Dwarf Geckos: Life in the Shadows

It is mid-afternoon and we roll our windows up as we drive through the dust and dry heat of Madanapalle, in southern Andhra Pradesh. Even in late January it is intensely hot on the plateau. Rocks and boulders glint in the sun. The asphalt is lined with dry grass and bramble. As we begin our climb towards Horsley Hills, brown scrub slowly gives way to eucalyptus plantations, and the temperature dips slightly. Below us, the landscape stretches out like a turbulent sea of dull green vegetation, breached by low islands of granitic rock.

We drive past a scattered group of tourists at the viewpoint and park the car at the side of the road. Biologist Ishan Agarwal, who I’m travelling with, hops out and crosses the road, climbing a few rocks to enter the forest. I follow, side-stepping some broken beer bottles. A few minutes later we are surrounded by boulders the size of small houses. Some of them almost touch the canopy.

The Mysore Plateau has a diversity of habitats, from grasslands and scrubland to montane evergreen forest, but its most obvious features are its rocks. If you’ve taken a train from Chennai to Bengaluru, they come into view towards the latter half of the journey, piled high on hills in an impossible game of Jenga. The rocks, and the Mysore Plateau, sit on the 3.4 billion-year-old crust of the Dharwar Craton that spreads its bedrock across Andhra Pradesh, Karnataka, and northern Tamil Nadu. Most of these rocks are schists, slates, and phyllites, formed by the early volcanism of the plateau and differential processes of cooling over geological time.

But even though these boulders are so old and so obvious — or maybe because they are — the biodiversity they support has been overlooked. That is what Ishan and his colleagues argue in a paper published in May 2020 in the Journal of Zoological Systems and Evolutionary Research. In it, they describe three new species of rock-dwelling dwarf geckos (genus Cnemaspis) from this region. All three species are endemic to rocky montane habitats. One of them, Cnemaspis graniticola, lives only in the cool shade of granite boulders, around Horsley Hills. Much like the lichen-covered boulder I was looking up at.

Given the cryptic nature of this species, however, I did not have very high hopes of seeing one today. Ishan’s “herping” stories always had an element of late-night adventure to them; intrepid explorers tramping about in dense vegetation with their torches, the ground sparkling with eyeshine. Today the sun was high in the sky and we were barely in a forest. But just a few minutes in, he shone his phone light into the crevice of a rock and pointed. “There – at the edge of that crevice”, he said. And there she was — a gravid female, bulging slightly at the belly. Mottled grayish-brown and sitting very still, she was hardly distinguishable from the rock underneath her. Her round pupils were black, ringed by golden irises. Her toes were like pine needles, segmented, unbelievably delicate. Individuals of this species are small, growing only up to about 4 cm, and she must have been the length of a forefinger. I moved in to get a better look, but leaves crunched noisily under my shoes. She lifted her snout, angled her head sideways, and vanished into a fold in the rock. It is a singularly peculiar — and privileged — feeling to witness a form of life that has not yet been officially introduced to the world. It felt like I had just been let in on a great secret.

But, of course, these animals have been around a while — 50 million years, as Ishan’s work finds. South Asian Cnemaspis originated in the cool, wet forests of the Western Ghats and were thought to be restricted to these habitats within peninsular India. But 10 species, including the three just mentioned, are now known from outside the Ghats, in warmer parts of south India. The three new species form a novel clade, or group, that diverged from their ancestors 15–25 million years ago, and Cnemaspis graniticola, the gecko we had just seen, has been isolated from its sister taxon in Kolar, Karnataka, just 60 kilometres away, for about seven million years. The two other species described, C. bangara and C. yelagiriensis are found in the Kolar Gold Fields and the Yelagiri Hills in Tamil Nadu, respectively. Unlike the Western Ghats, these habitats are hot and dry, and not considered very biodiverse. So how did these geckos, historically adapted to live in cooler environments, get to these drier landscapes? And how are they able to live in such hot habitats today?

“We think it’s the large granite boulder fields that act as climate refugia” says Ishan, “this is probably why these geckos have been able to persist in these warm and exposed areas.” When the ancestor of these species moved out of the Western Ghats millions of years ago, the peninsula was much wetter and more forested. As climate changed over time, the plateau opened up, becoming hotter and drier; today, daily air temperatures routinely exceed 35 degrees Celsius — much higher than what these small geckos can withstand. The temperatures inside and under boulders, however, can be almost 10 degrees lower than the surrounding environment, and are much more stable over time. Like most other tropical geckos, Cnemaspis have highly conserved thermal preferences and haven’t been able to evolve higher heat tolerances to live in warmer climes. Ishan and his colleagues propose that these rocks and boulders have acted as refugia from changing climates, providing pockets of cool, stable microhabitats for these geckos over millions of years.

Today, however, these ecosystems are under threat from a range of stressors. “These geckos are already living on the edge”, Ishan told me. “Their lethal temperature limits are below the daily maximum temperatures in this landscape across much of the year.” A rapidly changing climate is likely to exacerbate this, and even the boulder fields may not provide enough protection for these delicate, cool-temperature adapted geckos.

Added to that is our ever-growing need for granite. Quarrying in the region has already led to huge losses in rocky habitat, causing local extinctions and fragmenting the ranges of species that are found here. Worse still, many of these geckos are endemic to a single mountaintop or small hill range, which means we stand to lose millions of years of evolutionary history if these habitats are destroyed.

Building an inventory of species diversity and distribution, and documenting the biodiversity that is found here, is a critical first step in systematic conservation planning. The fact that this region does not have any charismatic megafauna has, according to Ishan, been another reason for it being overlooked. Studies like this reveal their hidden diversity. In doing so, they also offer new and interesting ways to understand the world.

We walked around for a little while longer and found two more granite dwarf geckos in shaded areas among the large rocks. They let us watch them for a few seconds before disappearing from view. Eventually, we made our way back to the car and began our descent. As we turned a bend, I caught a view of the plateau again, studded with boulders winking in the evening light. I couldn’t help but wonder what was in their shadows.