone may be synonymous with maleness; however, it may impose a number of costs, including negative energy balance, immunosuppression, and prostate cancer.

A discussion on male reproductive ecology ends our chapter because it draws from our discussions on reproductive effort and physiology. Readers are encouraged to utilize the information presented here to further discussion and research on human male reproduction, both the proximate mechanisms involved in addition to the evolutionary explanations for the ways we function and behave.

INTRODUCTION

Hormones do not fossilize. Yet, arguably, they are as important to understanding the evolution of Homo sapiens and other primates as any fossil specimen. The role of hormones in understanding human life history evolution emerges from how genes translate into phenotypes with considerable input from environmental cues. Most hormones are evolutionarily quite conservative, with very similar if not identical chemical structures between species. Many hormones that flow through the veins of humans are identical to those that flow through the most exotic vertebrate. Other hormones and receptors, however, can differ in subtle but important ways between species and even individuals. Hormonal variation, as reflected by circulating levels as well as chemical structure, are of central importance to the evolution of human life histories, both from a macro- and microevolutionary perspective.

The evolutionary significance of hormones is clearly evident in the multitude of functions that are served, including growth, reproduction, metabolism, and senescence, all of which are central to the evolution of human life histories. Hormones are inextricably involved in the optimal allocation of time and energy. Insulin, leptin, and cortisol, for example, initiate and manage the flow and assessment of energetic assets such as glucose and fat. Indeed, hormones are involved in life history trade-offs that influence many aspects of human health (Bribiescas and Ellison,2008). Testosterone, estradiol, and oxytocin affect behavioral patterns that result in differences in how individuals allocate their time, such as in the trade-off between mate seeking and parenting.

The purpose of this chapter is to provide readers with introductions to several topics central to a modern understanding of human evolutionary biology. Infectious pathogens have placed critical selective constraints on the evolution of our hominin ancestors, and our own species continues to coevolve with infectious organisms today. Our understanding of the processes that shaped this evolutionary struggle have changed, and now an adaptationist perspective offered by the discipline of evolutionary medicine helps to shed light on our vulnerabilities to infectious diseases and noninfectious degenerative diseases. It also aids in our understanding of the purpose and outcomes of our coevolutionary conflicts with the microscopic predators that parasitize us. So as to compete in these interactions, we have developed a marvelously complex immune system capable of dynamic, varied responses. Insight into these mechanisms provides fascinating examples of real-time Darwinian processes of survival and fitness maximization in the face of invading competitors within the human host. Interestingly the ontogeny and deployment of these responses are dependent on several factors, including genetic and ecological constraints.

The discussion offered below provides an introduction to evolutionary medicine with the specific purpose of better understanding human–pathogen coevolution and the development of human immune responses. A current, detailed description of human immunity is included, but this discussion is far from complete. Comparative aspects of evolutionary immunology are emphasized, as are the genetic and ecological sources of variation in these responses.

In review of a different text, Moses Hadas (1900–1966) remarked that “this book fills a much-needed gap.” Unlike the text he was referring to, the topic of human evolutionary biology deserves no such gap in our understanding. To identify one's place in nature and to appreciate how human evolution has been guided by the same evolutionary principles that guide other organisms is humbling and necessary. We are products of evolution, and this is reflected throughout our biology and behaviors.

The purpose of our text is to provide thorough and modern reviews of a wide range of pertinent aspects of human evolutionary biology and contemporary human biological variation. The history of research on human biological variation is a long one, and includes studies on general human adaptability, variations in growth patterns, body sizes and shapes, genetic diversity, and race concepts. More recently, the study of human biology has included analyses of reproductive physiology and behavior within evolutionary and ecological frameworks. Other advancements include the science of evolutionary medicine, in which evolutionary research on health and disease is used to elucidate medical research and practices.

The text before you is different from most others. Unlike traditional texts on evolutionary biology, our book focuses specifically on humans and the application of evolutionary theories on understanding modern human variation and adaptability. Unlike traditional texts on human biology, our book does not include detailed descriptions of all human physiological and anatomical systems.

Investigators utilizing human subjects for any reason in their research are charged with the vital responsibility of ensuring ethical standards of conduct. These standards are outlined by a number of governing bodies, particularly the Department of Health and Human Services, Office for Human Research Protections (http://www.hhs.gov/ohrp/) and Office for Civil Rights (http://www.hhs.gov/ocr/) in the United States. Human subjects committees, empowered by these governing bodies, are then designated at the institutional level to provide oversight of human use in biomedical and behavioral research projects. Principle investigators and their teams are then responsible for ensuring the safety and welfare of subjects and compliance with research protocols. This is initially accomplished by working with institutional review boards (human subjects committees in particular) in a detailed application process prior to the initiation of research.

Below is a brief introduction for newcomers to the human subjects committee application, including the informed consent. This general information has been compiled from a number of online sources (last accessed April, 2010). Application format is certainly specific to individual institutions. However, the general guidelines are usually the same, drawn from basic requirements laid forward by, among others, the US Department of Health and Human Services Code of Federal Regulations, Title 45 (Public Welfare), Part 46 (Protection of Human Subjects) (http://www.hhs.gov/ ohrp/humansubjects/guidance/45cfr46.htm) and the Health Insurance Portability and Accountability Act (HIPAA) (http://www.hhs.gov/ocr/hipaa/ and http:// privacyruleandresearch.nih.gov/).