Human-like maternal left-cradling bias in monkeys is altered by social pressure

Abstract

About 66-72% of human mothers cradle their infants on their left side. Given that left-cradling exposes the baby's face to the mother's left visual field (i.e., mainly projected to her right hemisphere) and is altered by emotional states such as stress, maternal left-cradling was interpreted as reflecting right-hemispheric dominance for emotional processing. Whether this phenomenon is unique to human evolution is still in debate. In the present study we followed 44 olive baboon (Papio anubis) mothers and their infants in different social groups. We found that a maternal cradling bias exists and is predominantly towards the left in a similar proportion as in humans, but shifts toward a right bias in mothers living in high density groups. The sensitivity of left-cradling to social pressure highlights its potential links with the mother's stress as reported in humans. Our finding clearly illustrates the phylogenetic continuity between humans and Old-World monkeys concerning this lateralization and its potential links with hemispheric specialization for emotions, inherited from a common ancestor 25-35 million years ago.

Figure 1

Maternal cradling in an adult female olive baboon. A baboon mother is cradling her baby on her left side. Photograph copyright: Eloïse Disarbois.

Figure 2

Plot depicting correlation between individual CBI from successive newborns. N = 18. The maternal cradling-side bias index (CBI) is the ratio (R − L)/(R + L), where R and L respectively represent the total right and left arm uses. It ranges from − 1 to 1. A negative ratio indicates a left side cradling bias, whereas a positive one indicates a right-cradling bias.

Figure 3

Effect of social density on maternal cradling-side bias. (A) Boxplot of the densities of social groups housing Papio anubis females (n = 40) regarding their cradling-side bias (i.e., based on z-score and excluding 4 ambiguously lateralized cases). (B) Boxplot of the cradling z-scores of Papio anubis females according to their social group structures: high density mono-male aviaries (t1), low density mono-male parks (t2) and a high density multi-male park (t3). Adult males are in beige and females and juveniles are in purple. The calculation of the z-score is based on the total left and right arm uses. It provides the direction of cradling side preference: left (< − 1.96) or right (> 1.96). (*P value < 0.05).