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Chapter 1: Introduction.

CHAPTER OBJECTIVES

* Describe the focus of this book.

* Provide a brief overview of the field of reproductive physiology.

* Establish a context for studying the reproductive physiology of mammals.

FOCUS OF THE BOOK

This book is about the reproductive physiology of mammals. Reproduction refers to the process of self-replication. Physiology is the branch of biology that deals with the functions and activities of organisms. Therefore, reproductive physiology can be defined as a branch of biology that deals with how organisms replicate. It is important to realize that an understanding of physiology requires some appreciation for anatomy, or the structure of organisms. Obviously it would be difficult, if not impossible, to explain how something works without knowing its parts and how they are related to each other. For example, one couldn't understand how an automobile moves without being aware of parts such as the engine, gears, drive train, and wheels, as well as knowledge of how these parts interact with each other. Likewise, it is impossible to understand how dogs reproduce without first learning something about the structure of their reproductive organs.

We will be concerned with the reproductive physiology of mammals. In general mammals are endothermic, homeothermic animals that are hair covered, and feed their young with milk produced by the female's mammary glands. Other defining characteristics include the presence of a diaphragm; three middle-ear ossicles; heterodont dentation (differentiated teeth); sweat, sebaceous and scent glands; a four-chambered heart; and a large cerebral cortex. A more precise account of what it means to be a mammal requires an understanding of how these animals fit into the overall classification system of animals. There is disagreement concerning the classification of animals. Figure 1-1 shows one widely accepted system. Mammals make up the Class Mammalia, which belongs to the Phylum Chordata. Mammals and other animals with backbones (fishes, amphibians, reptiles, and birds) comprise the Subphylum Vertebrata. Class Mammalia can be divided into two subclasses: Prototheria and Theria. The only prototherian mammals that exist today are members of the Order Monotremata. This order consists of two families of mammals that lay heavily yolked eggs with leathery shells. In addition, females of this subclass nourish their young with milk that is produced by mammary glands and secreted through pores that align the belly (e.g., duck-billed platypus, spiny anteater, and echidna). The other subclass of mammals Theria includes two infraclasses: Eutheria and Metatheria. There are approximately 270 species of metatherian mammals. These animals are also known as the marsupials. Females of these species give birth to extremely immature offspring, which then migrate from the birth canal to an abdominal pouch where they nurse until they become independent (e.g., opossums, kangaroos, wombats and koalas).

[FIGURE 1-1 OMITTED]

Most of the information presented in this book pertains to the eutherian mammals. This group is typically referred to as the placental mammals, but this terminology can lead to confusion because marsupials have placentas, albeit ones different from those of eutherian mammals. The placentas of eutherian mammals consist of two extra-embryonic membranes; that is, the allantois and the chorion. The placentas of marsupials consist only of tissue from the yolk sac, a structure similar to the yolks of avian eggs.

We will concern ourselves only with the few orders of eutherian mammals with which humans deal most often: Rodentia (e.g., rats and mice), Primata (e.g., monkeys and apes), Lagomorpha (e.g., rabbits and hares), Carnivora (e.g., dogs and cats), Perissodactlyla (e.g., horses), and Artiodactyla (e.g. cattle and sheep). It is important to bear in mind that our focus on reproductive physiology is an extremely narrow one. Vertebrate species account for only 2 percent of the known animal species living today, whereas mammalian species account for slightly more than 8 percent of the known vertebrate species (Figure 1-2). Thus mammals make up less than 0.2 percent of the known species of animals, and we will consider only a small fraction of these animals.

Much of the information presented in this book concerns domestic mammals, particularly those used in agriculture. However, when appropriate, discussions will be extended to include wild mammals since interest in managing the reproductive physiology of these animals has grown in recent years. We will also examine some of the more important principles of human reproduction. There are two major reasons for emphasizing livestock species. First, we know much more about the reproductive physiologies of livestock species than those of other animals (except laboratory rodents). Second, reproductive technologies are used extensively in livestock production. This provides opportunities to link fundamental scientific knowledge with practical applications of this knowledge. The primary goal of this book is to develop an understanding of the basic mechanisms of reproduction, but we will also consider how these mechanisms can be manipulated to enhance or reduce birth rates, and explore some of the ethical issues associated with these practices.

