UW News featured a press release about a recent fossil discovery that impacts how we view mammalian reproduction. This research was conducted by a number of current and former UW Biology members.

Which group of mammals has the more "primitive" reproductive strategy — marsupials, with their short gestation periods, or humans and other placental mammals, which have long gestation periods? For decades, biologists viewed marsupial reproduction as "more primitive." But scientists have discovered that a third group of mammals, the long-extinct multituberculates, had a long gestation period like placental mammals. Since multituberculates split off from the rest of the mammalian lineage before placentals and marsupials had evolved, these findings question the traditional view that marsupials were “less advanced” than their placental cousins. Their paper announcing this discovery was published last week in The American Naturalist.

Lead author on the paper is Dr. Lucas Weaver, a postdoctoral researcher at the University of Michigan. Dr. Weaver conducted this study as a doctoral student here at the University of Washington under co-author Dr. Gregory Wilson Mantilla, a UW professor of biology and curator of vertebrate paleontology at the UW Burke Museum of Natural History & Culture. Additional co-authors are former UW undergraduate researcher Henry Fulghum, now a graduate student at Indiana University; UW postdoctoral researcher Dr. David Grossnickle; UW graduate students William Brightly and Zoe Kulik; and Dr. Megan Whitney, a UW doctoral alum and current postdoctoral researcher at Harvard University. 

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 weaning periods, or lactation periods, during which young are nursed on milk from their mothers. 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.

For decades, biologists saw the marsupial way of reproduction as the more “primitive” state, and assumed that placentals had evolved their more “advanced” method after these two groups diverged from one another. But new research is testing that view. In a paper published July 18 in The American Naturalist, a team led by researchers at the University of Washington and its Burke Museum of Natural History and Culture present evidence that another group of mammals — the extinct multituberculates — likely reproduced in a placental-like manner. Since multituberculates split off from the rest of the mammalian lineage before placentals and marsupials evolved, these findings question the view that marsupials were “less advanced” than their placental cousins.

Read the full story in UW News.