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(CNN)The smallest known reptile in the world can fit on your fingertip.
It's known as Brookesia nana, or "nano-chameleon," and it's a blotchy brown lizard that usually doesn't surpass an inch in body length. Before 2012, biologists didn't know it existed.
But the researchers who discovered it are still stumped as to why it stays so small when most vertebrates grow in size as they mature.
Brookesia nana is different from any other vertebrate and other chameleon species. It can't change color, and it's more comfortable on the forest floor than in trees.
The unique chameleon is spotlighted in an article published last week in the journal Scientific Reports.
In 2012, leading herpetologist Frank Glaw and other German researchers traveled to the rainforests of Madagascar, one of the most biologically diverse areas on Earth, in search of previously unknown reptile and amphibian species.
What they found surprised them.
"We expected to find some new species, but not specifically the smallest chameleon," Glaw and Oliver Hawlitschek, study coauthor, said in an email to CNN.
Most male reptiles are larger than females, but the reverse is true with the ever-surprising Brookesia nana. While the male was just over 2 centimeters long -- a bit over ¾ inch -- the female was 2.89 centimeters, over an inch long. The females are likely larger than the males to accommodate the eggs they lay, Glaw and Hawlitschek said.
Why the 'nano-chameleon' is so unusual
It's unusual for an adult vertebrate to remain this small. As vertebrates, which include mammals and reptiles, evolved, so did their size -- extremely small animals like this chameleon were previously thought to "face physiological challenges that limit further size reductions," according to the paper.
Most vertebrates grow as they mature, but not these chameleons. They're an example of "paedomorphism," or the retention of juvenile traits in maturity. Scientists aren't quite sure why the Brookesia nana and other related species have stayed so small while most other vertebrates keep growing.
But this chameleon makes it work. To reproduce with the female, the male has a hemipenis -- an internal sex organ that is exposed during breeding -- that's larger in size relative to his body compared to other chameleons, the researchers said.
"Probably, the male needs particularly large hemipenes to be able to copulate at all," Glaw and Hawlitschek said.
The chameleon is likely endangered
Though little is known about these tiny chameleons, Glaw and Hawlitschek said they assume the reptiles are critically endangered due to massive deforestation in the Sorata region of Madagascar, where Brookesia nana is found.