Applied Anthropology, University of South Florida
April 17, 2020 3:00PM, Diamond 5
The three assumptions of Trivers-Willard hypothesis (TWH) are: 1) maternal condition is associated with offspring condition, 2) offspring condition differences remain until adulthood, and 3) offspring condition influences fitness differently by sex. TWH proposes that offspring sex-ratio can shift if it increases fitness and predicts lower proportions of male offspring under poor conditions. Attempts at testing TWH on macaques have yielded varied results. Here we test the first TWH assumption, that maternal condition is associated with offspring condition, by assessing relationships between maternal-dominance-rank (MDR) and male-offspring-growth. We also test TWH’s prediction of lower male offspring proportions in poor conditions, by comparing offspring sex-ratios among and between high-ranking and low-ranking mothers. Low MDR was associated with receiving more aggression (rs = -0.512, p= 0.000), and with producing male offspring with lower weight (rs = 0.600, p= 0.024) and lower BMI (rs = 0.613, p= 0.020). Low-ranking mothers produced a lower proportion of male offspring (n=9) than female offspring (n=22) (χ2 =5.452, p= 0.020); and a lower proportion of male offspring (29%), in comparison with medium-high ranking mothers (52.1%) (χ2 = 4.643, p= 0.031). Findings suggest that low MDR in macaques: 1) is associated with compromised male offspring growth, and 2) results in a lower proportion of male offspring. Relationships between MDR and offspring sex-ratio appear to become significant at the lowest third of the dominance-hierarchy, suggesting a stress-mediated biological threshold for shifting offspring sex-ratios. Results show that maternal stress during pregnancy disproportionately affects male offspring wellbeing.