Reproductive behaviors constitute the most important category of social behaviors, because without them, most species would not survive. These behaviors-which include courting, mating, parental behavior, and most aggressive behaviors-are the most striking categories of sexually dimorphic behaviors, that is, behaviors that differ in males and females (di + Morphus, "two forms"). As you will see, hormones that are present both before and after birth play a very special role in the development and control of sexually dimorphic behavior.
A person’s chromosomal sex is determined at the time of fertilization. However, this event is merely the first in a series of steps that culminate in the development of a male or female.
Production of Gametes and Fertilization
Gamete [gamm eet] A mature reproductive cell; a sperm or ovum.
Sex chromosome The X and Y chromosomes, which determine and organism’s gender. Normally, XX individuals are females and XY individuals are male.
Gonad [rhymes with moan ad] An ovary or testis.
Development of the Sex Organs
Men and women differ in many ways: Their bodies are different, parts of their brains are different, and their reproductive behaviors are different. The X chromosome and the twenty-two nonsex chromosomes found in the cells of both males and females contain all the information needed to develop the bodies of either sex. Exposure to sex hormones, both before and after birth, is responsible for our sexual dimorphism. What the Y chromosome does control is the development of the glands that produce the male sex hormones.
There are three general categories of sex organs: the gonads, the internal sex organs, and the external genitalia. The gonads-testes or ovaries-are the first to develop. They produce ova or sperms, and they secrete hormones. Through the fourth week of prenatal development, male and female fetuses are identical. Both sexes have a pair undifferentiated gonads, which have the potential of developing into either testes or ovaries. The factor that controls their development appears to be a single gene on the Y chromosome called SRY. This gene produces and enzyme called testis-determining factor, which causes the undifferentiated gonads to become testes. If the gene is not present, they become ovaries. Once gonads have been developed, a series of events is set into action that determines the individual’s gender. These events are directed by hormones, which affect sexual development in two ways. During prenatal development these hormones have organizational effects, which influence the development of a person’s sex organs and brain. These effects are permanent; once a particular path is followed in the course of development, there is no going back. The second role of sex hormones is their activational effect. These effects occur later in life, after the sex organs have developed. For example, hormones activate the production of sperms, make erection and ejaculation possible, and induce ovulation. Because the bodies of adult males and females have been organized differently, sex hormones will have different activational effects in the two sexes.
Internal Sex Organs
The precursor of the internal female sex organs, which develops into the fimbriae and Fallopian tubes, the uterus, and the inner two-thirds of the vagina, is called the Mullerian system. The precursor of the internal male sex organs, which develops into the epididymis, vas deferens, seminal vesicles, and prostate, is called the Wolffina System. The gender of the internal sex organs of a fetus is determined by the presence or absence of hormones secreted by the testes. If these hormones are present, the Wolffian system develops. If they are not, the Mullerian system develops. The Mullrian (female) system needs no hormonal stimulus from the gonads to develop; it just normally does so. In contrast, the cells of the Wolffian (male) system do not develop unless they are stipulated to do so by a hormone. Thus, testes secrete two types of hormones. The first, a peptide hormone called anti-Mullerinan hormones, does exactly what it name says: It prevents the Mullerian (female) system from developing. It therefor has a defeminizing effect. The second, a set of steroid hormones called androgens, stimulates the development of the Wolffian system. Androgens have a masculinizing effect. The precursor of the male internal sex organs-the Wolffian System-contains androgen receptors that are coupled to cellular mechanisms that promote growth and division. When molecules of androgens bind with these receptors, the epididymis, vas deferens, and the prostate develop and grow. In contrast, the cells of the Mullerinan system contain receptors for the anti-Mullerinan hormone that prevent growth and division. Thus, anti-Mullerinan hormone prevents the development of the female internal sex organs. Two genetic disorders: androgen insensitivity syndrome and persistent Mullerinian duct syndrome. The cause of androgen insensitivity