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Abstract ObjectiveCesarean delivery is often epidemiologically associated with childhood obesity. However, little attention is paid to post‐birth modulatory environments, and most studies are conducted in settings where obesity arises for a number of reasons in addition to birth mode. We therefore assess population differences in the relationship between birth mode and childhood growth using data from rural and peri‐urban Latin American indigenous populations, and test predictions developed using life history theory. MethodsChild height and weight were measured monthly in 80 Yucatec Maya and 58 Toba/Qom children aged 1‐48 months (2007‐2014, 3812 observations). Random‐effects linear mixed models were used to compare children's growth by population, sex, and birth mode, accounting for potential confounders. ResultsCesarean delivery rates were 47% (Toba/Qom) and 20% (Yucatec Maya). Childhood obesity and overweight rates were low in both populations. Cesarean‐delivered children had significantly greater weight gain (but similar height grain) compared to vaginally‐delivered children. By age 4, cesarean delivered Yucatec Maya girls and boys, and Toba/Qom boys (not girls), had significantly higher weight‐for‐age compared to vaginally‐delivered children from their own sex and population. ConclusionsThis provides one of the first attempts to document differences in children's growth patterns according to mode of birth in modernizing indigenous populations. Cesarean delivery is associated with young children's growth patterns, even in the absence of many obesity‐inducing factors. There are also population, age, and sex differences in the relationship between birth mode and childhood weight trajectories that warrant future investigation. 

Abstract ObjectivesWe quantified variation in fecal cortisol across reproductive periods in Azara's owl monkeys (Aotus azarae) to examine physiological mechanisms that may facilitate biparental care. Specifically, we evaluated evidence for the explanation that owl monkeys have hormonal mechanisms to mobilize energy during periods when each sex is investing heavily in reproduction, that is, the gestation period for females and the infant care period for males. Materials and methodsBetween 2011 and 2015, we monitored 10 groups of Azara's owl monkeys from a wild population in Formosa, Argentina and collected fecal samples from 26 adults (13 males, 13 females). Using enzyme‐linked immunosorbent assays, we quantified fecal cortisol as a proxy for evaluating stress responses, including energetic demands, on both sexes during periods of reproduction and parental care. ResultsMale cortisol was lowest during periods when they were caring for young infants (<3 months) compared with periods with older infants or no infant. Female cortisol was elevated during gestation compared with other periods. Mean fecal cortisol in both males and females was lower when an infant was present compared with when females were gestating. DiscussionOur results do not support the hypothesis that owl monkey males have elevated fecal cortisol during periods when they need to mobilize energy to provide intensive infant care. Our findings are also inconsistent with the Maternal Relief hypothesis. However, results from studies measuring fecal cortisol must be interpreted with care and alternative explanations, such as seasonal fluctuations in diet and thermoenergic demands, should be considered when drawing conclusions. 

To address claims of human exceptionalism, we determine where humans fit within the greater mammalian distribution of reproductive inequality. We show that humans exhibit lower reproductive skew (i.e., inequality in the number of surviving offspring) among males and smaller sex differences in reproductive skew than most other mammals, while nevertheless falling within the mammalian range. Additionally, female reproductive skew is higher in polygynous human populations than in polygynous nonhumans mammals on average. This patterning of skew can be attributed in part to the prevalence of monogamy in humans compared to the predominance of polygyny in nonhuman mammals, to the limited degree of polygyny in the human societies that practice it, and to the importance of unequally held rival resources to women’s fitness. The muted reproductive inequality observed in humans appears to be linked to several unusual characteristics of our species—including high levels of cooperation among males, high dependence on unequally held rival resources, complementarities between maternal and paternal investment, as well as social and legal institutions that enforce monogamous norms.