ON THE EDGE OF EXTINCTION - A - THE TIGER (Panthera tigris)

[GuateGojira]

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The tiger, evolution and taxonomy

In all these years that I have studied tigers, I have managed to reconstruct the entire evolutionary history of the tiger. This is a large summary of all the data that I have compiled and posted in several topics, especially those about the Ngandong tigers. Also I present a good recount of the taxonomical studies on the tigers, providing the three taxonomical groups proposed for the modern tigers.

It is a long post, read it, save it and discuss it.

Tiger evolution:
Tigers are part of the Panthera genus, which evolved about 4 million years ago, represented by Panthera blytheae like the first branch of this new form of predator. After this species, other great cats evolved, among them Panthera palaeosinensis and Panthera zdanskyi (The Longdang “tiger”), and with them the snow leopard (Panthera (Uncia) uncia) and the tiger (Panthera tigris); only the last ones exist in modern times. Jaguars-leopards-lions evolved more lately than these cats, although a large lion-leopard cat has been identified in the Laetoli area and dated about 3.5 million years ago.

The first fossils identified as “tigers” are found in central China, in areas like Honan, Yunnan, Hubei and Choukoutien. However these fossils are too fragmentary and need more studies in order to clarify they taxonomic status. The first “true” tiger fossils are found in Java, in the deposits of Ci Saat and Trinil, about 1.6 million years ago. This first tiger form is known as the Trinil tiger (Panthera tigris trinilensis). It was a primitive tiger with relative larger carnassial and was no larger than modern Javanese tigers.

At the early Pleistocene, the Sunda shelf (Sumatra, Java, Bali, Borneo) was connected to mainland by a land bridge, which was used by the Trinil tiger to enter into the Asian continent and replaced the primitive Chinese form. This was the first wave of the tiger evolution.

In the middle Pleistocene, the new tiger at mainland evolved in a new form known as the Wanhsien tiger (Panthera tigris acutidens). This tiger subspecies lived up to the late Pleistocene and varied in size from the small Sunda tigers (earlier forms) to the larger Amur-Bengal tigers (late forms). Meanwhile, the Sunda population evolved in a new form named Panthera tigris oxygnatha, that was not different from the Trinil form in size, but had a different masticatory system with more wide carnassial teeth (no long bones have been found). Meanwhile, the Wanhsien tigers expanded its territory to the north of Eurasia, reaching the area known as Beringia and up to Alaska. This other form of tiger was of the same size than modern Amur-Bengal tigers and coexisted with the cave lions (Panthera spelaea spelaea). However, as tigers are tied to the forest areas, they were unable to adapt themselves to the great steeps of Beringia and this population doesn’t prospered like the southern forms.

By the late Pleistocene about 100,000 years ago, the land bridge still connected the Sunda with the mainland, and the new large Wanhsien tigers began to expand its habitat and provoked a new wave that invaded the Sunda and replaced the original Sunda tigers. This new situation caused a new change in the Sunda ecology and with the large prey base at that moment, comparable with that of the richest areas of India, promoted the evolution of a new form known as the Ngandong tiger (Panthera tigris soloensis). This new subspecies was the largest tiger known in the fossil record and probably the largest Panthera member ever known. This giant had the same proportions than the large mainland tigers but its skull presented characteristics that were new in fossil records (elongated carnassials, smaller sagittal crest, and narrowed occiput) and that was the first time than the characteristics of the modern Java-Bali tigers are found in fossil records. Based on this, it is not clear if the mainland tigers completely replaced the old Sunda form or only mixed with it.

At about 70-80 thousand years ago, a cataclysmic event known as the Toba eruption destroyed all the north and central part of Sumatra and affected the climate of all Asia (probably the entire world), creating a series of extinctions and genetic bottlenecks among the surviving species. Tigers were no different as the species was nearly extinct after this event. However, the great cat is a very resilient species and recovered very well after this natural tragedy. A small population of Sunda tigers that lived in the eastern part of the Sunda shelf recovered and began to spread to the west area; these tigers were the first modern Java-Bali tigers, that retained the cranial characteristics of the Ngandong tiger but that began to suffer the island dwarfism.

