How much space do you need to evolve a new mammal? These worm-eating mice have the answer

It’s a long-standing mystery in biology: What is the smallest amount of land needed to evolve a new mammal? Since the 1980s, researchers thought the lower limit was 110,000 square kilometers—about the size of Cuba. Now, one team of scientists has deduced that an area one-tenth that size will do—and it could be smaller still. The finding is good news for conservationists and others concerned that climate change and habitat loss are going to speed up extinctions.

To discover the lower limit, the team turned to islands, whose isolated locations often make for an ideal laboratory—researchers can usually determine which animals arrived there and which evolved there. Lawrence Heaney, an evolutionary biogeographer at The Field Museum of Natural History in Chicago, Illinois, worked for years cataloging mammal diversity on the Philippines’s largest island, Luzon. He discovered that its 105,000 square kilometers hosted 66 mammal species, not including bats. Surely smaller islands could also allow new species to diversify, he thought. So he and his colleagues searched for just such a place. They settled on Mindoro, the seventh-largest island in the Philippines.

In 2013, they started an inventory of all the mammals there, including rats, mice, and the dwarf water buffalo. They set up live traps on the slopes of all five of Mindoro’s mountain ranges to catch the smaller ones, including a kind of long-snouted, earthworm-eating mouse native to the island, on which they focused their initial analysis. After comparing their DNA and looks, the scientists realized the mice represented four separate species—three living on their own mountains, and one occupying the lowlands below.

Furthermore, the genetic analysis suggests that the four species evolved from a single ancestor that landed on Mindoro about 2.8 million years ago. That means the island is the smallest place ever documented to have evolved new mammals from a single ancestor, Heaney and his colleagues report today in the Journal of Biogeography .

Rafe Brown, a biogeographer at The University of Kansas in Lawrence who was not involved with the study, is impressed with the work. “A lot of data show there’s lots of species on small islands,” he explains. But this is the only study to show “that species diversity had been generated on those islands.”

Heaney attributes this diversity to the mountain ranges that isolated mouse populations, enabling them to evolve their separate ways. Lizards and amphibians have long been known to speciate on smaller islands, but large mammals and birds typically need more space. “The minimum island size for speciation [gets] progressively larger for organisms” that move around more, says Jonathan Losos, an evolutionary ecologist at Washington University in St. Louis, Missouri, who was not involved with the study. “Below a certain size, islands aren’t big enough for isolation.”

Increasingly, conservationists are thinking about preserving the evolutionary potential of species they manage. Forty years ago, one calculated that it would take a preserve bigger than any existing one to do that. But this research shows that at least for small mammals, some protected areas are not too small.

So how small is too small? Brown isn’t sure. “A lot of these small islands have slipped through the cracks [of scholarship],” he says. “I’m sure we will find speciation on even smaller islands.”

*Correction, 16 May, 4:15 p.m.: This story has been updated to clarify that the mice evolved from a single ancestor.