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Biology LibreTexts

Animal Reproduction

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  • Asexual Reproduction

    Asexual reproduction involves one parent. As a result, the offspring tend to have the same genotype and phenotype; they lack variation.

    It is advantageous in a constant environment because a large number of offspring can be produced in a short time.


    Flatworms can divide into 2 halves; each half grows into a separate organism.

    Cnidarians undergo budding where a new individual grows from and then breaks off of the parent individual.

    Echinoderms can be cut to form new individuals.

    Insects- parthenogenesis (unfertilized egg develops)

    Sexual Reproduction

    Sexual reproduction involves the fusion of two gametes.

    Sperm and eggs are sometimes produced by the same individual and sometimes produced by different individuals.

    The sexes of vertebrates are separate but some vertebrates can change sex and others are both male and female.


    Sexual reproduction promotes variation because offspring inherit genes from two different parents.

    In addition, when an individual produces gametes, crossing-over and independent assortment mix genes from that individuals parents. Gametes are haploid; they contain some genes from the individuals mother and some from the father.

    Variation is advantageous to species in fluctuating environments.

    Reproductive cycles

    Reproductive cycles in many vertebrates and invertebrates are related to changes in day length.

    Day length is a reliable indicator of season for timing circannual events such as reproductive behavior and migration.

    In many species, the pineal gland secretes melatonin in the dark. Increasing day length (decreased melatonin levels) triggers reproductive behavior.


    External fertilization

    During external fertilization, many gametes are released into the water by each sex at the same time and place.

    This type of fertilization requires water because animal sperm must swim to the eggs. Water also protects the gametes from drying out.

    Species which have external fertilization are either aquatic or return to water for reproduction.

    Internal fertilization

    Internal fertilization is practiced by species that lay shelled eggs or have a period of internal embryonic development.

    This type of fertilization enables animals to reproduce in a terrestrial environment because it enables sperm to swim to the egg and it prevents gametes from drying out.

    The male often has a copulatory organ (a penis) for transferring sperm.


    The penis evolved independently in insects. It is found in all mammals.

    Males of some aquatic animals (ex: sharks, skates, rays) have specialized pelvic fins which allow the sperm to be passed to the female.

    Some reptiles and birds have a penis but most have a single opening for the reproductive, excretory, and digestive systems called a cloaca.

    Male Reproductive System

    Male Reproductive Structures

    Sperm are produced in the seminiferous tubules of the testes but mature in the epididymides (sing. epididymis). They are stored in the epididymides and vas deferens.

    The penis contains 3 masses of spongy erectile tissue with distensible blood spaces that produce erection when blood flow in the veins is inhibited. The spongy erectile tissues become engorged with blood. Stimulation is provided by the parasympathetic nervous system.

    An orgasm occurs as rhythmic muscular contractions compress the urethra and expel the semen. This expulsion is termed ejaculation.

    Fluids Added to Form Semen

    The prostate gland produces a secretion that buffers the vaginal pH, which is normally 3.5 to 4.

    The seminal vesicles add fructose to nourish the sperm and prostaglandins to promote contractions in the female.

    The bulbourethral gland (cowpers gland) produces mucus which lubricates the penis.

    Testes- human

    The testes are kept slightly cooler than body temperature because they are suspended outside the body within the scrotum.

    Each testis contains 250 to 300 lobules, and each of these contains one to three tightly coiled seminiferous tubules. Sperm are produced within the seminiferous tubules.

    The Leydig cells, located between the tubules, function to secrete testosterone.

    Sertoli cells are located within the tubules. They support, nourish and regulate (stimulate) cells that form sperm.


    Testosterone is secreted by Leydig cells in the testes. These cells are located outside the seminiferous tubules.

    Testosterone is also necessary for the development and function of the male reproductive tract.

    The formation of sperm (called spermatogenesis) is stimulated by testosterone.

    It promotes the development and maintenance of sexual behavior.

    It stimulates the development of secondary sexual characteristics such as beard growth, deepening of the voice, etc. and has growth-promoting effects.

    Negative Feedback Regulation of Sex Hormones

    Testosterone inhibits the hypothalamus and anterior pituitary. Inhibin, secreted by Sertoli cells also inhibits the hypothalamus and anterior pituitary.


    Sperm are produced in the seminiferous tubules in the testes. After meiosis is complete, sperm move from the lumen of the seminiferous tubule to the epididymis, where they become motile.

    Sperm production starts at puberty.

    Spermatogenesis is a continuous process. An average male produces approximately 1,000 sperm/second (30 billion/year).

    Each ejaculation should contain 200-300 million sperm.

    The testes are located outside the body because spermatogenesis requires a temperature cooler than the body core.