REPRODUCTIVE PHYSIOLOGY AS A SCIENTIFIC DISCIPLINE

Science can be thought of as a system for developing knowledge. This involves asking questions and constructing hypothetical answers that are either supported or rejected by experimentally-derived data. Scientists consider two types of questions: ultimate and proximate. Ultimate questions deal with "why?" These are the so-called big questions. Why does a robin fly? Why does a cow chew its cud? Answers to these types of questions require theoretical assumptions about the world. Biologists typically ground answers to why questions in evolution theory. According to this theory, robins fly and cows chew their cud because of natural selection. In other words, these traits either made these animals more fit (enhanced their ability to survive and reproduce), or didn't make them less fit. We won't concern ourselves very much with these types of questions. Rather, we will focus on proximate questions, or questions of "how?" How is the sex of an organism determined? How does a sheep become sexually mature? How does pregnancy occur?

In studying this subject, you will have to deal with three types of facts. First there are scientific names for cell types, reproductive organs, and the various chemical signals that regulate reproduction. The only way to learn this type of material is to memorize it. Think of this as a necessary means to build a vocabulary that will allow you to speak the language of reproductive physiology. The second type of fact you will encounter deals with measurements; for example, duration of pregnancy, length of an estrous cycle, dimensions of various reproductive organs, and so on. These types of facts help you achieve a frame of reference. For example, in planning a trip, it's crucial to know how far you will have to travel. Likewise, if you are planning to manage a herd of cows to produce beef, it is necessary to know the length of pregnancy, and so on. Finally, there are concepts. These are scientific claims (hypotheses) about how reproductive processes work. For example, you will read that estrogen causes a female to come into heat. This may seem like a well-accepted fact that virtually everyone accepts as truth, but it is really a hypothesis that is supported by abundant experimental data. A hypothesis is assumed to be true until there is good evidence to reject it. Although it is important to understand the scientific basis of this type of fact, it is beyond the scope of this book to provide detailed discussions of the scientific research supporting all the concepts presented in this book. Nevertheless, some experiments are particularly worthy of attention as instructional tools, and will be discussed where appropriate.

In their pursuit of answers to questions about how animals reproduce, reproductive physiologists use a variety of techniques and experimental approaches. When a particular species is first studied, much of the scientific work involves description; for example, a careful and thorough documentation of various reproductive characteristics (sexual behaviors, breeding patterns, number of offspring, and so on). Once the reproductive traits have been characterized, research typically turns to the study of the mechanisms controlling reproduction. These types of studies are experimental in nature. In other words, they are designed to test specific hypotheses about reproductive processes. For example, once descriptive information about the pattern of reproduction in sheep became available, it was possible to perform experiments that tested specific hypotheses regarding the mechanisms controlling this pattern of reproduction. Of particular importance are experiments that tested the hypothesis that day length determines the time of year that sheep engage in reproductive activity. These types of experiments can involve a variety of approaches, including the following:

* Anatomic (studies describing the gross and microscopic structures of reproductive tissues)

* Physiologic (surgical, pharmacologic, and hormonal studies of how reproductive tissues function)

* Biochemical (studies of chemical activities of cells within the reproductive system)

* Molecular Genetics (studies of how genes regulate activities of reproductive cells)

A CONCEPTUAL FRAMEWORK FOR REPRODUCTIVE PHYSIOLOGY

Every discipline of science is based on a set of basic assumptions about the world; that is, a conceptual framework. Such assumptions are law-like in the sense that they are rarely, if ever, challenged. When they are challenged it is because they no longer help us make sense of the world. In the biological sciences, the theory of evolution provides a set of foundational assumptions about why living things are the way they are. The basic assumption of evolution theory is that the distinguishing features of particular organisms originate in preexisting organisms, and that differences among organisms are the result of natural selection over successive generations. It is useful to establish some fundamental concepts concerning evolution and reproduction:

* Different species express different reproductive strategies.

