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Work in behavioural ecology has been and continues to be highly informative in the study of human reproductive variation and behavioural flexibility. The traditional focus of this approach has largely been on calibrations in reproductive behaviour in ultimate terms (i.e. a specific behaviour is adaptive and maximizes fitness in different environments). Yet, understanding the ultimate function of human behaviour as adaptive to local environments allows for the deeper study of the proximate psychological, social and physiological mechanisms that can shift both reproductive timing and corresponding reproductive ideals, giving greater insight into the factors that influence early childbearing. Therefore, in the thesis I aim to explore and confirm some of the mechanisms that impact on male and female reproductive timing. Firstly, I demonstrate that the impacts of social and environmental stressors such as the local sex ratio result in different response patterns from women with different socioeconomic backgrounds, the implication being that women with different life history trajectories have different strategic responses to environmental conditions in line with the predictions of life history theory. Secondly, I show that individuals’ subjective perceptions of their environment are just as important, and potentially more important, indicators of their fertility intentions than the often used objective indicators of environment quality such as deprivation. Thirdly, I show that individuals take risks in strategic ways that can be explained by evolutionary principles and that their future reproductive intentions are supported by pro-natal norms and are not due to deficiencies in their knowledge of safe sexual practice. Fourthly, I consider the evidence that kin networks help shape individuals’ psychology around life history strategies. Finally, I explore the causal pathways by which acute stress shifts individuals’ life history strategies and how this adjustment is moderated by an individual’s exposure to chronic childhood stressors. The findings resulting from this work merges with other research in the field of behavioural ecology, moving towards an integrated understanding of human reproductive and behavioural calibrations and exploring the ultimate and proximate questions of human reproductive variation. These findings highlight the importance of understanding life history trade-offs as central to reproductive scheduling. In addition it provides policy makers and health workers with an alternative way of understanding early childbearing, one that sees human behaviour within its adaptive evolutionary context.

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La Guyane présente la fécondité la plus élevée de l’ensemble des régions françaises et également des autres territoires d’Amérique du Sud et de la Caraïbe : 3,3 enfants par femme estimés en 2018.Cette thèse propose une analyse des principaux déterminants de la fécondité en Guyane, notamment aux zones frontalières, les plus natalistes. Ce travail s’appuie sur les résultats d’une enquête par questionnaire menée auprès de femmes et d’hommes résidant dans quatre communes frontalières avec le Suriname, à l’ouest, et une commune frontalière avec le Brésil, à l’est. Un complément a été réalisé dans le municipio d’Oiapoque, au Brésil, à la frontière guyanaise.Cette thèse présente tout d’abord le contexte historique et ethnologique qui préside à la composition actuelle de la population de Guyane. Puis elle examine les caractéristiques sociologiques de la société guyanaise d’aujourd’hui, et notamment des communes frontalières, où les populations sont économiquement et socialement précaires, peu instruites et fréquemment migrantes.Explorant comment interagissent les déterminants socio­économiques, le niveau d’utilisation de la contraception, l’âge à la première maternité, le nombre actuel d’enfants et les normes de fécondité, les résultats de la recherche montrent qu’il y a deux situations bien distinctes : à l’est, pour les femmes d’origine brésilienne, la transition démographique est pratiquement terminée ; à l’ouest, pour les femmes bushinenge, elle ne fait que commencer.Cette recherche repose sur l’hypothèse que le moment présent se situe, pour les femmes bushinenge, à la charnière entre fécondité non régulée et fécondité régulée. / French Guiana has the highest fertility level of all French regions and of other countries in South America and the Caribbean: on average, 3.3 children per woman estimated in 2018.This thesis aims to analyze the main determinants of fertility in French Guiana, particularly in border areas which present high birth rates. It draws on the results of a survey of women and men by questionnaire residing in four municipalities bordering Suriname in the West, and a border town with Brazil in the East. An additional survey was conducted in the municipality of Oiapoque in Brazil at the Guyanese border.Firstly, this thesis presents a historical and ethnological background that determine the current composition of the population of Guyana. Secondly, it examines the sociological characteristics of today’s Guyanese society, notably border municipalities where populations are economically and socially precarious, poorly educated and frequently migrating.Exploring the interaction between socioeconomic determinants, the level of contraceptive use, age at first birth, current number of children and fertility standards, our research findings show two different situations: demographic transition is almost complete in the East for women of Brazilian origin while in the West, for bushinenge women, demographic transition is just beginning.This research is based on the assumption that currently, fertility of bushinenge women is at a transition period between unregulated and regulated fertility.