- The highly biodiverse island of Luzon in the Philippines has yielded up two species of rats new to science.
- Both are found high up on Luzon’s mountains, where they’ve evolved to feed on the earthworms that abound in the lush, wet habitat.
- Researchers say they hope the new discoveries, the latest of dozens made here since 2000, will help shine a spotlight on the importance of conserving Luzon’s unique habitats and wildlife.
As you explore the high altitudes of Luzon Island in the Philippines, you’ll encounter plenty of earthworms. There’s an abundance of them. So the ecosystem did exactly what nature does to bring balance: it evolved predators. And the most common earthworm predators here are rats. Two of them, in fact, are new to science, having only just been described in a paper published in July.
“They’re quite bizarre,” says lead author Eric Rickart, a curator at the Natural History Museum of Utah, University of Utah. “They hop around on their sturdy hind legs and large hind feet, almost like little kangaroos. They have long, delicate snouts, and almost no chewing teeth.”
The scientists caught one of these new “tweezer-beaked hopping rats” when they set traps with a regularly employed bait: peanut butter. The first capture, however, happened quite by chance as the rat wasn’t interested in the peanut butter. However, it did slurp up an earthworm when the scientists offered it one as an experiment. Subsequently, when the team, led by the late Danilo Balete of Chicago’s Field Museum of Natural History, set traps with live, wriggling earthworms as bait, they came across the two new earthworm-loving rat species.
Scientists named the new species Rhynchomys mingan and Rhynchomys labo after the mountains they’re respectively found on, Mount Mingan and Mount Labo. Their genus name comes from the ancient Greek rhyncos for “snout,” due to the rats’ long pointed noses, and mys for “mouse.”
The rats catch earthworms by quietly hop-stalking little trails they make among the mud and humus of the forest. Once they detect an earthworm, they quickly pounce. They then brush the dirt off and swallow the worm whole, “like a long spaghetti,” according to the scientists.
“One of the things that’s striking to see was their reflexes — they’re lightning quick,” says Lawrence Heaney, a curator at the Field Museum and co-author of the study. “It later became obvious why they’re that fast. They’re trying to catch a nimble earthworm that’s partly in a hole.”
Island evolution
Luzon is the largest Philippine island and, at 27 million years old, one of the oldest oceanic islands in the world with never any direct dry-land connection to continental Asia. With volcanoes and mountain ranges, life in Luzon has rich resources and undisturbed geographical diversity to evolve uniquely. Today, all native animals and plants of Luzon are descendants of ancient species that colonized the island by crossing formidable water barriers. The tweezer-beaked hopping rats of the Rhynchomys genus evolved from a single ancient colonization event 7 million to 9 million years ago. They are found nowhere else on Earth.
These “earthworm rats” had to adapt to the prey available: earthworms. They quietly hop trails, dig deep in the soil, work through leaf litter, or go up the trees among orchids and other epiphytes, all in search of the best sustenance available. The worms are common in Luzon’s high, wet altitudes, with a forest floor covered in fallen plant material that takes a long time to decompose.
Scientists find Luzon particularly fascinating because it’s a natural laboratory for studying evolution.
“These interwoven processes occur everywhere, but they can be studied most readily on islands because of the effects of isolation,” Rickart says. As for the rats, they have adapted uniquely to take advantage of the rainforest’s abundance in earthworms and provide a great example of how evolution works as generations of a species are isolated in confined pockets.
Scientists have described 30 new species of mammals in Luzon since 2000 and expect to find more. In that same period, just one new mammal species was discovered in all of Europe, a land mass 92 times larger than Luzon. Rickart and his team say they believe Luzon has the world’s greatest concentration of endemic mammals on the planet.
“There are sky islands within the big island [of Luzon],” Heaney adds.
Conservation of the rats and biodiversity in Luzon
Only 6 percent of the tropical old-growth forests that blanketed the Philippines 500 years ago survive today. The country is losing its rainforests to oil palm plantations and other agricultural interests.
But the two new species are lucky; they live in high-elevation forests — wet, cold and steep — and are not currently threatened by agriculture or logging. Still, experts say geothermal energy development and mining could pose threats down the road. And it may not take much to endanger the new species.
“As far as we know, the two species are geographically restricted to small areas on isolated mountains, so any broad-scale disturbance to their habitat could be a great threat,” Rickart says.
The good news is that scientists have seen a steady rise in the growth of secondary forests, albeit a slow one, over the last 25 years. The earthworm rats don’t necessarily require old-growth forests but do need high-altitude habitats at elevations above 1,500 meters (about 5,000 feet).
“What we see is that as the forest regenerates, the native mammals move back in,” Heaney says.
Locals understand the importance of the forests, especially old-growth ones, against the pummeling of typhoons, according to the researchers. Without forests in higher lands, and the absorbent, mulch-carpeted mossy floors that act as intact watersheds, typhoons can cause extensive damage, including loss of life, due to erosion, landslides, mudslides and floods.
“The local populations and governments want to protect the land on which they depend for water and indirect resources,” Heaney says, adding that the researchers got a lot of support from the Philippines Department of Environment and Natural Resources.
The scientists say they hope the discovery of the tweezer-beaked hopping rats, and of all the other creatures here in recent decades, will put additional focus on conservation in Luzon. When a new species is discovered, it provides publicity and spark, Heaney says: “It often helps a lot to promote the establishment of new protected areas and national parks.” The docile earthworm-slurping rats, it turns out, may inadvertently promote conservation in their home range.
CITATIONS:
Rickart, E. A., Balete, D. S., Timm, R. M., Alviola, P. A., Esselstyn, J. A., & Heaney, L. R. (2019). Two new species of shrew-rats (Rhynchomys: Muridae: Rodentia) from Luzon Island, Philippines. Journal of Mammalogy, 100(4), 1112-1129, doi:10.1093/jmammal/gyz066
Heaney, L. R., Balete, D. S., & Rickart, E. A. (2016). The Mammals of Luzon Island: Biogeography and Natural History of a Philippine Fauna. Baltimore, MD: JHU Press.