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Reproduction

Reproduction. The Mammalian Strategy : Relatively few intrauterine young (higher survival rate) Nourish neonates with milk (high survival early; bonding) Young remains with mother (or parents) at minimum until weaned (parental protection; learned behaviors) . Reproduction.

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Reproduction

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  1. Reproduction The Mammalian Strategy: • Relatively few intrauterine young (higher survival rate) • Nourish neonates with milk (high survival early; bonding) • Young remains with mother (or parents) at minimum until weaned (parental protection; learned behaviors)

  2. Reproduction The Mammalian Strategy: • Amount of energy invested per young is lower than non-mammals; • Relatively few young produced but most survive to potentially reproduce

  3. Costs of Lactation

  4. Tradeoffs in Litter Size

  5. Reproductive Endocrinology“Crash Course” * Feedback mechanisms (environmental stimuli; hormone secretions)

  6. Reproductive Endocrinology“Crash Course” Ovarian Cycle Influenced by: • Follicle stimulating hormone (FSH) and luteinizing hormone (LH) secreted by pituitary • follicle growth which triggers ovary to secrete estrogen

  7. Reproductive Endocrinology“Crash Course” Ovarian Cycle Influenced by: 2) Estrogen secretion feeds-back to hypothalamus-pituitary; more LH secreted & less FSH • Ovulation & corpus luteum formation (spongy body which forms in place of ruptured follicle) • Corpus luteum secretes progesterone for uterine wall preparation

  8. Reproductive Endocrinology“Crash Course” Ovarian Cycle Influenced by: 3) No fertilization • Corpus luteum recedes to Corpus albicans • Progesterone & estrogen level drop • Begin again in cycle

  9. Reproductive Endocrinology“Crash Course” Ovarian Cycle Influenced by: 3) If fertilization occurs… • Corpus luteum continues to produce progesterone for maintaining pregnancy • Placenta soon assumes estrogen & progesterone secretion

  10. amnion chorion embryo allantois

  11. Four Major Parts of Embryonic Membranes • yolksac: part of primitive intestine lying external to embryo; forms from endoderm • No nutritional value • Portion of placenta in some cases (e.g., marsupials)

  12. Four Major Parts of Embryonic Membranes 2) amnion: forms from ectoderm & mesoderm around the embryo • Filled with serous fluid = prevent dessication/shock 3) allantois: out-pocket from hindgut of embryo • Movement of nutrients & O2 • Forms blood vessels for placenta

  13. Four Major Parts of Embryonic Membranes 4) chorion: outer embryonic layer (ectoderm); envelopes entire assemblage • villi • contact with uterine wall placenta: includes embryonic membranes & lining of uterine wall (endometerium)

  14. Types of Placenta • Placenta types based on villi distribution on chorion: • diffuse: villi scattered over entire surface of chorion = increased SA for absorption e.g., lemurs, perissodactyls, some artiodactyls 2) polycotyledonary: islands of villi scattered over chorion e.g., other artiodactyls such as bovids

  15. Types of Placenta • Placenta types based on villi distribution on chorion: 3) zonary: band of villi encircle center of blastocyst; lacking villi elsewhere e.g., carnivores 4) discoidal: regional restriction of villi e.g., most mammals

  16. discoidal zonary diffuse

  17. Types of Placenta B) Placenta type based on connection between villi & endometrium: • nondeciduate: loose fitting of villi with endometrium; villi pull free without disrupting endometrium during parturition (whales, ungulates) 2) deciduate: close fitting of villi-endometrium; villi pull free & cause erosion of endometrium during parturition (rodents, carnivores)

  18. Types of Placenta C) Placenta type based on degree of intimacy between embryonic & maternal parts: 1) choriovitelline: blastocyst lies in endometrium depression; does not embed 2) chorioallantoic: villi; blastocyst rests against endometrium at allantois-chorion contact point

  19. Types of Placenta C) Chorioallantoic Placenta Types: 1) epitheliochorial – lemurs, cetaceans, equids, suids - epithelial cells of chorion in contact with epithelial cells of uterus; villi in pockets in endometrium 2) syndesmochorial – artiodactyls - lacking uterine epithelial barrier; contact uterine tissue

  20. Types of Placenta C) Chorioallantoic Placenta Types: 3) endotheliochorial – carnivores - epithelial cells of chorion in contact lining of uterine capillaries 4) hemochorial – insectivores, bats, higher primates - villi in direct contact with maternal blood

  21. Types of Placenta C) Chorioallantoic Placenta Types: 5) hemoendothelial – lagomorphs, some rodents - lining of villi blood vessels only barrier to maternal blood

