Can disappearing tiger stripes save a disappearing population of tigers?

Sept. 16, 2021 (Huntsville, Ala.) –   While lions may be the king of the jungle, tigers take the title of largest living cat species. Tigers are most recognizable for their coats having dark vertical stripes on a light background of white, gold, or orange.

India is home to two-thirds of the world’s tigers, and although conservation groups in India have made much progress in recovering endangered tiger populations, several small, isolated populations of tigers still exist there. It was in one of these small populations that scientists discovered rare black tigers—a discovery that helped add to the genetic knowledge about coat color patterns in mammals. These black tigers also revealed important insights into the effect of small, isolated populations on Earth’s declining biodiversity. 

Over the course of several years, camera traps at the Similipal Tiger Reserve in eastern India caught images of a few rare tigers that appeared to be mostly black. This phenomenon, known as pseudomelanism, occurs when the dark stripes on the tiger are broadened so much that they fuse together.

Chris Kaelin, PhD and Greg Barsh, MD, PhD, from the HudsonAlpha Institute for Biotechnology were part of a team, led by Vinay Sagar and Uma Ramakrishnan, PhD, from Bangalore’s National Center of Biological Sciences, that set out to study the genetic basis of the rare stripe pattern in these tigers. Because the pseudomelanistic tigers live in a small population, this gave the team a unique opportunity to study a rare trait in a small population. The results from the study were published in PNAS.

Genome analysis of eight captive tigers revealed a single nucleotide variant in the Taqpep gene of all pseudomelanistic tigers. Further analysis indicated that the variant is a loss of function mutation in Taqpep, similar to mutations in other felines with unusual coat patterning. By constructing pedigrees of captive tigers, the team predicted that pseudomelanism in tigers is inherited as an autosomal recessive trait, meaning pseudomelanistic offspring must inherit one copy of the gene from each parent.

“We have no reason to think that dark colored tigers in Similipal are more successful than striped ones,” says Kaelin. “Instead, the high incidence of pseudomelanism likely reflects low genetic diversity of the Similipal population, a consequence of habitat fragmentation and lack of connectivity to other populations.” 

Small and isolated populations such as Similipal are at a heightened threat of extinction due to low genetic diversity. Conservation practice recommends rescuing small populations by introducing new individuals with a different genetic background into the population.

“The genetic isolation of Similipal tigers is possibly due to human-induced habitat fragmentation,” says Ramakrishnan. “It will be important to explore options to enhance connectivity for Similipal Tiger Reserve in the future.” 

Researchers from Indian Institute of Science Education and Research (Tirupati, India), Laboratory for the Conservation of Endangered Species (Hyderabad, India), Nandankanan Biological Park (Bhubaneswar, India), World Wide Fund for Nature-India (WWF India), Foundation for Ecological Research, Advocacy and Learning (India), Wildlife Institute of India, Odisha Forest Department (Bhubaneswar, India), National Tiger Conservation Authority – Wildlife Institute of India Tiger Cell, Wildlife Institute of India, and State Forest Research Institute (India) were also involved in this work.

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