Doctoral Dissertation Research: Seasonality of Fertility in the Laguna de Sinamaica, Venezuela

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Seasonal birth patterns have been observed in virtually all humanpopulations. While such patterns are known and well understood in manyanimal species, they are difficult to analyze in humans, since biologicalfactors contributing to birth rates are confounded with cultural factors.Taken as a whole, the conclusion of the many studies on birth seasonalityin humans is that while such seasonal variation is universal, the causes ofvariation are not universal. Most of the research that has attempted toexplain observed seasonal patterns in specific populations has focused ononly a few possible causal factors. In many cases factors such as diet,workload, and climate are shown to be correlated with fertility, but themechanisms linking them with fertility remain unexplained. The proposed study will analyze seasonal birth patterns in an indigenousAmerican population (the Anu of northwestern Venezuela). It will describeseasonal variation in proximate determinants of fertility, includingcharacteristics of ovarian cycles and fetal loss. These data will be usedin models of fecundability and fetal loss that together describeseasonality of fertility. The study includes analysis of 1346 urinesamples that were collected twice weekly during the seasons of most andfewest conceptions. The samples will be assayed to determine levels ofhuman chorionic gonadotropin (the hormone that indicates pregnancy) and ofurinary metabolites of the hormones estradiol and progesterone; the assaydata then will be used with interview data to determine proportion ofcycles ovulatory, length of cycles, rates of conception, rates of fetalloss, and levels of progesterone, as an indicator of luteal sufficiency. Previous investigations of seasonality of fertility have been basedlargely on factors specific to the population being studied. The proposedstudy uses a model of seasonality of fertility that can be applied to anypopulation. By combining existing models of fecundability and fetal loss,the impact of individual proximate determinants of fertility on seasonalfertility patterns can be more thoroughly analyzed than in previousstudies. Seasonal data on more distal determinants, such as diet,workload, spouse absence, and nutritional status, can then be evaluated interms of the information on the proximate determinants, shedding light onpossible linking mechanisms and opening avenues for further research.