* A reproductive strategy is an expression of an individual organism's genotype.

* Species express a particular reproductive strategy because individuals that expressed this strategy have greater reproductive fitness than others.

* Reproductive fitness of an individual is a function of how many of its offspring survive to reproduce; that is, how much of an individual's genes are spread through the population.

Keep these concepts in mind as you learn about the diverse reproductive traits discussed in this book. They will help provide insight into why mammals reproduce in so many different ways.

A GLOBAL CONTEXT FOR REPRODUCTIVE PHYSIOLOGY

Students frequently ask why they should study a particular subject. This is a fair question because we are more inclined to learn something when we find it relevant to our lives. Why should anyone study reproductive physiology? One reason is that this subject is concerned with a basic inclination of all living things, including humans. All one has to do is watch television, surf the Internet, or read a popular magazine to be reminded that humans are interested in reproduction and a wide array of subjects related to it. Reproduction has fascinated people for millennia. Another reason for understanding reproductive physiology is more practical; that is, knowledge of reproductive physiology can have a direct impact on the quality of our lives. Historically, humans have employed their understanding of reproduction to achieve two goals: 1) to limit growth of human populations and 2) to enhance food production. This has been the challenge of primitive societies as well as modern ones. Sustaining a human population with adequate food remains an important challenge and will continue to be as long as we humans inhabit this planet.

Agriculture and Human Population

According to paleontologists, humans have inhabited the earth for approximately 3 million years. For most of this time, humans accounted for what amounts to only a miniscule portion of the earth's ecology. However, during the past 10,000 years the human population has increased in an exponential manner (Figure 1-3). In other words, as the human population increases, the amount of time required for the population to double is decreasing. Today over 6 billion people inhabit the earth and they have had an impact on every square centimeter of the planet. What has been responsible for this change?

One of the fundamental laws of ecology is that the population of a particular species varies directly with available food supply. During the past 10,000 years, production of food increased due to agriculture. For every advance in humanity's ability to produce food, there has been a corresponding increase in population. For example, the introduction and use of high-yielding varieties of rice and other crops in the 1960s (the so-called Green Revolution) is largely responsible for the doubling of the world's population during the past 40 years. Geographer Jared Diamond explains how the development of agriculture stimulated population growth in early human societies. Most of the living matter on this planet is useless as food because it is indigestible, poisonous, low in nutritional value, tedious to prepare, or difficult or dangerous to hunt or gather. Typically only a small amount (0.1 percent) of biomass on a particular parcel of land is available as human food. By cultivating selected species of plants and animals, humans can make as much as 90 percent of the biomass available as food. As we shall see in later discussions, the major determinant of reproductive rate in healthy organisms is level of nutrition. Agriculture increases human populations by increasing the amount of edible food per unit of land.

[FIGURE 1-3 OMITTED]

Use of domestic animals contributes to the expansion of human populations via direct and indirect effects. Use of livestock directly affects population by providing sources of high-quality nutrients (particularly protein). The amount of food provided in the lifetime of a cow, ewe, or sow is much greater than that of wild animals because domestic animals are used as breeding stock to provide offspring that are killed annually for food. Domestic animals also affect human populations by enhancing production of crops. This effect is brought about by providing manure (fertilizer), as well as power for agricultural technologies (e.g., plows) that facilitate planting and harvesting. In addition to these direct effects, the use of domestic animals enhances human populations indirectly. Raising livestock and crops contribute to the development of a "settled existence," meaning that agriculturalists do not re-locate as frequently as hunter-gatherers or pastoralists. This less-mobile lifestyle leads to a shortened birthing interval, which results in greater population density.

