BRIEF COMMUNICATION A time to be born: Variation in the hour of birth in a rural population of Northern Argentina Carlye Chaney 1 | Laura G. Goetz 2 | Claudia Valeggia 1 1 Department of Anthropology, Yale University, New Haven, Connecticut 06511 2 Medical Scientist Program, College of Physicians and Surgeons, Columbia University, New York, New York 10032 Correspondence Carlye Chaney, Department of Anthropology, Yale University, 10 Sachem St., New Haven, CT 06511. Email: Carlye.Chaney@yale.edu Objectives: The present study aimed at investigating the timing of birth across the day in a rural population of indigenous and nonindigenous women in the province of Formosa, Argentina in order to explore the variation in patterns in a non-Western setting. Materials and methods: This study utilized birth record data transcribed from delivery room records at a rural hospital in the province of Formosa, northern Argentina. The sample included data for Criollo, Wichí, and Toba/Qom women (n 5 2421). Statistical analysis was conducted using directional statistics to identify a mean sample direction. Chi-square tests for homogeneity were also used to test for statistical significant differences between hours of the day. Results: The mean sample direction was 81.048, which equates to 5:24 AM when calculated as time on a 24-hr clock. Chi-squared analyses showed a statistically significant peak in births between 12:00 and 4:00 AM. Birth counts generally declined throughout the day until a statisti- cally significant trough around 5:00 PM. Discussion: This pattern may be associated with the circadian rhythms of hormone release, partic- ularly melatonin, on a proximate level. At the ultimate level, giving birth in the early hours of the morning may have been selected to time births when the mother could benefit from the predator protection and support provided by her social group as well as increased mother-infant bonding from a more peaceful environment. KEYWORDS birth, delivery, Toba/Qom, Wichí 1 | INTRODUCTION The expression that nothing good happens after midnight does not seem to apply to human births; peak hours for noninduced vaginal births fall between 1:00 and 7:00 A.M. followed by a decline through- out the day into the evening hours (Bernis &Verea, 2012; Heres, Pel, Borkent-Polet, Treffers, & Mirmiran, 2000; Kaiser & Halberg, 1962; King, 1955). Of course, human infants are born naturally at all hours of the day, but the observed clustering of births in the early morning hours has been proposed to be evolutionarily adaptive for the mother and newborn (Bernis & Varea, 2012; Honnebier & Nathanielsz, 1994; Jolly, 1972; Varea & Fern andez-Cerezo, 2014). A pattern of nocturnal births is observed in most primate species in captivity, excluding orangutans, nocturnal prosimians, and the owl monkey Aotus sp (Jolly, 1973). Nearly all primate species give birth dur- ing the time of day that they are least likely to be disturbed (Jolly, 1973), although few individual exceptions have been documented (DeLuycker, 2014; Douglas, 2014; Duboscq, Neumann, Perwitasari- Farajallah, & Engelhardt, 2008; Yang, Zhang, Huang, Garber, & Li, 2016). Giving birth during the sleeping-period may provide recovery time for both the mother and infant before the troop begins moving for the day (Bowden, Winter, & Ploog, 1967; Dunbar & Dunbar, 1974). It also gives the mother time alone with the infant before other mem- bers of the group discover her newborn, and this period to recover and nurse is important for the newborn’s survival in species where other members of the group show interest (and many times succeed) in han- dling the infant (Bowden et al., 1967; Dunbar & Dunbar, 1974). Birth during the early morning hours would have provided similar adaptive advantages to hunter-gatherer mothers across human evolu- tion. Giving birth when the group has reunited to rest after foraging or hunting during the day would increase the mother’s safety because labor and delivery activities would decrease awareness of her Am J Phys Anthropol. 2018;1–4. wileyonlinelibrary.com/journal/ajpa V C 2018 Wiley Periodicals, Inc. | 1 Received: 1 February 2018 | Revised: 25 March 2018 | Accepted: 27 March 2018 DOI: 10.1002/ajpa.23483 Am J Phys Anthropol. 2018;166:975–978. wileyonlinelibrary.com/journal/ajpa © 2018 Wiley Periodicals, Inc. 975