syndrome is a genetic mutation that prevents the formation of functioning androgen receptors. The lack of androgen receptors prevents the androgens from having a masculinizing affect; thus, the epididymis, vas deferens, seminal vesicles, and prostate fail to develop. The anti-Mullerian still has its defeminizing effect, preventing the female internal sex organs from developing. The uterus, fimbriae, and Fallopian tubes fail to develop, and the vagina is shallow. Their external genitalia are female, and at puberty they develop a woman’s body. Of course, lacking a uterus and ovaries, these people can not have children. Persistent Mullerian duct syndrome, when this syndrome occurs in genetic males, androgens have their masculinizing effect but not their defeminizing effect. The person is born with both sets of external sex organs, male and female. The presence of the additional sex organs usually interferes with normal functioning of the male sex organs. Turner’s syndrome have only one sex chromosome: an X chromosome. (Thus, instead of having XX cells, they have X0 cells—0 indicating a missing sex chromosome.) Because a Y chromosome is not present, testes do not develop. In addition, because two X chromosomes are needed to produce ovaries, these glands are not produced, either. But even though people with Turner’s syndrome have no gonads at all, they develop into females, with normal female internal sex organs and external genitalia—which proves that fetuses do not need ovaries or the hormones they produce to develop as females.
The external genitalia are the visible sex organs, including the penis and scrotum in males and the labia, clitoris and the outer part of the vagina in females. The external genitalia do not need to be stimulated by female sex hormones to become female; they just naturally develop that way. However, masculine development requires the presence of androgens—in particular, the presence of dihydrotestosterone.
The primary sex characteristics include the gonads, internal sex organs, and external genitalia. The
secondary sex characteristics, such as enlarged breasts and widened hips or a beard and deep voice do not appear until puberty. The onset of puberty occurs when cells in the hypothalamus secrete gonadotropin-releasing hormones (GnRH), which stimulate the production and release of two gonadotropic hormones by the anterior pituitary gland. The gonadotropic ("gonad-turning") hormones stimulate the gonads to produce their hormones, which are ultimately responsible for sexual maturation. The two gonadotropic hormones are follicle-stimulating hormone (FHS) and luteinizing hormone (LH), named for the affects they produce in the female (production of a follicle and its subsequent luteinization. In response to the gonadotropic hormones (usually called gonadotropins) the gonads secrete steriod sex hormones. The ovaries produce estradiol, one of a class of hormones known as estrogens. The gonadal steroids affect many parts of the body. Both estradiol and testosterone initiate closure of the growing portions of the bones and thus halt skeletal growth. Estradiol also causes breast development, growth of the lining of the uterus, changes in the deposition of body fat, and maturation of the female genitalia. Testosterone stimulates growth of facial, axillary (underarm), and pubic hair; lowers the voice; alters the hairline on the head (often causing baldness later in life); stimulates muscular development; and causes genital growth. If a man is treated with an estrogen, he will grow breasts, and his facial hair will become finer and softer. His voice will remain low, because the enlargement of the larynx is permanent. Conversely, a woman who receives a high levels of an androgen (usually, from a tumor that secretes androgens) will grow a beard, and her voice will become lower.
Gender is determined by the sex chromosomes: XX produces a female, and XY produces a male. Males are produced by the action of the SRY gene on the Y chromosome, which contains the code for the production of the testis-determining protein, which in turn causes the primitive gonads to become testes. The testes secrete two kinds of hormones that cause a male to develop. Testosterone (an androgen) stimulates the development of the Wolffian system (masculinization), and anti-Mullerian hormone suppresses the development of the Mullerian system (defeminization). Androgen insensitivity syndrome results from a hereditary defect in anti-Mullerian hormone receptors.
By default, the body is female ("Nature’s impulse…"); only by the actions of testicular hormones does it become male. Masculinization and defeminization are referred to as organizational effects of hormones; activational effects occur after development is complete. A person with Turner’s syndrome (X0) fails develop gonads but nevertheless develops female internal sex organs and external genitalia. The external genitalia develop the male form (masculinization).
