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REPRODUCTION IN MAMMALS
ABSTRACT
This study examined reproduction in mammals using man as a case study. In the study, some of the main features of reproduction in man are explored. After considering some of the various aspects of reproduction in man and the challenges faced, we move to consider in particular, pregnancy and childbirth. To this effect, a questionnaire was designed that addressed issues relating to pregnancy and childbirth. The primary audiences for the questionnaire were pregnant women/nursing mothers and medical practitioners, and the study area was Uhumwonde local government area of Edo state. The responses as analyzed revealed a number of findings in the study. Some of the findings reported include the need for pregnant women to exercise, and the importance of breastfeeding. The study, amidst other recommendations, stressed the need for women to be educated on contraceptive methods available for use as birth control measure after pregnancy.
CHAPTER ONE
INTRODUCTION
Some very basic facts of human life are:
1. People have no control over whether or not they become alive;
2. Once alive they strive to stay alive;
3. They have a strong and instinctive drive to reproduce;
4. They all eventually die.
Why are these facts ethically important? Human beings, as organisms, are subject to these facts of life, but uniquely in the living world, we are also able to rationalise and to moralise. We can readily understand that the basic instinct for life is present in other people too, and appreciate that we have a moral duty to protect that instinct in them. This has been termed the presumption in favour of life. Reproduction is the single most distinguishing characteristic of life. It is the driving force of evolution, which itself is responsible for the diversity of plant and animal species on Earth. We humans are living organisms and these forces act on us just as any other species with which we share the Earth. We feel the urge to reproduce at a very powerful biological level at various times in our lives. It is not surprising therefore, that over the course of history all aspects of human reproduction have been the subjects of intense moral and ethical debate.
1.1 BACKGROUND OF STUDY
The study of reproduction is a relatively new specialty, and developed in large part from a focus on body politics during second wave feminism in the 1970s. It was also in the 70s, that the first IVF baby was born, making “infertility,” more of a chronic condition as opposed to barrenness or sterility. As new technological advances made it possible for increasing numbers of infertile couples to conceive biological children, more people came forward to seek assistance with reproduction (Angela, 2011) Reproduction in humans and others mammals is the process by which sperm and egg cells are produced, brought together, join and develop into a new individual. Reproduction is the key to the continued existence of our species. Mammals reproduce by uniting sperm and egg internally in the female body by a process called internal fertilization. Many mammals reproduce only during particular seasons of the year; not so with humans. Men produce sperm more or less continuously and women ovulate (produce an egg) about once a month. The male reproductive tract consists of the paired gonads (organs that produce sex cells), the testes, where sperm are produced, plus accessory structures that store the sperm, produce secretions to activate them and finally conduct them to the inside of the female reproductive tract.( Richard, 2000).
Mousseau and Roff (1987) conducted a comprehensive review of the heritable variability of the morphological, behavioral, physiological, and life history phenotypes (i.e., measurable traits) that covary with survival and reproductive outcomes in wild, outbred animal populations. The analysis included 1120 heritability estimates-- the proportion of variability across individuals that appears to be due to genetic variability—across 75 invertebrate and vertebrate species. Although there was con-siderable variation ---across species, contexts, and phenotypes--- in the magnitude of the heritability estimate, their analysis indicated that “significant genetic vari- ance is maintained within most natural populations, even for traits closely affiliated with fitness” (Mousseau & Roff, 1987). The median heritability estimates were .26 for life history traits (e.g., age of maturation), .27 for physiological traits (e.g., cardiovascular capacity), .32 for behavioral traits (e.g., mating displays), and .53 for morphological traits (e.g., body size), values that are similar to those found in human populations (Plomin et al, 2001).
Kingsolver et al (2001) reviewed field studies of the relation between the types of traits analyzed by Mousseau and Roff (1987) and survival and re-productive outcomes in wild populations. As aptly described by Alexander, “lifetimes have evolved to maximize the like-lihood of genic survival through reproduction” (Alexander, 1987), and the focus of life history research is on the suite of phenotypic traits that defines the species’ maturational and reproductive pattern (Charnov, 1993; Roff, 1992). A suite of traits must be considered because of the trade-offs involved in the expression of one phenotype versus another (Williams, 1957). The trade-offs are commonly conceptualized in terms of a competitive allocation of resources (e.g., calories) to somatic effort or reproductive effort, (Alexander, 1987; Reznick, 1985, 1992; Williams, 1966). Somatic effort is traditionally defined as resources devoted to physical growth and to maintenance of physical systems during development and in adulthood (West, et al, 2001), although growth also involves the accumulation, as in increases in body size, of reproductive potential. Reproductive effort is expended during adulthood and is distributed among mating, parenting, and in some species nepotism, that is, investment in kin other than offspring (Emlen, 1995; Hamilton, 1964).