  22. Reproductive Patterns • Continuous embryonic development (“typical”) a) ova fertilized in oviduct b) zygote begins mitosis - descends towards uterus c) zygote reaches uterus – mitosis ongoing – reaches blastocyst stage as implanting into endometrium d) placental connection: uterus to embryo e) continual development until parturition

  23. Reproductive Physiology - Implantation of embryo in uterine wall for varying lengths of time - Embryo supplied with nutrients via the placenta

  24. Reproductive Patterns 2) Deviations from contiuous development strategy: a) Delayed Fertilization: ovulation & fertilization delayed until an extended time after copulation • Viable sperm retained in female • Ovulation occurs ~months after copulation • Common to many temperate bats (vespertilionids)

  25. Reproductive Patterns Example 2) Deviations from contiuous development strategy: a) Delayed Fertilization:

  26. Reproductive Patterns 2) Deviations from contiuous development strategy: b) Delayed Development: blastocyst embeds into endometrium & then becomes dormant; development delayed (e.g., bats)

  27. Reproductive Patterns Example 2) Deviations from contiuous development strategy: b) Delayed Development:

  28. Reproductive Patterns 2) Deviations from contiuous development strategy: c) Delayed Implantation: obligate & facultative examples e.g., weasels, seals, bears • Blastocyst forms but does not embed & ceases to develop • Floating blastocyst remains dormant 2 weeks to 1 year

  29. Reproductive Patterns 2) Deviations from contiuous development strategy: c) Delayed Implantation: e.g., Mustelaerminea (avg age at death = 1.5 to 2 yrs) *gestation period = 9-10 months

  30. Reproductive Patterns Mustelanivalis Delayed Implantation???? * NO (avg age at death = <1 yrs) * gestation period = 35-37 days • 2 litter per year possible • Relation to vole cycles

  31. Types of Breeding Seasons 1) Continuous – year round breeding; no seasonality; common to tropics 2) Restricted a) Regular – seasonal breeding; temperate regions b) Irregular – discontiuous breeding during rainfall, etc… desert/arid regions Optimal timing for: * mating (time with best availability of mates) * birth (time with abundant resources

  32. Gestation Period Mating Birthing Resources Fall Winter Spring Summer Seasonality to Mating & Parturition based on resource availability (i.e, mates or food)

  33. Mating Birthing Resources Fall Winter Spring Summer Body size relation to length of gestation period….What if mammal could “extend” the gestation period to birth in a more favorable time and/or insure mating opportunities? (e.g., weasels) Gestation Period Delay Major Development

  34. Reproduction Sexual Maturity (puberty) – age when capable of producing gametes influence onset/cessation (restricted) *environmental factors efficiency of reproduction (continuous)

  35. Influences on Puberty & Reproduction • Light (photoperiod) Rattusnorvegicus (continuous breeder) • normal light • continuous light = 6 days earlier than normal (FSH) • Constant dark = 16 days later than cont. light

  36. Influences on Puberty & Reproduction Results…. • #1-4 = reached puberty • >60% females = pregnant • Control = no reproduction/puberty *Light (photoperiod) linked to reproductive development • Light (photoperiod) Microtusarvalis (seasonal breeder) • breeds 21 Mar – 24 Jun • simulate photoperiod during (22 Sep – Dec) • Natural light • Artificial light • Uniform 16-h daylength • Uniform 8-h daylength until Nov, then 13-h day • Control (“out of season”)

  37. Influences on Puberty & Reproduction 2) Temperature rodents **Growth rates lowered due indirectly to low temps. Thus, results directly in delayed puberty

  38. Influences on Puberty & Reproduction • Nutrition – under-nutrition delays puberty in both females and males • Precipitation – deer in Texas (Knowlton) - “high” rainfall lead to shorter breeding season, more synchronous breeding & fawning - lower predation rates (functional response of coyotes) # prey consumed Prey density

  39. Influences on Puberty & Reproduction Whitten Effect: synchronized estrus cycles when male introduced into population of females Bruce Effect: implantation blocked, pregnancy aborted if females exposed to strange, new male * Male urine stimulates FSH & LH secretion (pheromones) • Social Effects/Density (examples from captive mice) Lee-Boot Effect: pseudo-pregnancy induced among crowded females; may go anestrus

  40. Readings • Reproductive Cycles & Life-History Strategies, pp. 354-356 • Litter Size & Reproductive “Seasons”, pp. 356-357 • Lactation and Postnatal Growth, pp. 359-363

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