Sexual maturity occurs when the hypothalamus begins secreting gonadotropin-releasing hormones, which stimulates the secretion of the follicle-stimulating hormone and luteinizing hormone by the anterior pituitary gland. These hormones stimulate the gonads to secrete their hormones, thus causing the genitals to mature and the body to develop the secondary sex characteristics (activational effects).
HORMONAL CONTROL OF SEXUAL BEHAVIOR
Hormonal Control of Female Reproductive Cycles
The reproductive cycle of female primates is called a menstrual cycle. Estrus cycles—estrus means "gadfly". The primary feature of menstrual cycles that distinguishes them from estrus cycles is the monthly growth and loss of the lining of the uterus. The sexual behavior of female mammals with estrous cycles is linked with ovulation, whereas most female primates can mate at any time during their menstrual cycle. A cycle begins with the secretion of gonadotropins by the anterior pituitary gland. These hormones (especially FSH) stimulate growth of ovarian follicles, small spheres of epithelial cells surrounding each ovum. As ovarian follicles mature, they secrete estradiol, which causes growth of the lining of the uterus in preparation for implantation of the ovum, should it be fertilized by a sperm. Feedback from the increasing level of estradiol eventually triggers the release of a surge of LH by the anterior pituitary gland.
The LH surge causes ovulation: The Ovarian follicle ruptures, releasing the ovum. Under the continued influence of LH the ruptured ovarian follicle becomes a corpus luteum ("yellow body"), which produces estradiol and progesterone. The latter hormone promotes pregnancy (gestation). It maintains the lining of the uterus, and it inhibits the ovaries from producing another follicle. Meanwhile, the ovum enters one of the Fallopian tubes and begins its progress toward the uterus. If it meets sperm cells during its travel down the Fallopian tube and becomes fertilized, it begins to divide, and several days later it attaches itself to the uterine wall.
If the ovum is not fertilized, or if it is fertilized too late to develop sufficiently by the time it gets to the uterus, the corpus luteum will stop producing estradiol and progesterone, and then the lining of the walls of the uterus will slough off. At this point, menstruation will commence.
Male sexual behavior is quite varied, although the essential features of intromission (entry of the penis into the female’s vagina), pelvic thrusting (rhythmic movement of the hindquarters, causing genital friction), and ejaculation (discharge of semen) are characteristic of all male mammals. After eight to fifteen intromissions approximately 1 minute apart (each lasting only about one-quarter of a second), the male will ejaculate.
The mammalian female has been described as the passive participant in copulation. It is true that in some species the female during the act of copulation is merely to assume a posture that exposes her genitals to the male. This behavior is called lordosis response. The female will also move her tail away (if she has one) and stand rigidly enough to support the weight of the male.
The sequence of estradiol followed by progesterone has three effects on female rats. Receptivity refers to their ability and willingness to copulate . Proceptivity refers to a female’s eagerness to copulate, as shown by the fact that she seeks out a male and engages in behaviors that tend to arouse his sexual interest. Attractiveness refers to physiological and behavioral changes that affect the male.
ORGANIZATIONAL EFFECTS OF ANDROGENS ON BEHAVIOR:
MASCULINIZATION AND DEFEMINIZATION
If a rodent’s brain is not exposed to androgens during a critical period of development, the animal will engage in female sexual behavior as an adult. If a male rat is castrated immediately after birth, permitted to grow to adulthood, and then given injections of estradiol and progesterone, it will respond to the presence of another male by arching its back and presenting its hindquarters. In other words, it will act as if it were a female. If a rodent brain is exposed to androgens during development, two phenomena occur: behavioral defeminization and behavioral masculinization. Behavioral defeminization refers to the organizational effect of androgens that prevents the animal from displaying the female sexual behavior in adulthood. Moore and her colleagues discovered that mothers spent much more time licking their male offspring and wondered whether this licking could have any effects on the sexual behavior of these offspring later in life.