According to David (2003), Reproductive activity takes on two general forms: in semelparity all reproductive potential is spent in one breeding episode, but in iteroparity reproductive potential is allocated across more than one breeding episode. Semelparity is a more risky strategy because reproduction during poor ecological conditions could result in extremely high offspring mortality rates, with no opportunity to reproduce under more favorable conditions. Semelparity is, however, favored when adult mortality is high and thus the probability of surviving to the next breeding season is low. Under these conditions, individuals that devote minimal resources to somatic effort in adulthood and maximal resources to reproductive effort will produce more off-spring than individuals that do not. In contrast, iteroparity is favored when juveniles and adults are likely to survive from one breeding season to the next (e.g., due to low predation risks) and juveniles are unlikely to reproduce successfully (Roff, 1992; Wittenberger, 1979). For these species, the current reproductive effort is bal-anced against the costs of this effort with respect to survival and future reproductive potential. As a result, during each breeding season iteroparous species invest more in maintenance and less in reproduction than semelparous species (Roff, 1992).
Reproduction involves costs associated with mating (e.g., finding mates), pro-ducing gametes and offspring (e.g., eggs), and for many species parental care (Roff, 1992). Mechanisms underlying the cost/benefit trade-offs involved in reproducing may be genetic or social/environmental, or they may represent a genotype by environment interaction (Reznick, Nunney, & Tessier, 2000). Social costs include those incurred during intrasexual competition over mates and are described later. Genetic trade-offs arise when the same gene or genes affect two or more life history traits (Williams, 1957). In many species, reproducing earlier in life is associated with a shorter life span (Reznick, 1992). The same genes that promote early re-production have the negative consequence of accelerating the onset of senescence and reducing the life span.
Life span is also influenced by more proximal reproductive costs, such as pro-ducing eggs, competing for mates, and caring for offspring, which can compromise the physical health and oftentimes the survival prospects of parents (Clutton- Brock, 1991; Steams, 1992). The underlying physiological mechanisms governing these cost/benefit trade-offs are not fully understood, but include the energetic demands of reproduction (e.g., parental care) and associated hormonal changes (Sinervo & Svensson, 1998). For example, the development of male secondary sexual char-acteristics needed to compete with other males (e.g., antlers) or to attract females (e.g., a bright plumage) requires an increase in testosterone levels which in turn can compromise the immune system and survival prospects of unhealthy males (Folstad & Karter, 1992; Saino & Møller, 1994; Saino, Møller, & Bolzern, 1995). Similarly, in the female collard flycatcher (Ficedula albicollis) large brood sizes are associated with a reduced production of antibodies for a common parasite; the result is increased infection rate and mortality rate (Nordling et al, 1998).
All other things being equal, evolution should result in a life history pattern in which females produce many, fast maturing offspring, that have an increased probability of surviving to reproduce (Williams, 1966). The fact that many species do not show this life history pattern indicates that the associated trade-offs are costly. These trade-offs include smaller and less competitive offspring that in turn suffer high mortality rates (Steams, 1992). Across species of plant, insect, fish, reptile, and mammal, offspring that are larger at time of hatching or birth have increased survival rates due, in part, to decreased predation risk and decreased risk of starvation (Roff, 1992). The trade-off is that females of these species produce fewer offspring than do females of related species that produce many smaller offspring. Thus fast maturation and large numbers of offspring are associated with low-quality offspring (i.e., high mortality risks and low competitiveness). High---quality---larger and more competitive---offspring come at a cost of fewer offspring produced during a reproductive life span. Many factors will influence whether a species tends toward a low-quality/high-quantity or high-quality/low-quantity reproductive pattern, including age-specific mortality risks (e.g., through predation), population stability or expansion, and intensity of competition with conspecifics (Mac Arthur & Wilson, 1967; Steams, 1992; Roff, 1992).
Species that produce fewer and larger offspring also tend to have slower rates of growth, higher levels of parental care, and longer life spans in comparison to related species that produce smaller but more offspring (Roff, 1992; Shine, 1978, 1989; Steams, 1992). This life history pattern is more common in iteroparous than in semelparous species and is associated with relatively low juvenile mortality rates and a low probability of reproducing at an early age (Roff, 1992). Low juvenile mortality is related to larger size at hatching or birth as well as to parental protection and provisioning (Clutton-Brock, 1991; Shine, 1978). As described later, a low probability of reproducing at an early age can result from reproductive competition with more mature individuals in the population. In this situation, delayed maturation can improve reproductive prospects through, for in-stance, an increase in body size. Large body size enables females to give birth to larger and thus more competitive offspring, and for males it facilitates male- male competition in adulthood (Carranza, 1996; Steams, 1992). In some species, developmental activity during the maturational period enables improvements in survival- and reproduction-related behavioral/cognitive competencies. Slow maturation and growth thus allows for the accumulation of more reproductive potential, through physical development and developmental activity, than is possible with faster maturing species. A sex difference in potential rate of reproduction can create a sex difference in relative emphasis on mating or on parenting. Most generally, the sex with the higher potential rate of reproduction invests more in mating effort than in parental effort, whereas the sex with the lower rate of reproduction invests more in parental effort than in mating effort (Clutton-Brock & Vincent, 1991). This pattern arises because members of the sex with the higher potential rate of reproduction can rejoin the mating pool more quickly than can members of the opposite sex. Under these conditions, individuals of the sex with the faster rate of reproduction will typically have a higher lifetime reproductive success if they rejoin the mating pool and compete for mates than if they parent (Parker & Simmons, 1996).
