Saint Petersburg University biologists: infants of most mammal species tend to keep their mothers on the left rather than on the right side and perceive them predominantly with the left eye. Thus, such a lateralization in mother-infant interaction due to brain asymmetries, which were previously considered to be a distinctive feature of maternal behavior in humans only, is actually of ancient evolutionary origin, and it helps species survive.

In the past five years, a group of scientists, headed by SPbU Associate Professor Yegor Malashichev and comprising SPbU junior research associates Karina Karenina and Andrey Gilev, has studied behavioral patterns of 175 mother-infant pairs of marine and terrestrial mammals with laterally placed eyes: walrus, horse, reindeer, saiga antelope, muskox, sheep, beluga whale, orca, eastern gray kangaroo and red kangaroo. These animals belong to the so called “follower-type” species in which females and their dependent infants usually move side-by-side on parallel paths.

The research findings supported the hypotheses that left-cradling bias is a distinctive feature for all mammal species, both human and non-human, and evolutionary is of ancient origin. These forms of lateralization (brain asymmetries) arise from the right hemisphere advantage for social processing.

Perceptual lateralization is vital for social behavior: the position of an infant on the mother’s left side may optimize maternal monitoring and helps species survive. The findings revealed that horse foals initiated more bonding behavior with mother when they kept her on the left side. Moreover, horse foals maintained special proximity to mother more successfully when perceiving her predominantly via the left eye-right hemisphere system, thus they didn’t lag behind or got lost. Similar findings were obtained for some other mammal species.

Interestingly, only in stressful, potentially threatening situations mothers tend to keep their infants in the left visual field that is related to an increased need to monitor the infant in an unsafe environment and derive optimal control of the infant state as the left eye-right hemisphere system provides higher accuracy and speed in many types of social responses.

  • Much is said and written about mother-infant interactions explaining why mothers tend to cradle babies on their left arm, - said SPbU Associate Professor Yegor Malashichev. – However, its evolutionary origin remains unknown. Our findings proved that these behavioral patterns due to brain asymmetries and information processing are of evolutionary ancient origin. What is more is that we proved one hypothesis explaining left-cradling bias that is lateralization of visual recognition. These brain asymmetries, typical to all mammals, help them survive in the wild.

SPbU scientists are eager to gain deeper insights into lateralization: there are some findings that similar behavioral patterns are typical to some birds (for example, Australian magpies), which can assume these forms of lateralization are common to vertebrates with advanced social behavior, for example fish, birds and some reptiles.

Moreover, we can test the hypothesis that the left eye-right hemisphere system has social behavioral functions on humans: scientists are likely to believe that left-cradle bias and interaction with the left eyes may activate and develop right hemisphere in children, which can further ensure his/her socializing and minimize the risk of neurodevelopmental disorders, for example autism.

Learn more: Lateralization of mother-infant interactions in a diverse range of mammal species, Nature Ecology & Evolution, January 9, 2017.