EFFECTS OF PHEROMONES
Hormones transmit messages from one part of the body (the secreting gland) to another (the target tissue). Another class of chemicals, called pheromones, carries messages from one animal to another.
Lee-Boot effect The increased incidence of false pregnancies seen in female animals that are housed together; caused by a pheromone in the animals’ urine; first observed in mice.
Whitten effect The synchronization of the menstrual or estrous cycles of a group of females, which occurs only in the presence of a pheromone in a male’s urine.
Vandenbergh effect The earlier onset of puberty seen in female animals that are housed with males; caused by a pheromone in the males urine; first observed in mice.
Bruce effect Termination of pregnancy caused by the odor of a pheromone in the urine of a male other than the one the impregnated the female; first identified mice.
Vomeronasal organ [voe mer oh nay zul] A sensory organ that detects the presence of certain chemicals, especially when a liquid is actively sniffed; mediates the effects of some pheromones.
Accessory olfactory bulb A neural structure located in the main olfactory bulb that receives information from the vomeronasal organ.
Medial nucleus of the amygdala [a mig da la] A nucleus that receives olfactory information from the olfactory bulb and accessory olfactory bulb; involved in the effects of odors and pheromones on reproductive behavior.
Human Sexual Behavior
Activational Effects of Sex Hormones in Men
Sexual Orientation and the Brain
Heredity and Sexual Orientation
Neural Control Of Sexual Behavior
Maternal Behavior of Rodents
Stimuli That Elicit and Maintain Maternal Behavior
Hormonal Control of Maternal Behavior
Neural Control of Maternal Behavior
Neural Control of Paternal Behavior
Many species must care for their offspring. Among most rodents, this duty falls to the mother, who must build a nest, deliver her own pups, clean them, keep them warm, nurse them, and retrieve them if they are moved out of the nest. They must even induce their pups’ urination and defecation, and their ingestion of the urine recycles water, which is often a scarce commodity.
Exposure to young pups stimulates maternal behavior within a few days. Apparently, the odor of the pups elicits handling and licking, whereas the sound of their distress calls elicits nest building. Unsensitized virgin female rats appeared to be repelled by the odor of the pups, but temporary deactivation of the olfactory system with zinc sulfate abolishes this aversion and causes the animals to begin caring for pups more quickly. The inhibitory affect of the odor of pups may be mediated by the accessory components of the olfactory system; cutting the vomeronasal nerve facilitates maternal behvior. Both components of the olfactory system project to the medial amygdala. Lesions of the medial amygdala or the stria terminalis also facilitate maternal responsiveness. Therefor, the inhibitory affects of olfaction on maternal behavior may be mediated by the pathway from the olfactory system to the medial amygdala to the medial preoptic area, via the stria terminalis.
Nest building appears to be facilitated by progesterone during pregnancy and by prolactin during the lactation period. Injections of progesterone and estradiol that duplicate the sequence that occurs during pregnancy facilitate maternal behavior, as does injections of prolactin directly into the brain.
The medial preoptic area is the most important forebrain structure for maternal behavior, and the ventral tegmental area of the midbrain is the most important brain stem structure. Neurons in the medial preoptic area send axons caudally through the ventral tegmental area toward more caudal regions of the brain stem. If these connections are interrupted bilaterally, rats cease providing maternal care.
Paternal behavior is relatively rare in mammalian species, but research indicates that sexual dimorphism of the MPA is less pronounced in monogamous species of voles (prairie voles), in which both the male and the female care fore the offspring. When male prairie voles are exposed to pups, the activity of the MPA (as measured by Fos production) is increased. In addition, lesions of the MPA disrupt paternal behavior of male rats.
Hormonal Control of Sexual Behavior
Neural Control of Sexual Behavior
9. Maternal behavior is influenced by hormones—primarily estradiol and prolactin—but depends also on the stimuli provided by the female’s offspring. Some of the neural circuitry that controls the behaviors involves pathways from the medial preoptic area to the ventral tegmental area. The medial preoptic area also appears to be involved in paternal behavior.