Maternal Social Status, Offspring 2D:4D Ratio and Postnatal Growth, in Macaca mulatta (Rhesus Macaques)

Early life exposure to stressors can disrupt growth and development, resulting in long-term compromised function and increased risk for disease throughout the lifecourse. Maternal exposure to psychosocial stressors (i.e., stressors derived from social status, social inequalities, and social interactions) during pregnancy has been associated with reduced fetal growth, adverse birth outcomes, and increased morbidity for the offspring later in life. Maternal hormonal responses to stress, such as fluctuations in glucocorticoids (e.g., cortisol) and androgens (e.g., testosterone), can result in increased developmental instability, interfere with offspring growth in-utero, and may alter developmental processes of sexual dimorphism. Second digit to fourth digit length (2D:4D ratio), an indicator of prenatal exposure to androgens, is a sexually dimorphic trait established in-utero, associated with birthweight and health during adulthood. Although relationships between prenatal stress, 2D:4D ratio, and adult health have been studied extensively, there is insufficient research concentrating on maternal psychosocial stress and offspring growth during infancy and juvenile periods. The purpose of this study was to examine how maternal social status may impact offspring developmental instability in-utero, and postnatal growth, in a non-human primate model. The constancy of a matrilineal and hierarchical social structure in Macaca mulatta (rhesus macaques), results in consistent exposure to the conditions of social status, throughout development and across generations. This makes female rhesus macaques an ideal model for studying social inequality on mothers, and its effects on offspring growth. I studied mothers (n = 98) and offspring (n = 110) from a social group living in a shared enclosure. To quantify maternal social status, I collected social-behavioral data (i.e., agonistic and affiliative interactions), and assessed maternal social status (i.e., dominance-rank, received-aggression, and received-affiliation). To quantify offspring developmental stability and sexually-dimorphic growth in-utero, I measured 2D:4D digit ratios, and bilateral asymmetry of 2D:4D ratios (i.e., differences in 2D:4D ratios between right and left hand). To assess offspring postnatal growth, I collected morphometric data (i.e., weight, crown-rump length, and BMI) at two points in time, five months apart, from infants (at 5 and 10 months), yearlings (at 17 and 22 months), and two-year-olds (at 29 and 34 months). Maternal low social status was characterized by low dominance-rank, exposure to higher levels of received-aggression, and lower exposure to received-affiliation. Mothers with low social status were exposed to detrimental social conditions and appeared to have diminished maternal condition. Low status mothers produced fewer male offspring than the high status mothers, suggesting higher vulnerability in males. Offspring of low status exhibited lower 2D:4D ratios and higher bilateral asymmetry of 2D:4D ratios, suggesting higher developmental instability and altered sexually-dimorphic growth in-utero (i.e., reduced masculinization in males, and increased masculinization in females). Both, maternal low social status, and higher developmental instability in-utero, predicted compromised postnatal growth in the offspring. The findings of this study suggest phenotypic integration during development, and provide further support to the notion that, social inequality can become incorporated into the biology of individuals. Low 2D:4D ratio, high bilateral asymmetry of 2D:4D ratios, and reduced growth, appear to be examples of embodiment of social inequality during early development, in rhesus macaques