ARTICLES https://doi.org/10.1038/s41559-018-0567-6 © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. 1 Department of Anthropology, University of Durham, Durham, UK. 2 Department of Obstetrics and Gynecology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA. 3 Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA. 4 Department of Pathology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA. 5 Department of Anthropology, University of Chittagong, Chittagong, Bangladesh. *e-mail: kesson.magid@durham.ac.uk G lobally, men in wealthy, developed regions generally have higher testosterone than those living in less affluent ones 1–3 . While some researchers link such variation to ‘ethnic’, ‘racial’ or genetic traits 4–6 , ecological and behavioural variables associ- ated with energy availability, such as abundant nutritional intake, pathogen load and sedentary lifestyles, also potentially contrib- ute to interpopulation differences in reproductive phenotypes 7–12 . Developmental exposure to energetic variables during childhood may further explain adult variation in reproductive steroid hormones. Evidence supporting this ‘developmental hypothesis’ connects early infancy, pre-birth or childhood experience with sex steroid levels in later infancy 13,14 , developmental timing as measured by adult height and pubertal age 15–18 , or adult reproductive function 3,18–21 . Migration studies support the developmental hypothesis. Children migrating from less to more affluent regions show rapid postnatal growth and earlier sexual maturation 19,22,23 . Levels of salivary progesterone, ovulation rates and menopausal age of Bangladeshi women who reached adulthood in more ecologically constrained environments were lower compared with those who migrated to a less challenging one 19,24,25 , and early-childhood migra- tion (age 0–8 versus 9–16 years) was associated with more robust ovarian function 19,23 . We lack comparable migrant studies among men, but—based on the above findings—we predicted that men with different life histories would express varying degrees of reproductive investment depending on differential developmental conditions. We expected that males encountering improved ecologies before or during devel- opmental transitions would invest in more costly reproductive effort associated with competition and/or sexual signalling, mediated by testosterone 26–28 . Based on ecological developmental histories, we presumed that individual trade-offs between testosterone-mediated traits and other energetic demands would lead to population-level differences. Considering male variation in reproductive function, hormonal variations in non-clinical populations are unlikely to impact fecundity 29,30 , but instead relate to trade-offs between traits associated with survivorship and reproductive effort 2,3,31–34 . We therefore designed a cross-cultural study to distinguish whether global variations in male reproductive phenotypes (mea- sured by salivary testosterone levels, pubertal age and stature) reflect: (1) developmentally plastic, organizational responses to childhood ecology or (2) current, activational responses to local ecology. We selected a generally homogenous, ethno-cultural group of Bangladeshis of Bengali ethnic origin, some of whom migrated from a less to more affluent region (specifically, Sylhet, northeast Bangladesh to London, United Kingdom). We assumed that there would be fewer ecological constraints on males in the United Kingdom compared with Bangladesh. Despite improvements, Bangladesh still ranks globally among the poor- est quartile of countries, with high indicators of maternal under- nutrition and stunting (36%) among children aged <5 years 35,36 . However, the Bangladeshi populations studied here originate from the land-owning, middle-class not normally subject to nutritional Childhood ecology influences salivary testosterone, pubertal age and stature of Bangladeshi UK migrant men Kesson Magid  1 *, Robert T. Chatterton 2,3,4 , Farid Uddin Ahamed 5 and Gillian R. Bentley 1 Male reproductive investment is energetically costly, and measures of human reproductive steroid hormones (testosterone), developmental tempo (pubertal timing) and growth (stature) correlate with local ecologies at the population level. It is unclear whether male reproductive investment in later life is ‘set’ during childhood development, mediated through adulthood, or var- ies by ethnicity. Applying a life-course model to Bangladeshi migrants to the United Kingdom, here we investigate plasticity in human male reproductive function resulting from childhood developmental conditions. We hypothesized that childhood ecol- ogy shapes adult trade-offs between reproductive investment and/or other fitness-related traits. We predicted correspondence between these traits and developmental timing of exposure to ecological constraints (Bangladesh) or conditions of surplus (United Kingdom). We compared: Bangladesh sedentees (n = 107); Bangladeshi men who migrated in childhood to the United Kingdom (n = 59); migrants who arrived in adulthood (n = 75); second-generation UK-born and raised children of Bangladeshi migrants (n = 56); and UK-born ethnic Europeans (n = 62). Migration before puberty predicted higher testosterone and an earlier recalled pubertal age compared with Bangladeshi sedentees or adult migrants, with more pronounced differences in men who arrived before the age of eight. Second-generation Bangladeshis were taller, with higher testosterone than sedentees and adult migrants, and higher waking testosterone than Europeans. Age-related testosterone profiles varied by group, declin- ing in UK migrants, increasing in sedentees, and having no significant relationship within UK-born groups. We conclude that male reproductive function apparently remains plastic late into childhood, is independent of Bengali or European ethnicity, and shapes physiological trade-offs later in life. NATURE ECOLOGY & EVOLUTION | www.nature.com/natecolevol