Boys are right-handed, girls are left ... Well at least this is true for sugar gliders (Petaurus breviceps) and grey short-tailed opossums (Monodelphis domestica). New research shows that handedness in marsupials is dependent on gender. This preference of one hand over another has developed despite the absence of a corpus collosum, the part of the brain which in placental mammals allows one half of the brain to communicate with the other.