Analysing the link between adaptability and species success of Elvira rat, Cutch rock rat

The fates of two native rat species – the Elvira rat (Cremnomys elvira), which is critically endangered and the more commonly seen Cutch rock rat (Cremnomys cutchicus), illustrate how species adaptability to a specific landscape impacts their chances for success, wildlife researchers studying the two species state.

“The rocky hillocks and granite inselbergs of Madurai district are not places most people associate with wildlife watching. Scrub vegetation clings to thin soils between exposed boulders; the landscape looks spent in the dry months, almost inhospitable. But for C. cutchicus, this is prime real estate. During nocturnal transect surveys across three sites in the rocky terrain south and east of Madurai, conducted between 8 p.m. and midnight across multiple evenings,” according to R. Brawin Kumar, a wildlife biologist studying lesser known species of fauna in Tamil Nadu. He noticed the Cutch rock rat in areas with “large, jointed granite blocks offering deep crevice systems.”

He also recorded the presence of the species in the Shervaroy Hills of Salem, which is home to the critically endangered sister species of the Cutch Rock Rat – the Elvira Rat. Unlike the former, the Elvira Rat is restricted to a specific Shonkinite rock formation in the Shervaroy Hills. Shonkinite is also one of the rarest geological rock types in the world.

“Interestingly, The Cutch rock rat was observed in adjacent rocky areas with different lithology, suggesting the two species may partially overlap in geographic space while partitioning microhabitat by rock type, a question that warrants dedicated survey efforts,” said Mr. Kumar.

The Cutch rock rat makes its home in the rock crevices and thrives in the granite rock’s thermal stability. “The Aravalli Range, the primary stronghold of C. cutchicus in Rajasthan, is one of the world’s oldest geological formations, believed to be between 1.5 and 3.2 billion years old, formed during the Precambrian era. The rocky outcrops of Tamil Nadu, while geologically distinct (comprising Archaean gneisses, granites, and the rare Shonkinite of Salem), are comparably ancient. This rat has not merely adapted to old rocks, it has co-existed with some of the oldest land surfaces on Earth,” he said.

Mr. Kumar believes the relative success of the Cutch rock rat in comparison to its sister species The Elvira rat is due to its adaptability to different rock types. “The Elvira rat is tied to Shonkinite, an extremely rare intrusive igneous rock found in only a handful of locations worldwide, with small, localised outcrops. The Elvira rat is found only in Salem and nowhere else in the world. The Cutch rock rat, by contrast, tolerates a wider variety of rocky substrates, granite, quartzite, basalt, Deccan Trap formations, and has therefore maintained a broader distribution,” he said.

However, he cautioned broader distribution does not mean the species is without threats, with the rock rat’s primary ecosystems in the Aravalli mountain range under serious threat over the coming decades due to mining, despite the species’ presence in Theerthamalai in Harur division, Salem and Madurai as well as in other medium-sized rocky hills.

“In Madurai and Salem, the southernmost confirmed field localities for this species based on recent observation, the threat profile is different. Here, the pressure comes not from mining but from urbanisation expanding into the rocky scrubland margins of the city, and from the conversion of rocky slopes near Salem for industrial use. The southern population of C. cutchicus, if genetically distinct from northern populations (a question unanswered by any published molecular study to date), could represent a conservation unit of independent significance,” he said.

Mr. Kumar, a recipient of the Tamil Nadu Environment Award in 2016 and the Tamil Nadu Green Champion Award in 2022 told The Hindu the rock rats are primary food sources for species such as the Bengal fox, as well as for a number of raptor species. “Their protection and presence in these landscapes is crucial for the local rocky ecosystems they inhabit,” he said.