Meanwhile, the entire mainland tiger population was reduced to a small patch in the south China-northern Indochina region. According with genetic studies, this small population gives origin to the entire modern mainland tiger population, with the South China tiger (Panthera tigris amoyensis) been the oldest and more primitive form. After this, tigers began to expand its territory to the north of China and to the Indochina region. Tigers invaded the center of Asia up to the Caspian region through the famous Silk Road and created the western population known as the Caspian tiger. However, this same population began to expand its territory following the rivers of north central Asia and reached the Baikal Lake and with time, the entire Russian Far East. This large tiger population remained together up to 200 years, when the modern human activities separate them and the population of the Amur area was isolated from that of East Asia, only the Amur population survived to the 20 century. The area of India was too dry through all this time, but about 12,000 years ago, at the end of the last ice age and the beginning of the Holocene, the tiger first entered to the Indian subcontinent and rapidly conquer the entire region, despite the presence of the Asian lion (Panthera leo persica) and the human (Homo sapiens). In fact, in India, the tiger evolved its most magnificent form, adapted from the northern areas of the Himalayas to the swamps of Sundarbans and the large forest of the Karnataka region. The Indian tigers developed a series of adaptations, that for the old hunters and naturalists represented different subspecies, however these where only clinal adaptations and new genetic studies shows that all the Indian-Nepal-Bhutan-Sundarbans tigers are of the same subspecies (Panthera tigris tigris).

Finally, what happen with the empty space between mainland and the Sunda shelf? Whell, in the time while the land bridge still existed (20-50,000 years ago), the mainland tigers began to travel to the south and the Sunda tigers to the northwest. Eventually, they join in the area that will be known as Sumatra. Here, something particular happened, the two subspecies mixed and they give origin to the modern Sumatran tiger. This new tiger present characteristics of both varieties but also evolved its own characteristics; recent craniological and genetic studies show that this is in fact, a different species, or at least, at the brink of been one.

At 10,000 years ago, already in the modern age, the tigers were already spread in the known places, from the Russian far east to the Malayan peninsula, from the Caspian see to the islands of Amoy in China, and from the Pakistani valley of the Indus through the south of eastern Asia and the Sunda islands of Sumatra, Java, Bali and Borneo (this last population get extinct before the history time).

Evolution in few words:
The fossil record show that the first "true" tiger lived in Java during the early-middle Pleistocene and it was the Panthera tigris trinilensis. However, this relative small tiger invaded mainland, replacing the primitive China tigers and gives origin to the large Wanhsien tiger (Panthera tigris acutidens). Latter by the upper Pleistocene, a second wave from China invaded the Sonda shelf (Sumatra, Java, Bali and Borneo, together), replacing the local tiger population (which in that time, was the Panthera tigris oxignata). So, by the final of the Pleistocene, there were two large tiger species, the large Wanhsien tiger (Panthera tigris acutidens) in mainland (China to Beringia) and the larger Ngandong tiger (Panthera tigris soloensis) from the Sonda shelf (Groves, 1992; Hertler & Volmer, 2007).

By end of the Pleistocene (75,000 - 108,000 ago), the great Toba eruption destroyed almost all the ecosystem of southern Asia and its species, forming several genetic bottle necks, and according with Luo et al. (2004), the last remnant tiger population that inhabit the north of Indochina, gives origin to all the modern mainland tigers (the South China-North Indochina tiger Panthera tigris amoyensis was the first of these new mainland tigers (Luo et al., 2004; Driscoll et al., 2009)). In other words, the large Wanhsien tiger is the direct ancestor of the entire modern mainland "Panthera tigris" (tigris, altaica, virgata, corbetti, amoyensis and jacksoni).

The island tigers (Java and Bali) are the last remnant of the giant Ngandong tiger, which already showed the particularly narrow occipital with is characteristic of the Javanese tiger. This suffer from Island dwarfism and by the early Holocene, they were already of the size of an average South China tiger. The Sumatran tiger seems to be a hybrid between the mainland population and the Javanese tigers that repopulated the area. The genetic studies of Cracraft et al. (1998) and Luo et al. (2004), together with the morphological analysis of Mazák & Groves (2006) and Mazák (2010) support the "species" status of Sumatran tigers (Panthera sumatrae).

Interesting as it is, the evolutionary history of the tiger is incredible and based in several re-colonization, not just one simple migration from north to south like is suggested by early studies (Mazák, 1981; Heptner & Sludskii, 1992). Old taxonomy like that of Pocock is now outdated and most be discarded, especially by the fact that he based his statements in very few specimens.

