The island of Luzon in the Philippines has the greatest concentration of unique mammals in the world and majority of these species cannot be found anywhere else on Earth.
In a 15-year-long study, which was reported in the journal Frontiers of Biogeography, Filipino and American researchers found that 93 percent of the non-flying mammals that are present in Luzon are found nowhere else in the world.
Of the 56 mammal species excluding bats that thrive on the island, for instance, 52 are endemic, which makes Philippines' largest and most populous island a biological treasure trove.
Between the years 2000 and 2012, researchers conducted a survey of non-flying mammals at 17 locations and discovered at least 28 new species, which include four species of tiny tree-mice characterized by very long whiskers that almost reach the animals' ankles and five species of shrew-looking mice that primarily feed on earthworms.
Most of the newly discovered species are found in a single mountain or mountain range, most of which has not yet been previously sampled.
Eric Rickart, from the Natural History Museum of Utah, who is part of the project, said that all of the newly discovered species are members of two branches on the tree of life that is confined to the Southeast Asian country.
"All 28 of the new species, and 20 of the species discovered prior to 2000, are members of two morphologically and ecologically diverse endemic clades ('cloud rats' and 'earthworm mice'), which strongly implies that species richness has primarily been the product of speciation within the island," the researchers wrote.
Nineteen of the species have already been described in scientific journals while researchers are still working for nine species to be formally described.
Lawrence Heaney, from The Field Museum in Chicago, explained that they started the study in Luzon in 2000 knowing that most of the native mammals there were unique to the island. The researchers wanted to know the reason for the island's biodiversity but did not expect that the project would double the known number of species on the island to 56.
One of the reasons behind the island's rich biodiversity is its size, which is comparable to Iceland and Cuba, providing it enough space to support the survival of species. The island has not also been connected to another landmass, which gives its species enough time to evolve to new ones and diversify further.
High forest-covered mountains within the island also serve as natural barriers that create separate ecosystems, which boosts speciation among the non-flying mammal species.