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

[peter]

GENETIC ANCESTRY OF THE EXTINCT JAVAN AND BALI TIGERS - CONCLUSIONS

1 - CONCLUSIONS

A few weeks ago, I posted a summary of a new and very interesting article on the evolution of tigers ('Genetic Ancestry of the Extinct Javan and Bali Tiger', Xue et al, 2015). The most important conclusions were:

1a - Earliest fossils and common ancestor

01 - The earliest tiger fossils found in China and Java date back to the early Pleistocene (2 million years ago).
02 - Molecular genetic imputation traces all living tigers back to a common ancestor as recent as 72 000 - 108 000 years ago.
03 - The Toba eruption in Sumatra 73 500 years ago may have contributed to the recent coalescence for modern tigers.

1b - Current tiger taxonomy

04 - Current tiger taxonomy recognizes 6 living subspecies.
05 - Mainland Asia tigers parse into 5 distinct groups (P.t. tigris, P.t. altaica, P.t. amoyensis, P.t. corbetti and P.t. jacksoni).
06 - P.t. tigris is genetically distinct from the other mainland subspecies, corresponding to an early divergance.

1c - Sunda tigers

07 - Bali, Java and Sumatra tigers (P.t. balica, P.t. sondaica and P.t. sumatrae) derived from a common matrilineal genetic lineage.
08 - All 3 Sunda tiger subspecies are genetically closer to each other than to mainland Asia tiger subspecies.
09 - All 3 Sunda tiger subspecies are distinctly different from each other, indicating a restriction or lack of matrilineal gene flow among the three islands. 
10 - The modern, now extinct, Javan tiger is not an autochthoneous descendant of a historic tiger population.

1d - Modern tiger evolution

11 - There once was a widespread tiger population from China (P.t. amoyensis) to the Sunda Shelf (P.t. balica, P.t. sondaica and P.t. sumatrae) that became isolated as a result of rising sea levels during interglacial periods.
12 - A second wave of expansion and divergence (P.t. tigris, P.t. corbetti, P.t. altaica, P.t. virgata and P.t. jacksoni) replaced much of the range of P.t. amoyensis and evolved into modern population in Indochina, the Indian Subcontinent, the Caucasus and the Russian Far East, where tigers fossils are only found from the Holocene. 
  
2 - QUESTIONS

The article is one of the best on tiger evolution I read. It answers a number of questions at a high level of abstraction in that it is now more or less clear in what period tigers and subspecies evolved.

The origin of tigers can be traced back to southern China about 2 million years ago, maybe a few hundred thousand years earlier. From China, they first spread south to the Sunda Shelf. This would make sense, as a large part of central and northern Asia was still covered with ice in the Pleistocene. At the end of the Pleistocene, maybe as a result of the Toba eruption, tigers probably became extinct in the Sunda Shelf. 

Some time later, a second wave followed. Tigers again spread south to the Sunda Shelf. When most of the Shelf was inundated, Sunda tigers became extinct in Palawan and Borneo, but they managed to survive on Bali, Java and Sumatra. It is likely tigers reached Sumatra later, because Sumatran tigers are different from Bali and Java tigers.

Many thousands of years later, just before the Holocene, tigers spread west from China to the Indian Subcontinent. Some time later, about 2 000 years, they colonized other parts of Asia. As a result of human expansion, they became extinct in most parts of mainland Asia. This happened between 1850-1980.  

Although the major developments are clear, some questions were not answered:

13 - Why are Sumatran tigers different from Bali and Javan tigers if all 3 inhabited a similar region?

It has been confirmed that Sunda tigers became isolated from mainland Asia tigers when the Sunda Shelf was inundated. This probably happened when the ice melted at the end of the Pleistocene or the beginning of the Holocene, well after the second wave had reached the Sunda Shelf. As Bali, Javan and Sumatran tigers also are distinctly different from each other, it is likely they became isolated when the sea level rose further. One would tend to think this probably happened in the last 10 000 years, but there's reason for doubt as Sumatran tigers are quite different from Bali and Javan tigers, This, in my opinion, points towards an earlier divergence. Unresolved, I'd say. 

14 - Did tigers cross the western part of the Himalayas to reach the Caspian region, did they travel west from China or were both routes used? If both routes were used, which was used first?