The Holocene tiger and its taxonomy:
The classification of Dr Vratislav Mazák (1981; 1983) is the most widespread among scientific documents and he stated that there is one tiger species and eight subspecies separated by its body size and pelage (although based in very few specimens). Latter, Dr Kitchener (1999), based in a craniological and pelage study, showed that the “differences” among the putative tiger subspecies are only clinal and that the sample used for the “subspecies” classification is ridiculously small and reflects no real variation. He stated that there are no significant differences to show that there are subspecies at all, but based in morphological evidence (few specimens again), stated that there are only three subspecies (1. Mainland, 2. Island and 3. The Caspian).

On the genetic side, Cracraft et al. (1998) proposed that there is not enough difference subspecies between the mainland tigers, but that the Sumatran tigers are different enough to separate them like a different species (not even subspecies). Luo et al. (2004; 2010) presented genetic evidence to show that there are in fact, enough genetic evidence to sustain the differentiation of five modern tiger subspecies and even proposed a sixth one (Malayan-jacksoni).

Latter, Mazák & Groves (2006), using a morphometric study, stated that there are enough differentiation between mainland tigers (Corbetti only) and the Sonda tigers, to classified them like a different species, but disproved the claim of Luo et al. (2004) about the separation of the Malayan tigers as a subspecies, because they don’t found any difference between them and those from Indochina. The final study of J. H. Mazák (2010) on the Craniometrical variation of tigers show, again, that there is great differences between the mainland group and the island group and that the Sumatran tigers were probably a hybrid between the two populations.

About the Caspian tigers, Driscoll et al. (2009) found that the Caspian tiger population was genetically undistinguishable from the Amur tigers and proposed to join the two groups into one subspecies (Virgata), discarding the statement of Kitchener (1999) that this group was a different subspecies. Besides, J. H. Mazák (2010) also found that the Caspian tigers had many morphological characteristics in common with all the other mainland tigers (disproving Kitchener (1999) again) and that the most different of all the mainland specimens were those from Amur, which were practically the most earlier in evolve.

Finally, Kitchener & Yamaguchi (2010) repeated the same claim from 1999, adding several complaints about all the previous studies, for example:

1. The genetic study of Luo et al. (2004) had not enough specimens and that they can’t explain why Bengal tigers are far away than Sumatran tigers in the genetic graphics.

2. The morphometric study of Mazák & Groves (2006) doesn’t use other mainland skulls, which according with Kitchener (1999) also present the narrow occiput of the Javanese tigers (although he used fewer specimens than those used by Mazák and Groves).

3. The differentiation of the Malayan tigers is invalid as they don’t present a holotype and the genetic evidence is not enough to establish it as a species.

However, they slightly mention the fact that Kitchener (1999) was wrong about the Caspian tigers and that his biogeographic analysis (Kitchener & Dungmore, 2000; repeated in 2010) failed in predict the populations of tigers in that area.

Here is what I can quote from memory right now and based in all this, I can state that there are several forms to assimilate all this information. I prepared three cases, which can be used for tiger specification:

Case 1 - The most simply form:
Two species:
* Mainland tiger: (Panthera tigris) - no subspecies.
* Island tiger: (Panthera sondaica) - no subespecies.

Case 2 - Based in morphometric and genetic analysis
Three species with subspecies (or two and one hybrid):
* Mainland tiger: (Panthera tigris):
- Bengal tiger (P. t. tigris).
- Caspian-Amur tiger (P. t. virgata).
- South China tiger (P. t. amoyensis).
- Indochina tiger (P. t. corbetti) - including jacksoni.
* Island tigers: (Panthera sondaica):
- Javan tiger (Panthera sondaica sondaica).
- Bali tiger (Panthera sodaica balica).
* Sumatran tiger: (Panthera sumatrae (tigris x sondaica)).

Case 3 - Proposed by Kitchener, with modifications:
One species (Panthera tigris) with two subspecies and one hybrid:
* Mainland tiger (Panthera tigris tigris) - only clinal variations.
* Island tiger (Panthera tigris sondaica).
* Sumatran tiger (Panthera tigris sumatrae (tigris x sondaica)). Reply