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Sex: Evolutionary, Hormonal, and Neural Bases. 12 Sex: Evolutionary, Hormonal, and Neural Bases. Sexual Behavior : Reproductive Behavior Can Be Divided into Four Stages The Neural Circuitry of the Brain Regulates Reproductive Behavior Pheromones Guide Reproductive Behavior in Many Species.
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12 Sex: Evolutionary, Hormonal, and Neural Bases Sexual Behavior: • Reproductive Behavior Can Be Divided into Four Stages • The Neural Circuitry of the Brain Regulates Reproductive Behavior • Pheromones Guide Reproductive Behavior in Many Species
12 Sex: Evolutionary, Hormonal, and Neural Bases Sexual Behavior: • The Hallmark of Human Sexual Behavior Is Diversity • An causal study by the use of animal to investigate the relationship of Brain & Behavior • Many Vertebrates Depend on Their Parents for Survival
12 Sex: Evolutionary, Hormonal, and Neural Bases Sexual Differentiation: • The Sex of an Individual is Determined Early in Life • How Should We Define Gender – by Genes, Gonads, Genitals, or the Brain? • Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior
12 Sex: Evolutionary, Hormonal, and Neural Bases Sexual Differentiation: • Social Influences Affect Sexual Differentiation of the Nervous System • Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood?
12 Reproductive Behavior Can Be Divided into Four Stages Four stages of reproductive behavior: • Sexual attraction • Appetitive behavior • Copulation • Postcopulatory behavior
12 Reproductive Behavior Can Be Divided into Four Stages Sexual attraction brings males and females together – may be synchronized with: • Physiological readiness to reproduce, indicated by odors reflecting estrogen levels in females • Learned associations, such as appearances
12 Reproductive Behavior Can Be Divided into Four Stages Appetitive behaviors establish, maintain, or promote sexual interaction. - appetitive (事前) vs. consumptive (當下) A proceptive female may approach males or perform “ear-wiggling.” Male behaviors include staying near the female, sniffing, singing, and nest-building.
12 Reproductive Behavior Can Be Divided into Four Stages Copulationinvolves one or more intromissions in which the male penis is inserted into the female vagina. - appetitive vs. consumptive Following stimulation the male ejaculates sperm-bearing semen into the female.
12 Reproductive Behavior Can Be Divided into Four Stages After first copulation a refractory phase follows – length varies with species. The Coolidge effect refers to the faster resumption of mating behavior, with a different partner. If a female is willing to copulate, she is sexually receptive, or in estrus.
12 Reproductive Behavior Can Be Divided into Four Stages Postcopulatory behavior varies across species. In a copulatory lock, occurring in dogs and some mice, the penis swells temporarily and cannot be withdrawn from the female. Postcopulatory behavior also includes extensive parenting of offspring.
12 Reproductive Behavior Can Be Divided into Four Stages Fertilization is the process of bringing gametes – sperm and ovum – together. A zygote is a fertilized ovum. External fertilization takes place outside the female’s body, as with fishes and frogs that release gametes into water.
12 Reproductive Behavior Can Be Divided into Four Stages Internal fertilization takes place inside the female’s body. Internal fertilization in birds occurs through the cloaca. Oviparous animals lay eggs, while viviparous animals have live young. Ovoviviparous animals carry young in eggs internally, until ready to hatch.
12 Reproductive Behavior Can Be Divided into Four Stages In rats, females ovulate, or release eggs, every 4–5 days.She displays proceptive behavior and adopts a posture called lordosis, allowing intromission. Pair bonds form between animals who live together before and long after copulation. (oxytocin)
12 Reproductive Behavior Can Be Divided into Four Stages Hormones are important in mating behaviors. A castrated male loses interest in mating as testosterone is no longer produced. Behavior is restored with testosterone treatment – the activational effect – hormones briefly activate behavior.
12 Reproductive Behavior Can Be Divided into Four Stages Estrogens produced at the beginning of the ovulatory cycle are important for female proceptive behavior. A female without ovaries will respond to a combination of estrogen and progesterone treatments.
12 The Neural Circuitry of the Brain Regulates Reproductive Behavior In female rats, the ventromedial hypothalamus (VMH) is crucial to the lordosis response through steroid actions. Estrogen increases dendritic trees of neurons in the VMH. Estrogen also stimulates progesterone receptors, which in turn contribute to lordosis through protein production.
12 The Neural Circuitry of the Brain Regulates Reproductive Behavior The VMH sends axons to the periaqueductal gray in the midbrain, which projects to the medullary reticular formation. This in turn projects to the spinal cord via the reticulospinal tract. As the male mounts, sensory information via the spinal cord evokes lordosis.
