Infant position in mother-calf dolphin pairs: formation locomotion with hydrodynamic benefits

Cetacean mother-calf dyads demonstrate formation locomotion that may provide hydrodynamic and/or social benefits. For example, echelon position (calf in close proximity of its mother’s mid-lateral flank) enables calves to increase swim performance at reduced locomotor effort compared to periods of solitary swimming. Yet cetacean mother-calf dyads are also observed in infant position (calf in close proximity underneath its mother’s tailstock), thus we wondered if infant positioning also provides hydrodynamic benefits to calves. Social and hydrodynamic benefits of infant position have both been hypothesized previously, but the logistical difficulties of conducting experiments on free-swimming mother-calf dolphin dyads have precluded empirical quantifications of kinematics until now. We demonstrated that 0 to 98 d old calves of the bottlenose dolphin Tursiops truncatus are afforded locomotor advantages while swimming in infant position with their mothers. Calves in infant position had reduced locomotor effort via a 24% reduction in fluke stroke amplitude compared to periods of solitary swimming. Yet stroke frequency (the other component of locomotor effort) and swim performance (swim speed) were not different between infant position and solitary swimming. Although swimming in infant position provided some hydrodynamic benefits to calves, these benefits paled in comparison to those afforded to calves maintaining echelon position. Given that echelon position provides the best hydrodynamic benefits, we speculate that older, stronger swimming calves are predominately in infant position because by relinquishing some hydrodynamic benefits of echelon position they gain important social benefits of infant position, particularly camouflage from predators, which undoubtedly plays a vital role in calf survival.