The success of agrarian societies is widely evident. Aside from a few remaining pastoral or hunter-gatherer societies, the vast majority of humans living today depend on agriculture for survival. In fact, the dominance of western civilization is largely attributed to the development of agriculture. For better or worse, development of agriculture in the West facilitated the development of advanced weaponry (steel swords, guns, and so on), ocean-going ships, political organization, writing, and epidemic diseases, developments which allowed Western peoples to conquer other peoples and spread their culture to almost every part of the globe.

Although agriculture permitted the development of technologically advanced societies, it also resulted in significant unintended consequences such as: 1) loss of natural habitat; 2) reduction in wild foods (primarily fish and shellfish); 3) loss of genetic diversity in wild species; 4) erosion of soil; 5) reduction in fossil fuel reserves; 6) pollution of air and water; 7) destruction of native species by invasions of alien species; and 8) atmospheric changes (ozone depletion, and greenhouse gases). Each of these problems can be directly attributed to increasing human populations along with an increased environmental impact of each individual. As agrarian populations expand they seek to increase food production by one or more of the following practices: securing more land, using more resources, and employing new technologies to increase per capita production of food. Unfortunately, these practices often enhance environmental degradation. Societies with expanding populations, limited resources, and minimal access to technologies that can boost food production are prone to such environmental problems. For example, the extensive deforestation of the Caribbean nation of Haiti is in part attributed to the desperate attempts of an impoverished people to meet their growing demands for fuel and food. It is important to emphasize that such problems are not confined to poor nations. All nations struggle with environmental problems stemming from increasing populations and/or increasing resource consumption.

Carrying Capacity of Earth

The preceding discussion sets the stage for explaining how a knowledge of reproductive physiology might be useful, if not essential, for stabilizing population growth and resource use. Before this can be made clear, it is necessary to discuss what it means to stabilize population growth and resource use. The concept of carrying capacity is helpful in this regard (Figure 1-4).

Charles Darwin's (1809-1882) insights into evolution were based in part by the ideas of economist Thomas Malthus (1766-1834). Both men came to realize that populations expand exponentially when resources (e.g., food) are not limited. They also realized that resources are never unlimited and that populations tend to increase more rapidly than resources. When an organism lives in an environment where there is a surplus of resources, population growth increases exponentially. As resources become more limiting (due to diminishing surpluses of resources), population growth decelerates. Eventually, the rate of resource consumption becomes equal to rate of resource replenishment, and the population stabilizes. The point at which this occurs is known as carrying capacity.

[FIGURE 1-4 OMITTED]

The carrying capacity concept may seem simple, but it can be quite difficult to determine, especially when it comes to estimating the earth's carrying capacity for humans. With respect to humans, carrying capacity is a function not only of available resources, but also of how such resources are consumed. The manner in which humans use resources is determined by population size, use of technology, and cultural attitudes. First, consider how human numbers affect carrying capacity. The carrying capacity of the earth today may be less than it was 10,000 years ago because a significant amount of some resources have been used and there are considerably more humans using the resources. On the other hand, because of certain technologies, we are able to use resources today that were not available to earlier human cultures (e.g., petroleum and electricity). This brings us to the second variable affecting carrying capacity; the ability to develop and employ new technologies. Although technologies can expand the resource base, they can also lead to more rapid depletion of resources. This depends to some extent on cultural attitudes, the third variable affecting carrying capacity. The types of technologies developed by a culture as well as the ways in which such technologies are used are related to its values. For example, cultures that value forests as sacred places are less likely to develop and use forest-clearing technologies than are cultures that value forests purely as resources to support human development. So, taking all of these variables into account, what then is the carrying capacity of the earth?

