50.1 Animals employ both sexual and asexual reproductive strategies.
Sexual and Asexual Reproduction
• Sexual reproduction occurs when the gametes from different sexes unite. (p. 1062)
• Asexual reproduction can occur through fission, budding, or parthenogenesis. (p. 1062)
• Hermaphroditism occurs when one individual has both testes and ovaries, although most require another individual in order to reproduce. (p. 1062-1063)
• In mammals, the sex of an individual is determined early in embryonic development. (p. 1063)
50.2 The evolution of reproduction among the vertebrates has led to internalization of fertilization and development.
Fertilization and Development
• Most fish and amphibians engage in external fertilization, while most other vertebrates, including mammals, engage in internal fertilization. (p. 1064-1065)
• Vertebrates utilizing internal fertilization follow one of three strategies -- oviparity, ovoviviparity, or viviparity -- based on the degree of dependence of the developing embryo on the mother. (p. 1064)
Fish and Amphibians
• While most bony fish utilize external fertilization, most cartilaginous fish utilize internal fertilization. (p. 1065)
• Most amphibians utilize external fertilization and develop in water into a larval stage before metamorphosing into the terrestrial adult form. (p. 1065)
Reptiles and Birds
• Most reptiles and birds exhibit oviparity and lay amniotic eggs which contain a fluid-filled cavity surrounded by a membrane called the amnion. (p. 1066)
• Birds are homeothermic and thus must incubate eggs as well as watch after and protect hatchlings until they are developed enough to survive on their own. (p. 1066)
Mammals
• Some mammals are seasonal breeders, while others undergo more frequent reproductive (estrous) cycles; still others are induced ovulators. (p. 1067)
• In mammalian estrous cycles, changes in follicle-stimulating hormone (FSH) and luteinizing hormone (LH) cause changes in egg cell development and ovarian hormone secretions. (p. 1067)
• Mammals fall into one of three categories of reproduction: monotremes, marsupials, or placentals. (p. 1067)
50.3 Male and female reproductive systems are specialized for different functions.
Structure and Function of the Male Reproductive System
• In human males, the scrotum contains two testes; each testis contains seminiferous tubules, which are the sites of sperm production. (p. 1068)
• Semen, which is composed of sperm and fluid from the seminal vesicles and the prostate gland, is ejaculated from the penis following erection and sexual stimulation. (p. 1070)
• FSH stimulates Sertoli cells to facilitate sperm development, and LH stimulates Leydig cells to secrete testosterone. (p. 1071)
Structure and Function of the Female Reproductive System
• In human females, the ovaries contain ovarian follicles, which each contain an egg cell. (p. 1072)
• The human menstrual cycle lasts approximately one month and is divided into two phases (follicular and luteal) which are separated by ovulation. (p. 1073)
• During each menstrual cycle, ovarian follicles begin development due to FSH stimulation and eventually release one secondary oocyte (ovulation) under LH stimulation. (pp. 1073-1075)
• During the follicular and luteal phases of the menstrual cycle, ovaries release hormones to stimulate endometrium thickening, preparing it for the potential implantation of a fertilized egg. (p. 1073-1075)
Birth Control
• Major forms of birth control include abstinence, sperm blockage, sperm destruction, prevention of ovulation, prevention of embryo implantation, and sterilization. (pp. 1076-1078)