Lateralization of spatial relationships between wild mother and infant orcas, Orcinus orca

Cooperative interactions have been argued to be a powerful factor mediating the evolution of lateralization in animals. Mother−infant asymmetric spatial relationships represent a case of social coordination among organisms. Although lateralized interactions between mothers and infants have been found in beluga whales, Delphinapterus leucas, whether this is the case in other cetaceans remains unknown. In the current study, we investigated mother-infant spatial laterality, more specifically, the lateral biases in an infant's position near its mother in wild orcas, Orcinus orca. Distances between the research boat and whales were categorized into three groups to test the influence of a potential threat on laterality expression. Observations on travelling individually identified mother-infant pairs showed group-level preference for the infant to be on the mother's right side when far from the boat. This bias reversed at close distance. At an intermediate distance, no significant side bias was found; however, when we considered only cases of apparent mother−calf pair avoidance of the boat for analysis, the left-sided bias was again observed. In contrast, when infants were socializing near mothers or when they followed older calves, the infants preferred the right side. We argue that these preferences are associated with right-hemispheric advantage in social responses, while the shift from right-sided to left-sided bias in potentially threatening situations is caused by role reversal between mother and infant in determining their relative position in the dyad. Cetaceans seem to share with primates the pronounced lateralization of parent−offspring relationships.