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Unformatted text preview: IB35ac Class 19: Fertility (cultural influences) What about all those babies that some cultures have? How does that happen? Is it genetic? Today we'll talk about the biology of human reproduction and how that gets differentially employed by people with many, just one, or no children. IB35 Human Biological Variation Class 19 Human Reproduction Fertility = the production of a live birth (marked by a birth) Fecundity = the biological capacity to reproduce (theoretical potential) Fecundity is crucial to fertility: fecundity interacts with various behavioral processes to determine the level of fertility. Topics for today:
Fertility: The Interface of Human Biology and Culture What are some cultural practices that affect Fertility? ->Contraceptive use Natural Fertility Populations
"fertility which exists...in the absence of deliberate birth control. The adjective "natural" is admittedly not ideal but we prefer it to "physiological" since the factors affecting natural fertility are not solely physiological. Social factors may also play a part sexual taboos, for example, during lactation. lactation Some of these factors may result in a reduction of fertility but this cannot be considered a form of birth control. Control may be said to exist when the behavior of the couple is bound to the number of children already born and is modified when this number reaches the maximum which the couple does not want to exceed. It is not the case [that control exists] for a taboo concerning lactation, which is independent of the number of children already born" (Louise Henry, 1961, page 81). ->Age increases, pregnancies decrease ->Gradual decline in number of pregnancies for natural-fertility populations ->Steeper decline in number of pregnancies for controlled-fertility populations Populations leave NF when... A large fraction of couples believes that control is possible ... believes that control is morally acceptable ... believes that control is desirable ... gains access to effective methods of control And, couples exert control in accordance with their achieved parity, which in turn implies that they have conscious norms about what represents a desirable family size
Wood, JW. 1994. Dynamics of Human Reproduction. Aldine de Gruyter, NY. Seasonality of human births ->More babies born in certain times of the year ->Due to: Agricultural cycles, holidays Total Fertility Rate ->Natural fertility VS Controlled fertility ->Difference in mean and variance 1 Examples of Natural Fertility ->!Kung San Bushmen ->Amish ->Hutterites Proximate Determinants of Natural Fertility Time Intervals of a Woman's Reproductive Life Start of ovulation Age at menarche Age at marriage At regular risk of Waiting time to conception pregnancy; not marriage Time added by fetal loss in the traditional sense Length of gestation Duration of lactational infecundability Age at menopause/onset of permanent sterility Unable to give birth
Wood, JW. 1994. Dynamics of Human Reproduction. Aldine de Gruyter, NY. Time Intervals of a Woman's Reproductive Life Human Reproductive Biology Male anatomy & spermatogenesis Female anatomy Menstruation & oogenesis Fertilization & implantation Lactation The importance of hormones Wood, JW. 1994. Dynamics of Human Reproduction. Aldine de Gruyter, NY. Theoretical Maximum for no. of children: 31 Testis (where sperm is produced) -> Epididymis Male Anatomy Female Anatomy SPERMATOGENESIS ->Process by which male spermatogonia develop into mature spermatozoa ->Initial stages occur within the testes ->Progress to the epididymis where the developing gametes mature and are stored until ejaculation 2 Creation of an ovum(egg cell) Shedding of the uterine lining Oogenesis
*Ruptured Follicle Oogenesis & Menstruation Fertilization and Implantation Birth Same as Estrogen Lactation The Importance of Hormones
Hormones are molecular triggers. Estradriol and Progesterone Produced in the Testosterone gonads FSH (follicle-stimulating hormone) LH (luteinizing hormone) Produced in the anterior pituitary Prolactin ->Suckling leads to production of prolactin ->Prolactin stimulates production of milk 3 The Importance of Hormones The Importance of Hormones Female hormones & the menstrual cycle The Importance of Hormones Female hormones & the menstrual cycle Proximate Determinants of Natural Fertility Age at menarche Age at marriage Waiting time to conception Time added by fetal loss Length of gestation Duration of lactational infecundability Age at menopause/onset of permanent sterility How can you max out the TFR? IBI = Interbirth interval Hutterite IBI is approximately 1.6 years. !Kung IBI is approximately 4 years. Wood, JW. 1994. Dynamics of Human Reproduction. Aldine de Gruyter, NY. 4 Trends in the Data Does subsistence explain the variation? Peasants, tribal horticulturalists, huntergatherers No significant interclass difference in the mean or variance in total fertility 90% of the variation falls between 4 and 8 All populations with TFR > 8 can be considered colonizing populations No populations with TFR < 8 can be reasonably classified as colonizing populations All populations with TFR < 4 are likely to have high prevalances of pathological sterility Declining human fertility rates worldwide The Demographic Transition Demography = the scientific study of human population, especially its dynamics, as reflected in fertility, mortality, and migration Dramatic decrease in the birthrate of populations undergoing industrialization/modernization Dysgenic? Things to think about New types of selection Culture's significant effect on human biological evolution
H Hormonal affects l ff t Fertility and fecundity Study of factors producing the accumulation and perpetuation of defective or disadvantageous genes and traits in offspring in a particular population or species 5 ...
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- Spring '08