For species with internal gestation and obligatory postpartum female care (e.g., suckling in mammals), the rate at which females can produce offspring is considerably lower than the potential rate of reproduction of some specific males (Clutton-Brock, 1991). In addition, internal gestation and the need for postnatal care results in a strong bias in mammalian females toward parental investment and results in a sex difference in the benefits of seeking additional mates (Trivers, 1972). Males can benefit, reproductively, from seeking and obtaining additional mates, whereas females cannot. In other words, males that compete for additional mates typically have more offspring than do males that do not compete and instead invest in parenting. Thus, the sex difference in reproductive rate, combined with offspring that can be effectively raised by the female, creates the potential for a large female--male difference in the mix of mating and parenting, and this difference is realized in 95-97% of mammalian species (Clutton-Brock, 1989). In these species, females can effectively provide the majority of parental care and do so. Female care, in turn, frees males to invest in mating effort, which typically takes the form of male--male competition over access to mates or for control of the resources (e.g., territory) that females need to raise their offspring.
1.2 STATEMENT OF THE PROBLEM
Fertility changes with age. Both male and females become fertile in their teens following puberty. For girls, the beginning of their reproductive years is marked by the onset of ovulation and menstruation. It is commonly understood that after menopause, women are no longer able to become pregnant. Generally, reproductive potential decreases as women get older, and fertility can be expected to end 5 to 10 years before menopause. In today’s society age-related infertility is becoming more common because, for a variety of reasons, many women wait until their 30s to begin their families. Even though women today are healthier and taking better care of themselves than ever before, improved health in their in later life does not offset the natural age-related infertility. It is important to understand that fertility declines as a woman’s ages due to the natural age-related decrease in the number of eggs that remain in the ovaries. This decline may take place much sooner that most women expect and thus hinder their chances of reproduction.
1.3 HYPOTHESIS OF THE STUDY
The hypothesis of this study is stated in the null as;
Not every pregnant woman knows what to do if her labour starts early.
1.4 AIM AND OBJECTIVES OF THE STUDY
The aim of this study is to outline the concept of reproduction in man and the challenges faced overtime. Some of the objectives include but not limited to;
1. Give an insightful introduction to the concept of reproduction.
2. Give an insight into some challenges of reproduction and how they can be managed.
1.5 SCOPE OF STUDY
This study, as the title implies, has used man as a case study, hence all attempt to make the subject of reproduction clearer has been duly focused on man. Some of the areas covered include pregnancy and child birth.
1.7 DEFINITION ODF TERMS
The following definition of terms has been provided to ensure uniformity throughout the study.
1. Reproduction: The sexual or asexual process by which organisms generate new individuals of the same kind; procreation.
2. Individual: An individual belonging to a group of organisms (or the entire group itself) having common characteristics and (usually) are capable of mating with one another to produce fertile offspring.
3. Fertility: The state of being fertile or fruitful; fruitfulness; productiveness.
4. Sperm: The male reproductive cell; the male gamete; “a sperm is mostly a nucleus surrounded by little other cellular material”.
5. Fertilization: the union of male and female gametes during sexual reproduction, to form a zygote.
6. Population: A group of individuals of the same species occupying a particular geographical area. Population may be relatively small and closed, as on an island or in a valley, or they may be more diffuse and without a clear boundary between them and a neighbouring population of the same species
7. Gene: A gene is a molecular unit of heredity of a living organism.
8. Traits: A distinguishing feature, as of a person’s character.
9. Phenotype: The physical appearance or biochemical characteristic of an organism as a result of the interaction of its genotype and the environment.
10. Heredity: The passing on of traits to offspring from its parents or ancestor. This is the process by which an offspring cell or an organism acquires or becomes predisposed to the characteristics of its parent cell or organism.
11. Genotype: The genetic constitution of an individual organism.
12. Offspring: the immediate descendants of an organism.
13. Mating: The meeting of individuals for sexual reproduction.
14. Breeding: The mating and reproduction of offsprings by animals.
15. Gametes: Gametes are reproductive cells that unite during sexual reproduction to form a new cell called a zygote.
16. Survival: A natural process resulting in the evolution of organisms best adapted to the environment.
17. Zygote: The cell formed by the union of two gametes, especially a fertilized ovum before cleavage. A zygote is the cell that results from fertilization.
18. Gestation: In mammals, the time between conception and birth, during which the embryo or fetus is developing in the uterus.
19. Menopause: Cesation of menstruation in the human female, occurring usually around the age of 50.
20. Evolution: A gradual process in which something changes into a different and usually more complex or better form.
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