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Past climate changes helped diversification of lizards in India, fossils show

  • Photos : Dr V. Deepak
    Photos : Dr V. Deepak

Climate change has always been a major driver that has shaped our planet’s biodiversity. Massive extinctions and severe adaptations are all a result of climate change. As we start to understand how the current change in climate is impacting us, effects of previous climate changes are hardly understood. In a rare study combining biology and paleontology, researchers from the Indian Institute of Science, Bengaluru, have shown how different climatic factors, present millions of years ago, have influenced the evolution of fan-throated lizards.

Fan-throated lizards (FTL) is a kind of agama found in India. Males of this species have an extendable dewlap or fan which is brightly coloured in some species. Until a few years ago, researchers thought that a single species of the genus Sitana, was present throughout the country. However, with increased efforts, 10 more species of Sitana and three more species of a new genus Sarada, were found over time. The study attributes this observed diversity to the onset of monsoon and subsequent aridification during the Miocene epoch--a geological period spanning from 23 million to 5.3 million years ago.

“In 2007, I saw some photographs of Sitana ponticeriana (fan-throated lizards) from Maharashtra that looked very different from those in Pondicherry. This was the first time when I got interested to study this group”, says Dr V. Deepak, the lead author of the study published in the journal Molecular Phylogenetics and Evolution. “These preliminary observations gave me the hint about the diversity within these lizards”, he recounts.

The researchers used genetic data of fan-throated lizards sampled from different regions where they were found. They then used tools to tell apart two or more distinct species tagged as a single species due to similarities in their looks and body characteristics. In their molecular dating they used five fossils and sequences of the different Fan-throated lizard species trying to understand the time at which different diversifications were observed.

“Molecular dating is a very useful tool if one understands the underlying principles. Typically, biologists rely on the information provided by paleontologists about the fossils. When paleontologists describe a fossil, they also estimate its age and the error associated with this number. Based on the morphological description of the fossil, paleontologists and neontologists (those who study living organisms as opposed to fossils) identify to which extant group (genus, family, subfamily) the fossils are related to. Given this information, a particular fossil can be placed in the molecular phylogeny to estimate the dates for the related species group of our interest”, informs Dr. Deepak about the methods used in this study.

In total, the team identified 15 species of fan-throated lizards, and discovered five potential new species of Sitana, and one new species of Sarada. They also found that the dry zone species of the Sarada-Sitana group diverged from the Sri Lankan wet zone sister genus Otocryptis at around 26 million years ago during early Miocene. Sitana and Sarada evolved as separate genera around 18 million years ago during mid-Miocene, which corresponds to the establishment of monsoon. During the late Miocene, around 11 million years ago, there was an increase in open habitats and a decline in rainforest habitats, with the initiation of aridification in South Asia. This helped Sarada and Sitana to further diversify into the present delimited species.

Also, 13 of the 15 delimited species were found from peninsular India. This greater diversity of fan throated lizards in peninsular India could be due to the availability of different types of habitat, compared to other relatively homogeneous dry zones of the subcontinent. Hence, climatic factors and landscape heterogeneity were found to have influenced current diversification  in this group.

The study has brought the evolutionary and conservation significance of the Indian arid zone, where lizard diversity is underestimated, to focus. Systematic documentation of lizards in this area is necessary, say the researchers. “I advocate taxonomy of different lizard species to be dealt with caution. One has to look at more than just traditional characters used in lizard systematics, examine as many museum specimens as possible before going out in the field. Zoological Survey of India has one of the oldest and widest arrays of collections in the country and this needs to be utilized”, recommends Dr. Deepak.

As we have entered Anthropocene, a human-influenced epoch, human-induced climate change has already started to take its toll on biodiversity. What does it mean for the fan-throated lizards? “These lizards are short-lived species with 1-2 years of lifespan. They typically breed just before monsoon when the habitats are dry and are more visible when they display and mate. After the breeding season that lasts up to 3 months, the females lay their eggs with 40 to 45 days of incubation period. This period roughly corresponds to the post-monsoon season when the young ones emerge. There is sufficient vegetation for them to hide and surplus insects to feed on. Therefore, the life cycle of these lizards is synchronized with the trends in current climatic conditions. If there is continuous disruption of the climatic cycle due to climate change it will indeed affect the long-term survival of many of the these species, particularly the range restricted species”, signs off Dr. Deepak.