The world's smallest known vertebrate is a frog the size of a housefly, a new study says.
At an average of 7.7 millimeters long, the newfound Paedophryne amauensis is a hair smaller than the previous record holder, the Southeast Asian fish species Paedocypris progenetica, whose females measure about 7.9 millimeters.
During recent field surveys in southern Papua New Guinea, scientists found P. amauensis and another new species of tiny frog, Paedophryne swiftorum, which measures about 8.6 millimeters.
It's obvious "they're adapting to fill a niche that nothing else is filling," he said.
Indeed, the frogs likely evolved their tiny sizes to eat tiny invertebrates, such as mites, that are ignored by bigger predators, said study co-author Christopher Austin, a biologist at Louisiana State University in Baton Rouge.
Tiny Frogs Hard to Catch
Discovered in 2010 but announced on Wednesday, all the species of the Paedophyrne genus are tiny and seem to live solely amid leaf litter on New Guinea's rain forest floor.
Scientists locate the teensy animals by listening for their calls and trying to zero in on the sources of the sounds—no mean feat, since the high pitch of the calls make their sources especially hard for human hearing to locate.
Austin and graduate student Eric Rittmeyer tried four times to find the frogs before exasperatedly grabbing a big handful of leaf litter and putting it in a plastic bag.
The scientists then combed through the contents until "eventually we saw this tiny thing hop off one of the leaves," Austin said.
The frogs are so small it's hard to see their earth-colored skin patterns with the naked eye, so Austin took pictures and then zoomed in, using a digital camera like a microscope.
But photographing the amphibians was just as challenging as finding them. When Austin brought the camera to his eye, the subject would often already be gone.
The new frogs are "incredibly good jumpers—they can jump 30 times [longer] than their body size," said Austin, whose study was published January 11 in the journal PLoS ONE.
New Guinea Minis Not Alone
As part of the research, Austin and colleagues also did a global genetic comparison of tiny frogs.
The team discovered that small frogs have evolved independently 11 times around the world, and almost exclusively in tropical rain forests, where the amphibians' skins won't dry out and food is plentiful.
(See related pictures: "Pea-Size Frog Found—Among World's Smallest.")
As the study says, "minute frogs are not mere oddities, but represent a previously unrecognized ecological guild."