Hopfenberg calculated the earth's carrying capacity for humans between 1960 and 2000 taking into account food production and patterns of food consumption, and compared these estimates to those based entirely on food production. What is striking about these data is that the carrying capacities based solely on food production are two to three times greater than those that take into account actual food production and food consumption patterns. For example, in the year 2000, the estimate of carrying capacity based on food production only is over 20 billion humans, whereas the estimate that takes into account how food is actually distributed and used is only 6 billion, the population of the Earth in 2000. The discrepancy between how many people can be adequately fed and how many are actually well fed is due primarily to food distribution patterns. Major problems include breakdown of normal production, disruption of transportation, and distribution due to wars or natural disasters (flooding and drought), and insufficient access by certain individuals and/or groups due to social, political, and economic conditions. For example, each year in the United States (a nation that produces more than enough food to meet the nutritional needs of its people) approximately 12 million children and 8 million adults suffer from a chronic shortage of nutrients needed for growth and good health.

The fact that the earth could support many more people than actually inhabit the planet may seem re-assuring. However, it is important to keep in mind that there is a limit to how many resources the earth can provide (Figure 1-5). Populations can exceed carrying capacity. When this occurs, the rate of resource consumption exceeds the rate of resource production, and shortages ensue. A decline in per capita consumption of resources can lead to a rapid drop (crash) in population size due to decreased reproductive rates and increased mortality rates. Resource shortages, along with increased competition for limited resources, leads to social and political problems, which tend to lower living standards. The drop in population will continue until it reaches a level that can be sustained by available resources. In cases where the population surge results in depletion of resources the population will continue to fall and eventually reach extinction.

To illustrate this concept, consider what life would be like in a world inhabited by 10 billion people, the projected global population in the year 2050 according to the World Bank and the United Nations. In order to meet the nutritional requirements of this many humans, Smil (1994) estimates that agricultural production of food energy would have to increase by 60 percent. This could be achieved by improving production efficiencies, reducing waste, and lowering the amount of fat in our diets. This is no simple task, and the implications may not be too appealing, especially to those of us living in wealthy societies. Currently, the wealthiest nations, with only 20 percent of the world's population, consume approximately 67 percent of all fossil fuels. Much of the technology responsible for enhanced food production during the past century is dependent on fossil fuels (e.g., heavy agricultural machinery, and inorganic nitrogen fertilizer). Unless there is a more equitable distribution of energy, it is unlikely that poor nations can adopt the petroleum-driven technologies required to meet growing demands for food. Moreover, a failure of rich nations to stabilize their energy use is likely to increase the economic gap between rich and poor nations, thereby exacerbating social and political unrest and leading to regional conflicts as well as refugee and immigration problems.

[FIGURE 1-5 OMITTED]

Even if energy distribution were more equitable and food production more efficient, these factors alone will not resolve problems related to population pressures. What happens if the population grows beyond 10 billion people? As noted previously, there is a limit to our ability to increase resources. At some point we reach the point of diminishing return; that is, the costs of increasing resources become greater than the gains derived from the resources. For example, even though the earth's reserves of petroleum seem vast, once 50 percent of the reserves are depleted (sometime between the years 2005 and 2010 according to some estimates), it becomes increasingly more costly to extract it. Thus the amount of oil that is actually available for our use is much less than that which exists in the ground.

As illustrated in Figure 1-5, once population size expands to a point where resource consumption exceeds resource replenishment, severe shortages ensue resulting in a crash in population size. No one knows when, or even if, this will occur on a global basis. However, over the millennia, such catastrophic changes have occurred on a regional basis with devastating consequences for some human societies; for example, the builders of the stone statues of Easter Island in the South Pacific Ocean, the Anaszi of the southwestern United States, and the Maya of Central America. Such crises are not unknown in more recent times. In 1994, the African nation of Rwanda experienced the third largest genocide since 1950 (the killing of 800,000 Tutsi's or 11 percent of the Rwandan population by the Hutu majority). The immediate or proximate cause of this violence was ethnic hatred instigated by politicians who were attempting to stay in power. However, the ultimate cause of this genocide may have been population pressure and scarcity of resources. In the early 1990s, Rwandan society was experiencing tremendous population expansion (3 percent annually), shrinking farm sizes, degradation of farm land, and disputes over access to scarce agricultural resources (land and water). Areas that experienced the most intensive violence were those where agricultural resources were most limited and food shortages most severe. This case should not be interpreted to mean that population pressure always leads to genocide or that other factors didn't contribute to this catastrophe. Certainly societies can and do choose different approaches to alleviate population pressures. Nevertheless, the events in Rwanda illustrate what can occur when available resources fail to sustain a population.

