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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)
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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
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Costs of Lactation
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Tradeoffs in Litter Size
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Reproductive Endocrinology “Crash Course” * Feedback mechanisms (environmental stimuli; hormone secretions)
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Reproductive Endocrinology “Crash Course” Ovarian Cycle Influenced by: 1)Follicle stimulating hormone (FSH) and luteinizing hormone (LH) secreted by pituitary follicle growth which triggers ovary to secrete estrogen
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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
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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
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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
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allantois chorion embryo amnion
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Four Major Parts of Embryonic Membranes 1)yolk sac: part of primitive intestine lying external to embryo; forms from endoderm No nutritional value Portion of placenta in some cases (e.g., marsupials)
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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 & O 2 Forms blood vessels for placenta
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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)
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Types of Placenta A)Placenta types based on villi distribution on chorion: 1)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
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Types of Placenta A)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
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discoidal zonary diffuse
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Types of Placenta B) Placenta type based on connection between villi & endometrium: 1)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)
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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
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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
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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
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Types of Placenta C) Chorioallantoic Placenta Types: 5) hemoendothelial – lagomorphs, some rodents - lining of villi blood vessels only barrier to maternal blood
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Reproductive Patterns 1)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
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Reproductive Physiology - Implantation of embryo in uterine wall for varying lengths of time - Embryo supplied with nutrients via the placenta
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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)
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Reproductive Patterns Fallcopulation WinterSperm storage Early springovulation Spring-summerEmbryo develops after fertilization 2) Deviations from contiuous development strategy: a) Delayed Fertilization: Example
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Reproductive Patterns 2) Deviations from contiuous development strategy: b) Delayed Development: blastocyst embeds into endometrium & then becomes dormant; development delayed (e.g., bats)
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Reproductive Patterns 2) Deviations from contiuous development strategy: b) Delayed Development: Late summerBlastocyst forms Summer-FallBlastocyst dormant Late fall- early winter Development begins Early springparturition Example
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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
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Reproductive Patterns Summer (Jun-Jul) 2004 Mating March 2005Implantation (8-9 mo delay) Spring (Apr-May) 2005 Parturition Summer (Jun-Jul) 2005 Mating (including 2005 females 2) Deviations from contiuous development strategy: c) Delayed Implantation: e.g., Mustela erminea (avg age at death = 1.5 to 2 yrs) *gestation period = 9-10 months
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Reproductive Patterns Spring-Summer (Apr-May) 2004 Mating Spring-Summer (May-Jun) 2004 Parturition Summer (Jul-Aug) 2004 Mating? Sexually Mature 2004 Females Summer-Fall Aug-Sep 2004 Parturition (2 nd litter) Mustela nivalis Delayed Implantation???? * NO (avg age at death = <1 yrs) * gestation period = 35- 37 days 2 litter per year possible Relation to vole cycles
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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
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Seasonality to Mating & Parturition based on resource availability (i.e, mates or food) FallWinterSpringSummer MatingBirthing Resources Gestation Period
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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) FallWinterSpringSummer MatingBirthing Resources Gestation Period DelayMajor Development
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Reproduction Sexual Maturity (puberty) – age when capable of producing gametes influence onset/cessation (restricted) *environmental factors efficiency of reproduction (continuous)
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Influences on Puberty & Reproduction 1)Light (photoperiod) Rattus norvegicus (continuous breeder) normal light continuous light = 6 days earlier than normal (FSH) Constant dark = 16 days later than cont. light
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Influences on Puberty & Reproduction 1)Light (photoperiod) Microtus arvalis (seasonal breeder) breeds 21 Mar – 24 Jun simulate photoperiod during (22 Sep – Dec) 1)Natural light 2)Artificial light 3)Uniform 16-h daylength 4)Uniform 8-h daylength until Nov, then 13-h day 5)Control (“out of season”) Results…. #1-4 = reached puberty >60% females = pregnant Control = no reproduction/puberty *Light (photoperiod) linked to reproductive development
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Influences on Puberty & Reproduction 2) Temperature rodents TempPuberty1 st Estrus Experimental Animals -3 o C33 days61 days Control21 o C26 days38 days **Growth rates lowered due indirectly to low temps. Thus, results directly in delayed puberty
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Influences on Puberty & Reproduction 3)Nutrition – under-nutrition delays puberty in both females and males 4)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
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Influences on Puberty & Reproduction 5)Social Effects/Density (examples from captive mice) Lee-Boot Effect: pseudo- pregnancy induced among crowded females; may go anestrus 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)
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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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