Early expression of manual lateralization in bipedal marsupials

Robust lateralization in forelimb use has recently been found in bipedal, but not quadrupedal, marsupial mammals. The link between bipedality and handedness, occurring in both marsupials and primates, remains to be investigated. To shed light on the developmental origins of marsupial manual lateralization, infants of macropod marsupials were examined before and shortly after the acquisition of habitual bipedal posture and locomotion. Forelimb preferences were assessed in natural, not artificially evoked, behaviors of infant red-necked wallaby in the wild and infant eastern gray kangaroo in free-ranging captivity. Pouch young of both species showed population-level left-forelimb preference when manipulating food objects, such as leaves and grass blades. This result provides the first report of lateralization in pouch young marsupials and rare evidence of lateralized manual activity in early mammalian ontogenesis. Young-at-foot juveniles of eastern gray kangaroo preferred to use the left forelimb to manipulate the mother's pouch edge as previously shown for red-necked wallaby. In both species, the direction of biases in manipulative behavior for young-at-foot and pouch young was the same as in adults. Forelimb preferences in offspring were positively correlated with the forelimb preferences of their mothers. Our results strongly suggest that the emergence of individual and population-level forelimb preferences in macropod infants precedes the onset of independent standing and locomotion. In all probability, manual lateralization in bipedal marsupials, such as kangaroos and wallabies, is not determined by the acquisition of habitual bipedality in the course of ontogenesis.