Figure 12.5 Neural Circuits for Reproduction in Rodents (Part 1)
12 The Neural Circuitry of the Brain Regulates Reproductive Behavior In male rats, the medial preoptic area (mPOA) coordinates male copulatory behavior. The mPOA sends axons to the ventral midbrain, then to the basal ganglia to coordinate mounting. Axons also project through brainstem nuclei to the spinal cord.
12 The Neural Circuitry of the Brain Regulates Reproductive Behavior One nucleus, the paragigantocellular nucleus (PGN)has fibers that normally inhibit the erection response – mPOA signals counteract the inhibition. The vomeronasal organ (VMO) detects chemicals called pheromones, which activate male arousal. VMO information is sent to the medial amygdala, and in turn to the mPOA.
Figure 12.5 Neural Circuits for Reproduction in Rodents (Part 2)
12 Pheromones Guide Reproductive Behavior in Many Species Hormones can signal readiness within an animal or provide information between animals. Examples: F prostaglandin in goldfish, pheromones in urine of mice and prairie voles Musth is a period of increased activity in male elephants – sexual status is marked by different pheromones.
12 The Hallmark of Human Sexual Behavior Is Diversity Humans differ from other species in reproductive behaviors. Humans can report subjective reactions, such as orgasm. The four phases of response are excitement, plateau, orgasm, and resolution – during the excitement phase, the phallus becomes erect.
12 The Hallmark of Human Sexual Behavior Is Diversity Men and women differ in sexual response: • Women have a greater variety of responses, with three typical patterns • Men, but not women, tend to have an absolute refractory phase following orgasm
12 The Hallmark of Human Sexual Behavior Is Diversity In double-blind tests, neither researcher nor subject knows which treatment is received. A low dose of androgens can stimulate interest and activity in males, and in some cases, in women.
12 The Hallmark of Human Sexual Behavior Is Diversity Pheromones may affect human reproductive behaviors - menstrual cycles of women living together may synchronize. The major histocompatibility complex (MHC) is a group of immune-related genes with many alleles. Women prefer the smell of men with MHCs not too similar to their own.
12 Many Vertebrates Depend on Their Parents for Survival Precocial animals are born with well-developed sensory and motor systems. Altricial animals are less developed at birth. Both males and females may be involved in care of the young.
12 Many Vertebrates Depend on Their Parents for Survival Rat mothers show four behaviors: nest building, crouching over pups, retrieving pups, and nursing. During pregnancy, exposure to hormones will prepare her brain to display these maternal behaviors immediately after giving birth.
12 Many Vertebrates Depend on Their Parents for Survival A parabiotic preparation shows the effects of maternal hormones. A nonpregnant female exposed to the circulating hormones of a pregnant rat will display the same maternal behaviors.
12 The Sex of an Individual Is Determined Early in Life Sex determination is the early developmental event that decides if an individual will be male or female. If the sperm that enters the egg has an Y chromosome, the offspring is male – if an X chromosome, the offspring is female.
12 The Sex of an Individual Is Determined Early in Life The early indifferent gonads begin to change into ovaries or testes in the first month. SRY gene– sex-determining region on the Y chromosome – is responsible for the development of testes Without an SRY gene, an ovary forms.
12 The Sex of an Individual Is Determined Early in Life Hormones secreted by gonads,mainly from the testes, direct sexual differentiation. Embryos have early tissues for both male and female structures. The wolffian ducts and the müllerian ducts connect the gonads to the body wall.
12 The Sex of an Individual Is Determined Early in Life In females, the müllerian ducts develop into the fallopian tubes, the uterus, and vagina – only part of the wolffian ducts remains. In males, the wolffian ducts develop into the epididymis, vas deferens, and seminal vesicles, and the müllerian ducts shrink.
12 The Sex of an Individual Is Determined Early in Life Two hormones from the testes make the system masculine: • Testosterone – promotes the development of the wolffian system • Anti-müllerian hormone (AMH) – induces the regression of the müllerian system
12 The Sex of an Individual Is Determined Early in Life Testosterone also masculinizes other structures: Other tissues form the prostate gland, scrotum, and penis. 5α-reductaseis an enzyme that converts testosterone into the more powerful dihydrotestosterone (DHT), necessary to form genitalia.
12 The Sex of an Individual Is Determined Early in Life In Turner’s syndrome, a person only has one sex chromosome – a single X. The individual develops as a female – without the SRY gene, no masculinizing effects take place.
