Chapter 46 Animal Reproduction Introduction 46 1 BOTH ASEXUAL AND SEXUAL REPRODUCTION OCCUR IN THE ANIMAL KINGDOM There are 2 modes of animal reproduction sexual and asexual Sexual Reproduction occurs with the fusion of haploid gametes which form a diploid Zygote The Animal that develops from a zygote can mature to produce gametes by mitosis Eggs female large and non motile are fertilized by sperm male small motile Asexual Reproduction occurs in which individuals are produced via mitotic division and not through fusion of egg and sperm MECHANISMS OF ASEXUAL REPRODUCTION There are several simple forms of asexual reproduction only found in invertebrates they are Budding New individuals arise from outgrowths of existing ones Fission Separation of a parent organism into 2 offspring Fragmentation Breaking of the body into several pieces followed by Regeneration regrowth of lost body parts Parthenogenesis Egg develops without being fertilized Common in bees wasps ants Haploids are male drones while diploid adults are formed from fertilized eggs SEXUAL REPRODUCTION AN EVOLUTIONARY ENIGMA Sexual reproduction occurs to enhance reproductive success or survival However there is a two fold cost to sexual reproduction However many species and organisms find it advantageous The advantage of Sexual Reproduction is argued to be an increase through genetic variation through Meiotic recombination The new genetic combinations that arise through sexual reproduction can speed up adaptation REPRODUCTIVE CYCLES Most animals exhibit cycles in reproductive activity and this is usually related to changing seasons which trigger the release or inhibition of hormones The changing environmental cues allows the organisms to determine when it is best to reproduce and allows it to conserve energy Seasonal Cues often are the best indicator but this is being disrupted due to climate change Animals that alternate between asexual and sexual reproduction also use reproductive cycles Asexual Reproduction occurring in stable environments while Sexual Reproduction occurs in unstable environments VARIATION IN PATTERNS OF SEXUAL REPRODUCTION Species that members who find it difficult to find mates have evolved and adapted hermaphroditism which is one organism containing both reproductive systems and this allows any 2 members of the population to mate 46 2 FERTILZATION DEPENDS ON MECHANISMS THAT BRING TOGETHER SPERM AND EGGS OF THE SAME SPECIES The union of sperm and egg FERTILIZATION can be internal or external External fertilization involves the release of eggs into the environment while Internal fertilization occurs when sperm is deposited in or near the female reproductive tract Moist Habitats makes External Fertilization advantageous However External fertilization requires synchronization One form of external fertilization is SPAWNING which is when individuals from the same population release gametes at the same time into the water and this is triggered by chemical signals that one generates due to the environment INTERNAL FERTILIZATION IS AN ADAPTATION THAN ENABLES SPERM TO REACH AN EGG EVEN WHEN THE ENVIRONMENT IS DRY Animals may use pheromones chemicals to attract mates ENSURING THE SURVIVAL OF OFFSPRING Internal fertilization is typically associated with the production of fewer gametes than external fertilization but it is usually more successful mainly due to the fact that internally fertilized zygotes have the protection of their parents ANIMALS THAT ARE INTERNALLY FERTILIZED TEND TO RECIEVE MORE CARE FROM THEIR OFFSPRING GAMETE PRODICTION AND DELIVERY Sexual reproduction relies on cells that are precursors for egg and sperm and remain dormant during the formation of the embryo However when activated they become amplified and make the number of cells available for making eggs or sperm more GONADS organs that produce gametes Some organisms have more elaborate reproductive systems that have accessory tubes and glands o carry nourish and protect gametes and sometimes embryos Many insect species have females that have SPERMATHECAE sperm storage sites that allow for the fertilization to occur only at the right place and the right time Many non mammalian vertebrates have a CLOACA a single opening to the digestive urinary and reproductive tract 46 3 REPRODUCTIVE ORGANS SPRODUCE AND TRANSPORT GAMETES HUMAN MALE REPRODUCTIVE ANATOMY The human male s external reproductive organs are the scrotum and penis The internal reproductive organs consist of GONADS which produce hormones and sperm Testes Testes are the male gonads that produce sperm in highly coiled tubes called SEMINFEROUS TUBES The Scrotum ensures a stable environment for the Testes to produce sperm by maintain a temperature about 2 degrees C below Normal Body temperature Ducts From the Seminferous tubles the Sperm pass into the EPIDIDYMIS During ejaculation sperm is propelled through the VAS DEFERENS that extends behind the urinary bladder to join a duct from the seminal vesicle and the duct final opens into the urethra outlet Accessory Glands The SEMINAL VESICLES PROSTATE GLAND AND BULBOURETHRAL GLANDS produce secretions to combine with sperm to form semen fluid The prostate gland secrets its products directly into the urethra through small ducts Bulbourethral Glands secrete mucous to neutralize acidic urine in the urethra in order to preserve the semen Penis The Human penis contains the urethra and 3 cylinders of spongy erectile tissue During arousal the issue fills with blood from the arteries and as this increases pressure which seals off the veins and make it possible to maintain an erection Some animals have bones to stiffen the penis for mating raccoons walruses whales The Glans forms a thick head and the prepuce foreskin covers the head HUMAN FEMALE REPRODUCTIVE ANATOMY The external reproductive structures are the clitoris and 2 sets of labia which surround the clitoris and vaginal opening The internal organs consist of gonads which produce eggs and hormones Ovaries The female gonads are a pair of ovaries that flank the uterus Each ovary is packed with follicles each consisting of an oocyte partially developed eggs The surrounding cells provide support and nourishment Oviducts and Uterus An oviduct is the fallopian tube and extends from the uterus to each ovary Cilia in the oviduct help collect the egg and with wavelike contractions the egg is pushed down to the uterus womb The inner lining of the uterus the ENDOMETRIUM is richly supplied with blood