Worm-Eating Mice Display Unique Evolutionary Trend in the Philippines

New species can emerge from surprisingly small places.

Mindoro is known more for its white sand beaches, not its earthworm-eating mice. A new study may change that rep, at least among scientists and people fascinated by evolution. As the seventh largest island in the Philippines, it’s actually quite small by geographic standards, which makes these weird, new mice all the more extraordinary.

Mindoro is the smallest island in the world that can support the emergence of new mammals, according to new research. The island mice actually represent four species of mice that evolved from a common ancestor, scientists announced Tuesday in the Journal of Biogeography. This discovery pushes the limit regarding what we know about island biology: There’s a question of how small an island can be and still display species diversification. Now we know that multiple species can emerge in a zone of about 4,802 square miles — about two-thirds the size of Connecticut.

One of the earthworm mice found on Mindoro.

D.S. Balete

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“There are many islands that have species that arrived from somewhere else and that subsequently changed into something distinctive,” study co-lead author Lawrence Heaney, Ph.D. announced in a statement. “Rather, the key to this study is whether a single species that arrived from somewhere else has produced multiple species that all evolved within the given island from the single ancestral species.”

Heaney and his team had previously discovered speciation of earthworm mice — a group called Apomys — on a larger island in the Philippines called Luzon. Apomys are endemic to the Phillippines and, with Luzon as their clue, scientists turned their eyes to Mindoro. Across four field sessions, they collected mice and analyzed their DNA. Their genetics revealed that the mice belonged to four species, three of which had never been identified.

The original mice, Heaney hypothesizes, likely came from Luzon and colonized Mindoro about 2.8 million years ago. From that critter, three new species emerged — each that are now live on their own separate mountains. As an island, Mindoro likely placed unique selective pressures on the mice that created the dramatic emergence of new species. What those pressures were, however, is a topic that the scientists are looking to explore.

The authors also say that this study changes how they think about biodiversity conservation: “When we think about how to design protected areas, we need to think about the topography of the Earth, not just a flat map,” Heaney says. “That these mice evolved on their own separate mountains within a limited geographic area tells us that mountains are important.”

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