- A new study shows that wild populations of Indian star tortoises, one of the most trafficked species in the world, exist as two genetically distinct populations – one in northwestern India and the other in southern India.
- This study also shows that the biogeography of wild Indian star tortoises is well-structured and that the two populations have moderate to high genetic diversity; this contradicts previous findings.
- Integrating genetic data into the management of Indian star tortoises can leverage information on biological traits and natural genetic variation so that seized individuals can be released in environments where they have a higher chance of survival.
The Indian star tortoise is rampantly smuggled to serve the illegal pet and animal parts trade. It is endemic to the Indian subcontinent and inhabits two disjoined arid regions – northwestern India (including parts of Pakistan) and southern India (including parts of Sri Lanka). Hundreds of news reports on star tortoises have appeared worldwide over the five years since it was declared highly endangered and was awarded the highest level of protection under the Convention on International Trade in Endangered Species (CITES). Between October 2021 and February 2022, over 5,000 star tortoises in India alone were seized by government authorities, from smugglers.
Most trafficked animals seized by authorities end up in captivity in zoos or wildlife centres that often lack the expertise to care for them. Many others disappear back into the wildlife trade or are released unscientifically into nearby wildlife sanctuaries.
To help curb their unscientific release in India, a team of scientists from the Wildlife Institute of India and Panjab University has used genetic testing and measurements of physical features to understand the species’ distribution. The results of their work show that wild populations of star tortoises in India boast moderate to high genetic diversity, despite decades of illegal poaching.
The authors say that these differences may mean that star tortoises are locally adapted to either the northwestern or southern habitats in their range. Therefore, integrating genetic information into the management of this highly trafficked species could help in evidence-based conservation and better rehabilitation decisions for seized tortoises.
Could the two populations be grouped as separate subspecies?
Using samples from 82 tortoises across 14 locations in both northwest and south India, the study authors extracted DNA from tissue samples and used polymerase chain reaction (PCR) techniques to amplify and sequence certain genes. This involves creating more copies of a specific gene or stretch of DNA (amplifying), whose sequence of nucleotides or bases is then determined (sequencing).
One set of genes was used to understand the demography – the broad historic changes in the size, structure and movements of populations. This was done to confirm if the northwestern and southern star tortoises were one continuous population or not. In addition to this, morphometry data on the physical features of the tortoises in the form of measurements of body parts were also collected.
The results of the genetic studies revealed that the northwestern and southern populations of star tortoises are genetically so different, that they may be better classified as subspecies. This distribution pattern of the tortoises likely arose due to an intense dry period in the late Miocene followed by an increase in monsoon seasonality in the Pleistocene that aided the spread of grasslands across the Indian subcontinent. During this time, the star tortoise spread across India into its current northern and southern habitats. Later climatic events, that led to the formation and shrinkage of humid forest patches across southwest India split the star tortoises into the northwestern and southern populations roughly 2,000 years ago. These two populations seem to have been fairly stable until 100-200 years ago, when the trade in star tortoises probably began.
The two populations have distinctly different radiating patterns on their shells, with the southern ones having fewer ‘rays’ and a higher contrast in the patterns than their northwestern counterparts. Besides this, northwestern star tortoises are generally larger. Although females are larger than males in both populations, the southern females are similar in size to the northwestern males.
Surprising results on genetic diversity
Another set of genetic markers called microsatellites was also used to determine the genetic “health” of the wild star tortoise populations. Previous studies indicated degraded biogeographic structure and poor genetic diversity in star tortoise populations. One reason for this disparity in results could be that the previous studies relied on genetic samples from zoos, captive populations or pet stores, whereas this study was on wild populations in native habitats.
The northwestern population was found to have moderate genetic diversity. This may be due to their relatively homogenous habitat consisting of arid semi-deserts to semi-arid savannas bound by the Aravalli hills in the north to the Kutch and Kathiawar peninsula in the south, which probably caused the population to become locally adapted.
In contrast, the southern population exhibited high genetic diversity and has been described as a “hotspot of genetic diversity” with four distinct and partially overlapping sub-populations. Since the southern population is spread across dry zones in central and southern peninsular India and Sri Lanka, it covers several ancient hill ranges with highly variable landscapes and climates.
How do these results impact management strategies?
Currently, seized star tortoises in India are released into wildlife sanctuaries such as the Chinnar wildlife sanctuary and Sathyamangalam wildlife sanctuary.
However, the authors of this study warn against continuing this practice without information on the origins of the seized animals – whether they belong to the northwestern or southern populations.
“The naturally deep historical divergence in the northwestern and southern star tortoise populations should be maintained to minimise the impact of genetic admixture due to anthropogenic factors such as illegal trade and release. Care must be taken to release seized tortoises in their closest genetic landscape based on the reference database of wild star tortoises and not intermix the two groups.” the authors said.
In essence, releasing northwestern star tortoises in southern territories or vice-versa, can impact their survival or that of their offspring as these tortoises may be genetically more adapted to their original environment. However, the pet trade mostly deals with juvenile and sub-adult star tortoises, whose physical features are not developed enough to indicate if they belong to the northwestern or southern populations. The authors recommend genetic testing before taking any decisions on releasing them.
“Our study presents a reference database that can be used for scientific release to guide effective management and conservation. Integrating genetic intelligence into the management of this highly trafficked species will aid in shifting towards evidence-based conservation, leveraging the species’ biological traits and natural genetic variation,” the study adds.
“These are interesting and unexpected results for a species that have been in trade for a long time as a pet, both within India and internationally,” says Uttara Mendiratta, Affiliate Scientist, Wildlife Conservation Society (WCS) India. “The prudent warning by the authors against releasing seized Indian star tortoises without considering their origins is well-founded. Furthermore, the availability of this genetic data could empower enforcement agencies to pinpoint collection landscapes and hotspots. These results also open up a discussion about the scenarios under which these distinct populations have been able to endure despite widespread trade in the species.”
Banner image: An Indian star tortoise in Polonnaruwa, Sri Lanka. There are two genetically different populations of the species in India – one in northwestern India, bordering into Pakistan and the other in southern India, bordering into Sri Lanka. Photo by Jon Gudorf Photography/Flickr.