Why do most marsupials have limited vocal abilities?

Most marsupials exhibit limited vocal abilities compared to placental mammals, a characteristic that may stem from their relatively simpler brains. Because their brains are smaller and lack a corpus callosum—the structure that connects the two cerebral hemispheres in placental mammals—their behavioral patterns and vocalizations are generally less variable.1

While they are not entirely silent, most marsupials do not produce loud sounds of excitement or distress, nor do they typically utter cries of hunger or grunts of contentment. Their vocalizing is notably restricted, though there are exceptions, such as the dramatic rutting roars of male koalas. This limitation is considered a peculiarity of their evolutionary development, which has resulted in a different neurological and behavioral profile than that of placental mammals.1