New study challenges old views on what’s ’primitive’ in mammalian reproduction

It's hard to imagine life on Earth without mammals. They swim in the depths of the ocean, hop across deserts in Australia and travel to the moon. This diversity can be deceiving, at least when it comes to how mammals create the next generation. Based on how they reproduce, nearly all mammals alive today fall into one of two categories: placental mammals and marsupials. Placentals, including humans, whales and rodents, have long gestation periods. They give birth to well-developed young-with all major organs and structures in place-and have relatively short lactation periods during which young are nursed on milk from their mothers. Study abstract: Multituberculate Mammals Show Evidence of a Life History Strategy Similar to That of Placentals, Not Marsupials Marsupials, like kangaroos and opossums, are the opposite: They have short gestation periods-giving birth to young that are little more than fetuses-and long lactation periods during which offspring spend weeks or months nursing and growing within the mother's pouch, or marsupium.