- Fan-throated lizards are found only on the Indian subcontinent, where they evolved under the influence of a changing climate that modified habitats.
- The lizards originated 26 million years ago from a forest-dwelling ancestor and adapted to a dry environment.
- They are a diverse group of at least 15 species in India and 18 in the subcontinent.
Fan-throated lizards are small, colourful reptiles found in the dry shrublands and coastal areas of South Asia. Males have a loose patch of skin drooping from their throats – it doesn’t sound like much but comes into its own when the lizards’ dating game begins. That’s when males scamper up a rock, strike a cobra pose and unfurl the loose skin into a beautiful fan. This fan, called a ‘dewlap’, earns the lizards their name and comes in hues of metallic black, orange, blue and cream – or all in one.
Since their discovery from India in 1829, a detailed assessment of the diversity of fan-throated lizards found in the country has been due. Scientists have now searched in almost all of their likely habitats and concluded that there are at least 15 different species in India. These belong to two genera – Sitana and Sarada. Though six species (five Sitana and one Sarada) out of the 15 are still to be described, their discovery is already giving insights into the evolution of this group of lizards.
V. Deepak and Praveen Karanth from the Indian Institute of Science compared the physical traits of several lizards to differentiate between the species. They examined 465 individuals for body dimensions, dewlap colour, type and size, and the number of scales on the body. They then sequenced a few genes from over 100 individuals to identify ‘cryptic species’ – species that look similar but are genetically distinct.
The leap from forests to grasslands
Using the sequences and other information that they had, the researchers constructed a family tree tracing the lizards’ evolution. Fan-throated lizards evolved about 26 million years ago when they split from their closest living relatives, the kangaroo lizards (Otocryptis). The two groups evolved from an ancestral form that lived in moist forests millions of years ago.
When cool, humid forests gave way to hot, dry grasslands, fan-throated lizards adapted to the changing environment but their kangaroo cousins failed to do so. They did not adapt to dry conditions and are now restricted to small pockets of wet forests, says Deepak, who led the study and is now based at the Natural History Museum in London.
Kangaroo lizards are represented today by only three species, of which one is found in India – Otocryptis beddomei, which is restricted to the southern Western Ghats. On the other hand, there are 18 different species of fan-throated lizards on the Indian subcontinent.
In sync with the changing environment
The diversification of fan-throated lizards in a changing landscape overlapped with a major shift in climate – the strengthening of monsoon. Before the monsoon developed as a season, rains were more evenly distributed throughout the year, explains Deepak. But with its arrival came seasonal rains and a dry climate for the rest of the year. This influenced habitats and shaped the evolution of fan-throated lizards.
Geography was key to the present diversity of fan-throated lizards as well. Physical barriers such as rivers, hills and valleys isolate populations, driving the evolution of new species. Fragments of old forests can act as barriers for these dry-adapted lizards too, says Deepak. “That’s why peninsular India is much more exciting,” and home to most fan-throated lizard species found in India.
“This is a great piece of work for many reasons,” said Jesse Grismer of Auburn University in the US. “But most of all because it brings attention to a group that needed a thorough re-evaluation and simultaneously draws attention to the ‘hidden’ arid-adapted biodiversity of India.” It combines what we know about their natural history with evolutionary genetics “to arrive at an integrative understanding of the biodiversity represented by fan-throated lizards,” Grismer adds.
Citation:
• Deepak V., & Karanth P. (2018). Aridification driven diversification of fan-throated lizards from the Indian subcontinent. Molecular Phylogenetics and Evolution, 120: 53–62, DOI: https://doi.org/10.1016/j.ympev.2017.11.016
Banner image: A fan-throated lizard in mating display. Photo by V. Deepak.