Challenges for Reproductive Physiology

The previous discussion illuminates two challenges for the field of reproductive physiology: 1) increasing efficiency of food production and 2) reducing birth rates of humans. Reproductive physiologists who study livestock reproduction have sought to increase production efficiencies of dairy and meat-producing animals. During the previous century, research with livestock has lead to the development of various technologies that enhance reproductive efficiency; for example, artificial insemination and estrous synchronization. Such practices have increased the amount of food a particular animal produces during its lifetime as well as the amount of animal-derived products a farmer can produce. Therefore, fewer livestock (and fewer farmers) are required to sustain a particular level of production. Meanwhile, reproductive physiologists interested in human reproduction have developed birth control technologies that can be used to lower birth rates. Together, these technologies can be used to stabilize population growth and resource use.

As noted earlier, livestock have always played important roles in agriculture. However, in recent years, some people have questioned whether production of animal-derived foods is an effective means to enhance the nutrition of humans on a global basis. Meat and dairy products are luxury foods that only the wealthiest nations can afford to include in daily diets. David Pimental notes that on a global basis, only 30 percent of protein in human diets is derived from animals. In wealthy nations such as the United States, 70 percent of the protein consumed by humans is from animal-derived foods. On the average, 4 kg of plant-derived protein is required to produce 1 kg of animal protein. Thus, a diet that is rich in animal protein requires large amounts of crops to sustain livestock production. In the United States, the annual per capita consumption of meat is 120 kg. In order to support this diet, 91 percent of crops suitable for human consumption (cereal, legume, and vegetable protein) are fed to livestock. The earth may not have enough resources to sustain this type of diet on a global basis. In order to supply this amount of animal protein to the current population, all of the world's grain harvest would have to be fed to livestock. Moreover, such a heavy dependence on animal protein would result in a 30 percent reduction in total amount of protein available for human consumption (due to the inefficiency of converting plant to animal protein).

Clearly, a diet rich in foods derived from grain-fed livestock is impractical and highly unsustainable. However, this does not mean that livestock should be excluded from agriculture. Animal protein is of high quality and can be produced in ways that are less wasteful than those currently employed in industrialized nations. For example, a more moderate use of global grain supplies could support a global per capital consumption of 40 kg of meat per year, an amount that would vastly improve human diets in many regions of the world. Whatever system of livestock production is adopted, knowledge of reproductive physiology will be essential for the efficient management of resources required to sustain production of animal-derived foods.

As mentioned earlier, human invention can have a positive effect on carrying capacity. Knowledge of reproductive physiology can help bring about equilibrium between human populations and resource availability. However, having the knowledge and means to accomplish these goals doesn't guarantee success. Cultural attitudes determine how technology is used (or whether or not it is used at all). People have to be willing and able to use a particular technology. For example, the lower birth rates of industrialized nations has been attributed to not only birth control technologies, but also policies that empower women to the extent that they have access to and are free to use such technologies.

