If you thought that when it comes to mammals, there’s “nothing new under the sun” for researchers to discover, you might be surprised to know that several new species of mammals are discovered and described every year.
Within the past five years, Jake Esselstyn, curator of mammals at the LSU Museum of Natural Science and an assistant professor in the Department of Biological Sciences, has described a whopping six new species of murids! Murids include most kinds of rats, mice, and gerbils.
Jake’s most recent discovery Gracilimus radix is an omnivorous rat from Sulawesi, a new genus and species known only from Mount Gandangdewata, a remote and rarely climbed mountain in the Quarles Range of the island of Sulawesi in Indonesia. Gracilimus is particularly small and has gray–brown fur, small rounded ears and pale orange enamel on its teeth. Unlike its closest relatives, Gracilimus radix feeds on both plant and animal matter.
Keep reading to discover more about how Dr. Esselstyn is discovering new species of rats in remote locations around the world.
By Jake Esselstyn
Most mammals are rodents or bats. These mostly nocturnal groups of species, which often go unnoticed, represent nearly 60% of the roughly 6,000 known species of mammals. Globally, mammal diversity is not yet well documented. Every year, several new species of mammals are discovered and described. Sometimes they are discovered residing in museum drawers, like those at the LSU Museum of Natural Science. Sometimes they are discovered during exploratory expeditions, where scientists collect specimens for present and future research projects. While some of these new discoveries are big charismatic mammals like primates, more often than not they are small, nocturnal species that live in the tropics, where diversity is highest.
In my research, I seek to answer questions about the evolutionary history of mammals on the tropical islands of Southeast Asia. The region’s rodent fauna is incredibly diverse, and is mostly comprised of rats. There are more than 400 species of rats in the region, and they’re all endemic. Not only that, most of these species are endemic to a single island, and many are endemic to a single mountain peak. This means that when you survey two neighboring mountains, it’s quite possible you will find very distinctive rat faunas (all the rats living in that region). As a result, it is difficult to predict what you might catch on a particular mountain that has not yet been surveyed. The suspense this creates when I set out on an expedition is one of my favorite aspects of research. In some sense, I never know what I’m going to find, and sometimes the findings are astonishing.
Video describing the Esselstyn lab research.
In 2012, I set out on an expedition with colleagues from Indonesia and Australia. We were headed to Mt. Gandangdewata, one of the biggest mountains on Sulawesi, a large island in central Indonesia. No mammalogist had ever conducted surveys on this mountain, so we had only vague expectations of what we might find. Our wildest fantasies of discovery never approached what we would discover among the mountain’s rat fauna.
Our surveys lasted almost three months. We sampled sites from near the coast up to 8,500 feet above sea level. During this time, we lived in tents, worked closely with local people, and spent our days trapping rats and preparing specimens for museum research collections. Over the course of these surveys, we captured and prepared specimens of about 23 species of rats. Several of these represented new species and genera. Since that expedition, my colleagues and I have described (in published research papers) three new species of rats from Mt. Gandangdewata. Each of these species is so distinct from any other known species that we also named a new genus in each case.
Destroyer of Worms
The first rat we described, Paucidentomys vermidax, is unlike any other known rodent. Most rodents are famous for having a robust set of teeth that serve two functions – incisors for gnawing and molars for grinding. This new rat, however, feeds exclusively on earthworms. Because of this, over evolutionary time, it has lost nearly all of its teeth. Some call it the “gummy rat,” because like a toothless old man, it can eat only mushy stuff. The scientific name we gave this species translates roughly as “few toothed mouse, destroyer of worms.”
A Swimming Rat
The second rat we described was a really lucky discovery. We obtained only one specimen (for museum research) of this species, which we named Waiomys mamasae. It was captured by hand by one of our local guides – we never caught it in any of our traps! Why not? It’s an amphibious species, living on both land and in water, that feeds on aquatic insects, and our traps just aren’t suited for that. We should have brought crawfish traps! While there are a few other rats known to live an amphibious lifestyle, the closest known species live in New Guinea, nearly 1,250 miles to the east. So we had no expectation of finding a species that lives this way on Mt. Gandangdewata. But the local people we hired as guides knew the rat and even had a name for it! While it’s not terribly unusual for local people to know species that scientists have yet to discover, we had no expectation of finding a swimming rat.
The scientific name we gave this water rat honors the local knowledge of the species. “Wai” means “water” in the local language and “Mamasa” is the name of the local regency and people that live around Mt. Gandangdewata.
A Top 10 Rat
The most recent species we described from this same expedition, in a paper published in March 2016, is known as the slender root rat, scientifically Gracilimus radix. This mouse is small, with a delicate body and limbs. It feeds on both plant and animal matter, unlike the other two new species mentioned above, both of which are carnivorous (i.e., they feed on animal matter). The genetic data we collected from this and many other species indicate that the slender rat’s closest relative is the water rat we also discovered on Mt. Gandangdewata.
Our genetic research indicates that various groups of rats across Southeast Asia have evolved a carnivorous diet many times. Carnivorous diets always evolve from omnivorous ancestors, but omnivores never seem to evolve from carnivorous ancestors, except in one case – the slender root rat. As far as we can tell, this is the only case of this evolutionary reversal of diet, from feeding only on animal matter to feeding on both plant and animal matter. Ecologically “specialized” species commonly evolve from more “generalized” ancestors, but the reverse rarely happens. Perhaps this is why the International Institute for Species Exploration named Gracilimus radix as one of 2017’s top ten new species!
Stay tuned for news of more new mammal species. We’re not yet done describing the results of our survey of Mt. Gandangdewata!
Photos taken by Kevin C. Rowe, senior curator of mammals, Museum Victoria, and Jake Esselstyn, LSU.