A research expedition has found seven new species of frog in India’s Western Ghats mountains. Of those seven species, four fall into the "miniature" size category (under 0.7 inches, or 18 mm) – small enough to fit on an adult human's thumbnail.

All these frogs belong to the genus Nyctibatrachus, only found in the Western Ghats. The findings published Wednesday in the PeerJ journal increase the number of known species in that genus to 35 – and, scientists say, indicate a need for further research and conservation efforts.

“[O]ur discovery of several new species, particularly of easily overlooked miniaturized forms, reiterates that the known amphibian diversity of the Western Ghats of India still remains underestimated,” concluded the research team, led by SD Biju, an amphibian biologist at the University of New Delhi.

These seven frogs are especially easy to miss. “They were probably overlooked by researchers because of their extremely small size, secretive habitats and insect-like calls,” Sonali Garg, another member of the research team, told BBC.

While other members of this genus – typically known as night frogs – make their homes near streams, the seven newly discovered species prefer leaf litter or forest vegetation. They also emit insect-like calls.

While their tiny size made them even tougher to spot, that’s also the reason they’re especially interesting to researchers. “We were surprised to find that the miniature forms are in fact locally abundant and fairly common,” Ms. Garg, a PhD student at the University of New Delhi, said.

In recent years, other “miniature” frogs have turned up in tropical regions. The smallest of the frogs found in India measured 0.48 inches (12.2 mm), so it can’t take the title of “world’s smallest vertebrate” from a 0.3 inch-frog (7.7 mm) discovered in Papua New Guinea in 2009.

But it’s still likely to intensify a question that Husna Haq, reporting for The Christian Science Monitor, posed in 2015: “How did these frogs get so small?”

One explanation is a theory known as island dwarfism or insular dwarfism, which suggests that when animals colonize islands or other isolated areas, large species tend to get smaller over subsequent generations, possibly due to a more limited supply of food.

It’s not yet clear whether this process caused the Western Ghats night frogs to downsize over the millennia. But most scientists agree that the region has seen a remarkable burst of speciation over the centuries. Conservation International lists the region as part of one of 35 “biodiversity hot spots,” with a high percentage of plants and animals found nowhere else on Earth.

However, that designation also implies a serious threat. The Critical Ecosystem Partnership Fund estimates that only one-third of the region’s original forest “remains in pristine condition,” and warns that “its forests face tremendous population pressure and have been dramatically impacted by demands for timber and agricultural land.”

The newly discovered frogs may be feeling this pressure. Dr. Biju said in a statement that “over 32 percent, that is one-third of the Western Ghats frogs are already threatened with extinction." “Out of the seven new species, five are facing considerable anthropogenic threats and require immediate conservation prioritization.”

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Scientists consider amphibians an important gauge of an ecosystem’s overall health, because they’re exposed to both air and water. Biju and his colleagues hope that their work will inspire and inform efforts to protect these animals.

“Apart from big animals like [the] Tiger and elephants,” he told the Press Trust of India, “there is a need to conserve this tiny amphibian also as they have been ignored. It is a very cute and small animal.”