a. To reproduce, male and female animals must form pairs of the same species, of the correct age, where both are physiologically ready to reproduce.

b. As explained in Section 24.6, species are adapted to slightly different niches, so hybrids between them are usually not well adapted to either niche.

c. Animals have evolved mechanisms, many of them behavioral, that keep individuals of different species from mating.

d. The most primitive mechanisms for bringing the correct gametes together are chemical.

e. Sexual protists, algae, and fungi are best described as having mating types, rather than morphologically different sexes, and each type usually produces a sex pheromone that attracts gametes of the other type.

f. For example, large female gametes of the mold Allomyces produce the pheromone sirenin, which attracts the smaller, motile male gametes (Figure 51.1).

g. Sex pheromones in animals induce aggregation and complex behaviors.

h. For example, male and female insects find each other due to their sensitivity to tiny amounts of very volatile pheromones that diffuse over long distances.

i. Many species use pheromones to initiate complex sexual behaviors.

j. For example, male danaid butterflies (Figure 51.2) have scent scales on their hind wings and hairpencils on their abdomens, which they rub on females to induce them to alight and copulate.

a. In many species, simple chemical signals are not enough to induce mating, and complicated rituals that lead to pairing and mating have evolved.

b. Gary Neuchterlein and Deborah Bultron have described the ceremonies of the Western Greme, a North American water bird; see Figure 51.3.

c. By exchanging chemical and other signals, males and females of the same species recognize each other as acceptable mates.

a. Using humans as the model organism, vertebrate reproductive structures are reviewed in this section.

b. The human male reproductive system (Figure 51.4) is designed for delivering a mass of viable sperm to the vicinity of viable eggs.

c. Male sperm formation occurs in the seminiferous tubules in each testis, two of which are suspended outside of the body cavity in the scrotum.

d. The seminiferous tubules empty into collecting ducts that terminate in the epididymis on the surface of each testis; each epididymis leads into a vas deferens.

e. The vas deferentia run up the spermatic cord, through the body cavity around the urinary bladder, and join the seminal vesicles, which secrete about two thirds of the ejaculate in the form of amino acids, fructose, and mucus.

f. Prostaglandins in the seminal fluid stimulate contraction of the uterus and uterine tubes, helping to propel the sperm toward the oviducts.

g. The ducts of each seminal vesicle and vas deferens fuse into ejaculatory ducts that run through the prostate gland, which contributes about a third of the seminal fluid.

h. A common urethra, which carries both urine and sperm, runs through the penis, which is the embryonic homolog of the female clitoris.

i. The bulbourethral gland (Cowper's gland) just below the urethra produces a clear lubricating fluid upon sexual arousal.

j. The female reproductive system (Figure 51.5) produces ova, guides sperm toward fertilization, and protects and nourishes a developing embryo.

k. Eggs develop in two ovaries; one ovary releases an ovum into the abdominal cavity about once every 28 days.

l. A released ovum is guided into a funnel-like opening of the oviduct, or uterine tube or Fallopian tube, and cilia there sweep the ovum toward the uterus (womb), where an embryo normally develops.

m. Fertilization normally occurs in the upper part of the oviduct, though on rare occasions, a fertilized egg escapes into the abdominal cavity and implants on the peritoneum, causing an ectopic pregnancy.

n. The uterus opens through a narrow passage, the cervix, into the vagina, a muscular tube that leads to the outside of the body.

o. The introitus, or external opening of the vagina, is flanked by the labia minora and labia majora.

p. The clitoris lies anterior to the vaginal opening.

q. In virgin females, the introitus may be partially or completely covered by the hymen, a thin membrane that surrounds or bridges the introitus.

r. Vestibular glands (Bartholin's glands), homologs of the male Cowper's glands, open into the introitus, but they secrete relatively little lubricant compared with glands in the vaginal walls.

s. The female urethra opens between the clitoris and introitus, and is separate from the reproductive tubes.

t. A developing mammalian embryo begins with a set of indifferent genital structures, which can develop into either male or female genitals (Figure 51.6).

u. Femaleness is the body's default setting; genitals will always develop along the female pathway unless they are given male-specific chemical signals during specific times in development.

v. Under genetic and hormonal instructions, the primordia of the external genitalia take one developmental path or the other; the internal reproductive organs develop from another set of indifferent primordia.

w. Sidebar 51.1 describes variation in genital structures among mammals.

a. Until about 1970, the physiology of human sexual arousal and intercourse had not been examined in detail.

b. In this section, copulation in mice is examined and described as a set of fixed-action patterns (Figure 51.7).

c. A female mouse ready to mate will assume a posture called lordosis, with her hindquarters raised and her tail to one side.

d. A male attempting to mate will then go through a series of mounting movements that result in intromission–the insertion of his penis into her vagina–followed by slow and deep pelvic thrusts.

e. This series of actions is repeated several times before the male finally ejaculates.

f. T. E. McGill studied copulation in mice and found that each strain has a characteristic copulatory sequence, with considerable variation in the number of mounts and timing of ejaculation.

g. By studying hybrid strains, McGill found that some features of the behavior are inherited in a simple way and determined by single genes, but that most features are complex.

a. The work of Alfred Kinsey, William Masters, and Virginia Johnson, among others, has opened human sexuality to scientific study.

b. Masters and Johnson concluded that men and women have a common sexual arousal cycle that can be divided into four phases: excitement, plateau, orgasm, and resolution (Figure 51.8).

c. A variety of stimuli, especially visual ones, can induce sexual excitement in humans.

d. Humans produce sex pheromones that do not have odors and other scents that may excite others sexually.

e. Tactile (touch) stimuli are also important in human sexual arousal.

f. In men, the excitement phase is indicated by erection of the penis, which becomes engorged with blood (Figure 51.9).

g. In women, the excitement phase is marked by secretion of a lubricating fluid from the vaginal walls, erection of the nipples, and engorgement and swelling of the clitoris and surrounding tissues.

h. During the plateau phase, both the male and female genitals become more swollen and engorged with blood.

i. The orgasm phase entails a release of tension that is concentrated in regular contractions of the perineal muscles between the legs just below the external genitals.

j. In mature males, orgasm is usually accompanied by ejaculation, though the two events are controlled by different muscles and may occur separately in juveniles.

k. During the resolution phase, blood drains from the engorged tissues and they return to their normal size.

l. Sexual stimuli activate certain pleasure centers in the mammalian brain; this probably evolved as a mechanism for rewarding reproductive behavior, though masturbation (self-stimulation) can result in more intense orgasms for many people.

51.6 Gonads produce steroid sex hormones under control of the anterior pituitary gland.

51.7 The gonadotropins stimulate spermatogenesis and androgen production in males.

51.8 An interplay of hormones controls the female reproductive cycle.

51.9 Other mammals ovulate on different schedules from those of humans.

51.10 A mammalian embryo is attached to its mother through a placenta.

51.11 New hormonal pathways become active during pregnancy.