Remarkable rats: "... most [of these Filipino rodents] look nothing like anyone's typical image of a rat. They range from large, furry, long-tailed leaf-eaters to tiny, mouse-like creatures that feed on soft-bodied soil invertebrates.".

• Author: Heaney, Lawrence R.
• Position: Focus on Nature

BIOLOGISTS are an odd lot--we often get intensely excited about things that leave other people shaking their heads. Who else would spend a couple of months each year in remote tropical mountain ranges, eagerly racing through a fog-shrouded forest at dawn to see what beautiful rats were caught in their traps overnight?

"Come again?" you might ask. "Did you say 'beautiful rats?' They're filthy disease-carrying rodents! Surely you mean beautiful birds or butterflies!" Not so, as the rats of the Philippines are marvelous, a superb example of how biological diversity is produced. There are at least 60 species, 58 of which live in no other country, and most look nothing like anyone's typical image of a rat. They range from large, furry, long-tailed leaf-eaters to tiny, mouse-like creatures that feed on soft-bodied soil invertebrates.

For at least 200 years, even before naturalist Charles Darwin's famous visit to the Galapagos Islands, scientists have known that oceanic islands usually have unique--often bizarre plant and animal species that are quite different from species outside of their island home. Oceanic islands are those that have had no dry-land connection to a continent. When a small animal or plant population reaches them, it finds itself with abundant resources and few or no competitors or predators. Over millions of years, the original species increases from one to many, evolving in response to the available (and often unusual) resources. A classic example to illustrate this process, called adaptive radiation, has emerged from our studies of the weird and wonderful rodents of the Philippines.

Scientists classify species based on their position in the tree of life. For more than 20 years, my American and Filipino collaborators and I have been working to understand why the Philippine archipelago has unusually large numbers of species that exist nowhere else, and that have such distinctive--even odd--ways of making a living. DNA studies have confirmed what we tentatively surmised from anatomical research a decade ago: 60 species are what taxonomists technically would call rats because they are on that branch of the tree of life. Yet, the DNA data show that most of the 60 species are members of just a pair of groups (or two complete "branches") that are confined to the Philippines. They are the descendants of two different ancestral populations that originally came from the Asian mainland 15-20,000,000 years ago.

Most members of one...