Chapter 12 Summary & Outline

Sexual Behavior

Reproductive Behavior Can Be Divided into Four Stages

• Reproductive behaviors are divided into four stages: sexual attraction, appetitive behavior, copulation, and postcopulatory behavior, including parental behaviors in some species. Review Figure 12.1
• Although reproductive behaviors vary widely in form between species, their ultimate goal is the successful fusion of male and female gametes, and the generation of a maximal number of viable offspring.

Study questions: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10

The Neural Circuitry of the Brain Regulates Reproductive Behavior

• In the female rat, a steroid-sensitive lordosis circuit extends from the ventromedial hypothalamus (VMH) to the spinal cord, via the periaqueductal gray and medullary reticular formation.
• In the male rat, neurons of the medial preoptic area (mPOA) exert descending control of sexual behavior, integrating inputs from the medial amygdala and vomeronasal organ (VNO). These projections, via ventral midbrain and brainstem nuclei, terminate on motoneurons involved in copulation. Review Figure 12.6

Study questions: 11 | 12 | 13 | 14 | 15 | 16

Pheromones Guide Reproductive Behavior in Many Species

• Sex steroids act on circuits within the nervous system to coordinate reproductive behaviors with gonadal functions such as ovulation. In many species, pheromone signals between partners coordinate reproductive behavior.

Study questions: 17 | 18 | 19 | 20

The Hallmark of Human Sexual Behavior Is Diversity

• Human copulatory behavior is remarkably varied. Most men show a single copulatory pattern; women show much more varied sexual responses. The classic model of sexuality emphasizes four stages: (1) increasing excitement, (2) plateau, (3) orgasm, and (4) resolution. Modern models identify emotional factors and desire as crucial aspects of female sexuality, whereas male sexuality may involve feelings of power. However, male and female sexuality overlap and are heavily influenced by sociocultural factors. Review Figure 12.9, Web Activities 12.1 and 12.2
• In humans, very low levels of testosterone are required for either men or women to display a full interest in sex, but additional testosterone has no additional effect. Therefore, there is no correlation between circulating androgen levels and reproductive behaviors in men. Nor is there any strong correlation between copulatory behavior and stage of the menstrual cycle in women.
• There is evidence both for and against the idea that humans respond to pheromonal cues, so pheromones play only a subtle role, if any, in human sexuality.

Study questions: 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28

Many Vertebrates Depend on Their Parents for Survival

• Parental behavior is a crucial aspect of reproduction and is significantly influenced by hormones. Brain mechanisms for parental behavior show considerable overlap with mechanisms implicated in sexual behavior. Review Figures 12.11 and 12.12

Study questions: 29

Sexual Differentiation

The Sex of an Individual Is Determined Early in Life

• In birds and mammals, genetic sex determines whether testes or ovaries develop, and hormonal secretions from the gonads determine whether the rest of the body, including the brain, develops in a feminine or masculine fashion. In the presence of testicular secretions, a male develops; in the absence of testicular secretions, a female develops. Review Figures 12.13 and 12.14

Study questions: 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43

How Should We Define Gender—by Genes, Gonads, Genitals, or the Brain?

• People can be classified on the basis of their sex chromosomes, their genitalia, or the gender they identify with. The options within each of these categories are complex and sometimes overlap, so attempts to classify all individuals into just two gender groups oversimplify the real situation.

Study questions: 44 | 45 | 46

Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

• The brains of vertebrates are masculinized by the presence of testicular steroids during early development. Such organizational effects of steroids permanently alter the structure and function of the brain and therefore permanently alter the behavior of the individual. Review Figure 12.17, Web Activity 12.3
• Among the prominent examples of sexual dimorphism in the nervous system, gonadal steroids have been shown to alter characteristics such as neuronal survival, structure, and synaptic connections. Review Figures 12.18–12.20

Study questions: 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 59 | 60 | 61 | 62

Social Influences Affect Sexual Differentiation of the Nervous System

• Several regions of the human brain are sexually dimorphic. However, we do not know whether these dimorphisms are generated by fetal steroid levels or by sex differences in the early social environment.

Study questions: 63

Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood?

• Although no perfect animal model of sexual orientation has been developed, all research indicates that sexual orientation is determined early in life and, especially in men, is not a matter of individual choice. Review Figure 12.23

Study questions: 64 | 65 | 66 | 67 | 68 | 69