Animal Reproduction Chapter 46

Asexual Reproduction Can be found in invertebrates. Fission: parent separates into two or more individuals. Budding: new individual splits off from an existing one. Fragmentation/Regeneration: body breaks into several pieces, which develop into a complete adult. Advantage is that animals do not need a mate, and offspring are produced in a short amount of time.

Reproductive Cycles and Patterns Most animals have reproductive cycles; conserves resources and favors offspring survival. Cycles are controlled by a combination of hormonal and environmental (temperature, day length, lunar cycle) cues. In animals that can reproduce sexually or asexually, asexual reproduction often occurs under favorable conditions and sexual reproduction during times of environmental stress.

Hormonal Control in Humans In males, androgens are responsible for sex characteristics (development of reproductive organs, deepening of voice, hair growth, aggressive behavior, etc). LH -- lutenizing hormone; goes to testes to stimulate testosterone production. FSH – follicle-stimulating hormone; travels to testes to increase sperm production. Negative feedback loop keeps hormone levels constant in humans (monitored by hypothalamus).

Hormonal Control in Humans (cont) Female primates have a menstrual cycle. Endometrium (uterine lining) thickens prior to ovulation (release of egg from the ovary). If pregnancy does not occur, the endometrium is shed through the vagina (menstruation). Usually ranges from 20 - 40 days with an average of 28 days; Ovulation tends to occur at 14th day.

Menstrual Cycle Three phases of the menstrual cycle: 1. Menstrual flow phase (Day 1-4) -- most of the endometrium is lost from the uterus. 2. Proliferative/ovulatory phase (Day 5-15) – endometrium thickens and vascularizes (increased blood flow). a. Pituitary gland releases more FSH; causes ova to develop in the ovary, forming follicles; one developed follicle secretes estrogen. b. Increased estrogen levels inhibits FSH in the pituitary; more LH is produced, which causes follicle to rupture; Ovum releases from the ovary to travel down the oviduct.

Menstrual Cycle (cont) 3. Secretory/Luteal phase (Day 16-28) – endometrium readied for pregnancy, then prepares to shed. a. Ruptured follicle in ovary develops into the corpus luteum, secretes estrogen and progesterone (prepares uterus for implantation) until Day 20. If embryo settles into endometrium, hormones will maintain the uterine lining. b. Otherwise, hypothalamus cuts LH production; corpus luteum breaks down; estrogen and progesterone levels decrease. c. Reduced blood flow to the endometrium causes it to break down and pass out of the uterus. When estrogen drops, FSH will start to increase, and next cycle will start.

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