All societies struggle with issues concerning human reproduction and resource use. Over the centuries many societies have collapsed because of an inability to cope with population pressure and resource use. Jared Diamond reminds us that the success of a society (whether or not it survives) depends largely on how it chooses to cope with prevailing environmental conditions. Values play an important role in this regard. The colonies of Norse Vikings failed in Greenland after several hundred years because the colonists retained European values, which proved to be incompatible with the ecology of Greenland (e.g., devoting great effort to raising cattle without adequate forages). In contrast, the small (1.8 square mile) island of Tikopia, located in the Southwest Pacific Ocean, has been occupied continuously for 3,000 years, largely because its inhabitants adopted practices that limit population growth (e.g., abstinence, abortion, and ritual suicide) and restricted their use of essential resources (e.g., invoking food taboos that prevent over-fishing). This is not to say that changing values is the only key to survival. The survival of a society also depends on its ability to recognize and retain values that have been responsible for its success. The path society should take is not always clear. Sometimes, people don't foresee the negative consequences of their lifestyles, or they realize the consequences too late to circumvent disaster. Moreover, conflicts between those who want to retain traditional values (conservatives) and those who advocate changing values (progressives) are common in all societies and give rise to contentious issues that can prevent or delay decisive action. Finally, it should be noted that in spite of having awareness of environmental problems, some societies may lack the political power to choose how they will live. For example, an indigenous society that occupies a rainforest may not have the political power to resist the logging and development that destroys the habitats on which they rely for survival.

The previous discussion highlights the fact that the science of reproduction takes place in a social context. In other words, the values of society influence research on mammalian reproduction and the knowledge that we gain from these studies can have an impact on society. Therefore, it should not be surprising that there are numerous social issues concerning how this research should be conducted as well as how research results should be applied. Ultimately these are ethical issues. In the concluding chapter of this book we will examine ethical aspects of some of the more controversial issues associated with reproductive physiology.

SUMMARY OF MAJOR CONCEPTS

* Reproductive physiology is the study of the reproductive functions and activities of organisms.

* As a branch of biology, reproductive physiology is based on the assumption that a species expresses a particular reproductive strategy because of natural selection.

* Research in reproductive physiology focuses on describing patterns of reproduction as well as characterizing the mechanisms responsible for these patterns.

* Reproductive physiology can serve societies by developing technologies that help sustain the carrying capacity of Earth.

DISCUSSION

1. Imagine that you are a wildlife biologist supervising a project to establish a population of wolves on an island. The island has viable populations of other animals on which wolf packs can prey. Describe expected changes in the size of the wolf population over the next 30 years, assuming that no other animals are brought to the island after the initial stocking of wolves. Explain these changes.

2. Some people argue that new advances in agriculture (e.g., biotechnology) should be pursued in order to keep pace with a growing human population. Others argue that this should not be done because agriculture cannot keep up with population expansion. What are some of the strengths and weaknesses of these opposing views?

3. Consider the following questions: a) Why do humans have sex without reproducing? b) How do humans have sex without reproducing? c) Should humans have sex without reproducing? How do these questions differ?

REFERENCES

Cohen, J.E. Population growth and Earth's human carrying capacity. Science 269:341-346.

Diamond, J. 2005. Collapse. New York: Viking Penguin.

Diamond, J. 1998. Guns, Germs, and Steel. New York: W.W. Norton and Co.

Hopfenberg, R. Human carrying capacity is determined by food availability. Population and Environment 25:109-117.

Klare, M.T. 2001. Resource Wars. New York: Henry Holt and Co.

Pimental, D. W. Dritschilo, J. Krummel, J. Kutzman. 1975. Energy and land constraints in food protein production. Science 190:754-761.

Smil, V. 1994. How many people can the Earth feed? Population and Development Review 20:255-292.

United Nations. 1992. Long-Range World Population Projections: Two Centuries of Population Growth 1950-2150. New York: United Nations.

World Bank. 1992. World Development Report. New York: Oxford University Press.
FIGURE 1-2

The percentage of known animals species
that are vertebrates (a) and the percentage
of known vertebrate species that are mammals (b).

(a) Percentage of Animals that are Vertebrates   2%
(b) Percentage of Vertebrates that are Mammals   8.6%

Note: Table made from pie chart.
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Author:Schillo, Keith K.
Publication:Reproductive Physiology of Mammals, From Farm to Field and Beyond
Article Type:Work overview
Geographic Code:1USA
Date:Jan 1, 2009
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Next Article:Chapter 2: Life, reproduction, and sex.
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