    The head of a sperm contains the nucleus and acrosome. The acrosome contains enzymes that play a role in breaking down the barrier surrounding the egg.

    The middle piece contains mitochondria needed to generate ATP necessary to operate the flagella.

    The tail contains microtubules arranged in the 9+2 arrangement typical of eukaryotic cells.

    Female Reproductive System

    Female Reproductive Structures

    Gametes are produced by ovaries. The ovaries are suspended in the abdominal cavity by ligaments and mesenteries.

    Oviducts serve as passageways from ovaries to uterus. They are not connected to the ovaries.

    The uterus lies in a nearly horizontally position on top of the bladder. The myometrium is a muscle layer; the endometrium is the lining. The outer layer of endometrium is shed during menstruation.

    The cervix is muscular sphincter between uterus and vagina.

    The vagina is amuscular structure that functions to receive the penis and also as the birth canal. It opens between urethra and anus. Bacteria in the vagina convert glycogen (a carbohydrate) to lactic acid, making it slightly acidic.

    The clitoris is homologous with the glans penis in males; it contains erectile tissue and a glans.

    Labia are folds of skin near the opening of the vagina. labia majora are fleshy and covered with pubic hair. They are homologous with the scrotum in the male. Labia minora are homologous with the shaft of the penis in male.

    The external genitalia of females are the vulva.

    Oogenesis: Formation of Egg Cells

    Primary oocytes begin to form in females before they are born (during the 3rd month of embryonic development). Development stops before the first meiotic division is completed. Females are born with all the primary oocytes that they will ever possess.

    The first meiotic division is completed at about the time of ovulation; secondary oocytes are therefore released at ovulation, not eggs.

    The second meiotic division occurs after fertilization.

    An infant may have as many as 2 million primary oocytes.

    Approximately 300,000-400,000 primary oocytes remain at time of puberty; 400 mature over the reproductive years.

    Ovarian Follicle

    The oocyte and cells surrounding it are the follicle.

    The follicle moves to the surface and erupts, releasing the oocyte.

    After ovulation, the follicle develops into a corpus luteum, which lasts only about 10 days if the secondary oocyte is not fertilized.

    The secondary oocyte is swept into the oviduct by beating cilia and movement of the oviduct. Cilia and peristalsis move the oocyte through the oviduct.

    Fertilization occurs in oviduct.

    The secondary oocyte must be fertilized within 36 hours. Sperm survive in the female reproductive tract for approximately 48 hours. It takes 3 days to reach uterus.

    Below: Rabbit ovary X 40

    The Role of Hormones in Females

    Sex hormones produce secondary sexual characteristics.

    Most female mammals come into "heat" or estrus and mating occurs at this time.

    Humans and other primates have a menstrual cycle; there is no period of estrus nor is there a mating season.

    During the menstrual cycle, the uterine lining thickens to prepare for pregnancy but is sloughed off if pregnancy does not occur. These events repeat themselves approximately every twenty-eight days.

    Menstrual Cycle

    The events leading up to ovulation are called the follicular phase. During this phase of the menstrual cycle, FSH promotes the development of a follicle. The follicle secretes estrogen.

    Estrogen promotes thickening of the uterine lining and inhibits the anterior pituitary from secreting FSH.

    As the follicle grows larger it secretes larger amounts of estrogen. Although estrogen inhibits the anterior pituitary, high levels stimulate the hypothalamus.

    The hypothalamus responds to stimulation by releasing GnRH. High levels of GnRH stimulate the anterior pituitary to produce an "LH surge" (large amount of LH).

    The LH surge triggers ovulation.

    The luteal phase occurs after ovulation and is associated with the maintenance of the uterus. LH causes the follicle cells to become the corpus luteum, a gland which secretes estrogen and progesterone.

    Progesterone promotes the development of the uterine lining and it inhibits the anterior pituitary from secreting LH.

    The corpus luteum degenerates because the level of LH has decreased. It can no longer produce estrogen and progesterone. Progesterone is necessary for the maintenance of the uterine lining.

    Without it, the lining sloughs off, producing menstrual flow. The cycle ends as it began, with low levels of hormones; the hypothalamus and pituitary are not inhibited.

    If Pregnancy Occurs:

    The placenta produces HCG (human chorionic gonadotropin) which maintains the corpus luteum until the placenta can secrete its own estrogen and progesterone. Estrogen and progesterone continue to shut down the anterior pituitary; no new follicles are produced and the cycle stops.

    As the placenta develops, it begins to secrete its own estrogen and progesterone to maintain the lining of the uterus. The corpus luteum is no longer needed to produce these hormones.


    Around age 45-50, the ovaries gradually stop responding to pituitary hormones (FSH, LH).

    As a result, follicle production stops and the menstrual cycle stops.