Today on 26th January 2023 we are celebrating the 74th Republic Day and this year we are also going to celebrate 50 years of Project Tiger. Being a country having the highest population of Tigers in the world, we are successful in bringing back our tigers from the brink of extinction. Celebrating both, I am presenting a two-part article on tiger evolution and its distribution.
Tigers are one of the biggest and most efficient predators on earth, they are equipped with enormous strength and specialized bodies to kill prey even bigger in size. Sometimes we wonder how they have become the strongest predator during the course of evolution, and how they are distributed and adopted in different types of environments.
Like all other cats Tigers are also the members of family Felidae (Cat family) and belong to the genus Panthera (roaring cats) which primarily includes cats that can roar, tigers, leopards, lions, and jaguars. The genus does not include Puma and Cheetah, the other big cats that cannot roar. The species belonging to the genus of big cats which is known as Panthera are differentiated from other cats by the presence of free-floating hyoid bones in their throat which enables them to roar. However, later studies have suggested that the ability to ‘Roar’ is generated by the ‘Larynx’ which is commonly known as Voice Box. After new research, Snow Leopard is also included in this genus, though they can’t roar.
The subspecies of Tigers
As discussed in my previous blogs like many animals, tigers too have subspecies and initially, nine subspecies have been identified. Out of these, six subspecies namely the Royal Bengal Tiger, Siberian Tiger, The South China Tiger, The Sumatran Tiger, the Indo-Chinese Tiger, and the Malayan Tiger are surviving in the wild, while three subspecies, The Bali Tiger, The Caspian Tiger, and The Javan Tiger are believed to be extinct. New research has suggested that this nine subspecies concept is not correct and after 2017 IUCN recognized only two tiger subspecies, commonly referred to as the Continental Tigers and the Sunda Island Tiger. The Sumatran and extinct Java and Bali tigers come under the Sunda Island subspecies and all others are Continental Tigers.
Evolution of Tigers
Dormaalocyon latouri was considered the ancestor of all modern-day carnivores. It was a small, tree-dwelling mammal found around 55 million years ago. The fossils of this creature are near the root of today’s lineage of modern-day carnivorous mammals that were first found in the Belgian locality of Dormaal after which it was named. Tigers belongs to the Panthera genus which was branched away from the other Felidae about five million years ago. The tigers separated as a different species from this genus around two million years ago, before lions, leopards, and jaguars. The earliest fossil of a tiger which is smaller than the tigers found in South Asia but larger than the Sumatran Tigers is found in Henan in China. The evidence showed that the tigers were distributed widely in China and Southeast Asia by the early Pleistocene (2 Million Years Ago). They were spread southwards into the Indian Subcontinent and the Caspian region, and northwards into Russia, Japan, and Berengia by the late Pleistocene (1 million years ago). Tigers have been distributed in Java and possibly Borneo by the onset of the Holocene (10,000 years ago). They might have colonized the Caspian region through India. As per Molecular genetic studies, the most recent tiger ancestor is only 72,000 – 108,000 years old.
Range expansion or radiation of tigers
The range expansion of tigers was shaped by two factors driven by climatic fluctuations: alterations in sea level and changing vegetation patterns. The earth during the course of its history has experienced several glacial and interglacial periods which helped shape the distribution of various animal species. In the Pleistocene geological epoch, which began at two million years ago and lasted approximately 10,000 years ago during the advent of the Holocene epoch, the climate of the earth underwent periodic fluctuations with alternated cool-dry periods (glacial) and warm moist periods (Interglacial). Each of these climatic spells lasted for several thousand years, giving enough time to animals to respond to the changing environment. During the glacial period, most of the water on earth froze due to which the sea level dropped exposing land bridges that provided a connection between mainland Asia and the nearby islands. The cold and dry climate caused forest areas to shrink and grasslands to expand.
During the warm spell, frozen water melts which resulted in the raising sea level and submerging of land connections. This climate favors forest growth and pushes back grassland expansion.
These fluctuations in climate and vegetation led to the growth of numerous forms of ungulates, such as deer, antelopes, cattle, and wild pigs. According to the famous ecologist Mel Sunquist and his colleagues the Tiger has evolved and radiated as a hunter of these ungulates. When tigers were spreading in search of prey bases and suitable habitats their expansion was halted by a few ecological factors. These ecological factors were the expansion of deserts, rising sea levels, and loss of land bridges, though tigers are good swimmers and can swim 8 km. long water channels these factors have halted their expansion into the forests of Borneo, Sri Lanka, and North America.
During the Holocene epoch over the last 10,000 years, human societies have also started altering the landscapes and vegetation patterns through farming and fire. At present humans play a major role in determining where tiger have to live. The Tigers historic distribution covered about 30 modern-day countries. The present distribution is greatly restricted by the effect of human impact. They are extirpated from more than half of the countries distributed in the past. At present tigers are present only in 13 countries.
Tigers in India
Evidence suggests that tigers may have colonized the Indian subcontinent around 12,000 years ago. Even they have entered the West Asian region from India through Afghanistan. Tigers failed to colonize Sri Lanka before the sea level rose. They also failed to enter Borneo forests due to rising sea levels. Tigers have covered most of India except parts of North Western India but their current distribution is largely restricted. According to a study by Sanderson in 2000, the distributional range of Tigers in South Asia covers less than 350,000 sq km. having shrunk by 90 percent of its original distribution. Even the healthy breeding population of tigers is less than one percent. Though due to conservation effort, the population of tigers has increased during the last few years still the shrinking tiger habitat is a matter of concern.
We have learned about the evolution of tigers and their distribution in this part, in our second part we will learn how tigers are equipped to be the most efficient hunters on the planet.