Motivation IV Sexual Motivation Sexual Reproduction Reproduction is necessary for the survival of the species. Some organisms (e.g., bacteria) reproduce asexually. Sexual reproduction allows the genes of two individuals to be mixed to produce individuals with new genetic combinations. This increases the chances that at least some combinations will be found that will survive under given conditions. Genetics and Sex Genes carried on the chromosomes code for proteins that ultimately determine bodily characteristics and function. Chromosomes come in pairs. One member of each pair comes from the male, one from the female. The sex of the individual is specified by the sex chromosomes, which come in X and Y types. Females have two X chromosomes, males have one X and one Y chromosome. 1
Sources of the X and Y Chromosomes Females produce eggs, whereas males produce sperm. In either case, during production, each egg or sperm receives only one member of each gene pair, yielding only half the normal complement of chromosomes. Because females have only X-X chromosome pairs, all eggs have only an X chromosome. Because males have X-Y chromosome pairs, half of the sperm have X chromosomes and half have Y chromosomes. Fertilization and Genetic Sex During fertilization, egg and sperm combine, each contributing their half of the total chromosome complement. Genetic sex is determined by the sperm: Sperm X plus egg X = X-X = female Sperm Y plus egg X = X-Y = male However, this is not the whole story. Genotype Versus Phenotype The genotype is the characteristic programmed by the genes. The phenotype is the characteristic actually expressed in the individual. They are not always the same, because of the environment in which the phenotype develops (including other genes as well as hormones, drugs, infections, nutrition, etc.). 2
Sexual Development The sex chromosomes specify how the gonads develop whether into ovaries or testes. Other than that, early in fetal development, males and females have the same proto sex-organs, including external genitalia. In genetic males, the testes begin to secrete testosterone and other male sex hormones (androgens). Organizing Influence of Sex Hormones in Early Development In genetic males, sex hormones suppress further development of female internal organs and promote the development of male internal organs (seminal vesicles, prostate, etc.). External genitalia develop further into penis and scrotum. Brain development is also affected. In genetic females, absence of testosterone suppresses further development of male internal organs and allows female internal organs to develop. External genitalia develop further into clitoris, labia, and part of vagina. Sex Hormones and Sex-typical Behavior Presence or absence of high levels of testosterone during fetal development changes the wiring of the brain in some areas (e.g., pre- optic area of the hypothalamus). Female rats exposed to testosterone late in development (brain is still developing) behave sexually like males after puberty. Male rats deprived of testosterone late in development behave sexually like females after puberty. During development, testosterone biases the brain to favor male-typical as opposed to female-typical behavior. 3
Androgen Insensitivity Syndrome Genetic error renders cells of body unable to respond to androgens. Consequently, genetic males develop externally as females, but lack internal female organs. (They do, however, have a shallow vagina.) Typically, these individuals develop sexual interest in the phenotypically opposite sex (i.e., males). Thus, sexual preference is not directly influenced by the genes. Early Exposure to Androgens in Females Genetic females are sometimes exposed to relatively high levels of androgens during fetal development. Depending on how soon and how much, this can change development toward the male plan. Girls with this condition generally develop heterosexual interests at puberty, but in childhood tend to be described as tomboys. Sexual Orientation and Hormones In humans, determination of sexual orientation is complex and poorly understood at present. In all probability, genetic, hormonal (during fetal development), and environmental factors all contribute. At puberty, when sex hormone levels rise, sexual interest awakens. However, sexual orientation may already have been determined (not a matter of choice ). 4
Sexual Motivation Sexual reproduction carries with it a downside unlike bacteria, sexual reproducers must find a partner of the opposite sex. Sexual attraction to members of the opposite sex fosters this. Differences in reproductive equipment yield different needs in the two sexes. Female Needs Females are able to produce only a few offspring, and human children are extremely dependent for a relatively long period. It is difficult to provide for many highly dependent children while at the same time caring for them, especially while pregnant. Females must live in a world inhabited by generally larger and more powerful males. Must be able to attract a mate. Female Strategies Select a mate based on his apparent abilities to provide and protect. Often these will be somewhat older than herself. Power is more important than physical attractiveness. Be concerned about her own physical appearance as this is important in attracting a mate. Find a mate willing to stick around and help (long-term commitment). Be concerned about the relationship and work to maintain it. Maintain strong ties with maternal side. (Receive support and protection.) 5
Male Needs Males are capable of having a great number of children. However, to do so they must impregnate females. This places them in competition with other males. Reproductive success is more likely with young, healthy adult females who can bear children and have the energy to care for them. Successful reproduction requires that at least some of his children reach adulthood. Males need to know that the children they invest their resources in are theirs. Male Strategies Look for females of reproductive age who are relatively young and healthy (attraction to physical beauty). Successfully compete with other males for access to females, resources. Survival and success may depend on forming alliances with other males, being seen as a valuable contributor. Best reproductive strategy will depend on how difficult it is for his offspring to survive without him. Prevent other males from having sexual relations with his mate. 6