Mainland Asia tigers evolved into 5 distinct subspecies in the last 12 000 years or so. Indian tigers are different from the 4 other mainland subspecies, because they diverged a bit earlier. If we assume those close to China (P.t. corbetti and P.t. jacksoni) evolved before the others (P.t. altaica and P.t. virgata), the question is which of the two most remote regions (the Caspian region and northeast China) was colonized first. Although some researchers think it was the Caspian region, there's reason for doubt. One reason is tigers apparently reached Japan well before the Holocene started, which points towards an early wave directed to the north. Another is northeast China is closer to southern China than the Caspian region. It's also easier to reach that region.

I know Amur and Caspian tigers are one and the same, but genetic similarity doesn't answer the questions on both. J.F. Brandt (1856) wrote Amur tigers traveling west were seen well west of Lake Baikal, but he also said Caspian tigers, following a more southerly route, were seen traveling east. Those in the southern part of the route were reddish in colour as well as a bit smaller than those following a different route. Based on what I read, I'd say it is more than likely Caspian tigers expanded to the east and Amur tigers expanded to the west at about the same time. Apparently, they sometimes met and bred, especially in the northern part of the Trans-Baikal route close to the origin of rivers flowing north. As a result of the harsh climate and the lack of prey animals in winter, many of these tigers would have been wanderers by nature. Brandt wrote some followed wild boars and reindeer. In this way, they reached regions where tigers were never seen before. This happened in the 18th and 19th century.   

The question on how tigers reached the Caspian region is difficult to answer. Did they cross the Indus from western India and travel north and west, did they cross the western part of the Himalayas from northwest India or did China tigers travel west? Not one option can be excluded, I think. There are persistent rumours about tigers inhabiting remote and elevated regions in Afghanistan. It is also known they are able to breed at an altitude of 10 000 - 12 000 feet in Bhutan. If tigers are able to survive and breed at that altitude, one can't exclude wanderers from Kumaon and Nepal following corridors to the north. My guess is tigers also crossed the Indus River. Finally, there is no question tigers crossed the region between the Caspian Sea and Lake Baikal in both directions. 

Although genetically almost similar, Caspian and Amur tigers also are quite different. Caspian tigers, more than other subspecies, have quite short snouts. This was so clear, it was noticed in living animals. The Caspian tigers in the Berlin Zoo were described as short-faced tigers. In Amur tigers, however, the snout is longer and straighter than in other subspecies. Same (straighter) for the sagittal crest. Many Caspian tiger skulls have a very vaulted profile. Not so in the Amur skulls I saw. If I was asked to get to a conclusion based on the drawings I saw and skulls I measured, my guess for now would be Caspian tigers, although distinct, are closer to Indian tigers than to Amur tigers.                

15 - In what way are Malayan tigers (P.t. jacksoni) different from P.t. corbetti and P.t. amoyensis?

This question will be addressed in the next post. For now, I can say P.t. jacksoni is an enigma to me. Apart from size, I wonder in what respects they are different from P.t. corbetti and P.t. sumatrae.   

16 - What is the reason that subspecies, without losing specific characters, change in size almost overnight (a century or less)? In what way does size affect the unique characters?

This question, as far as I know, was never addressed. A pity, as it is known that tigers in some regions in mainland Asia are quite a bit larger or smaller than those in nearby regions. I also noticed they quickly changed in size in some regions in a century only.

Genetically, Sunderban tigers and Central India tigers are one and the same. I read descriptions of Sunderban tigers in different books. Although a bit smaller than their relatives in Central India, large animals were not unknown. In the last 50 years or so, Sunderban tigers suddenly changed. Many of the animals darted and weighed were unhealthy and they were even smaller than many Sumatran tigers. The change in size is so outspoken, that some biologists thought they had developed into a distinct subspecies. My guess is the change is a result of a loss of size foremost, but I wouldn't be surprised to find they developed unique characters in the last century.

When Vietnam was French Indochina, it was promoted as a paradise for hunters in the USA. Many hunters came over and tried their luck in the first decades of the last century. Most visited Annam on the east coast. To their surprise, tigers easily surpassing your average Indian tiger in all respects were not uncommon. Although their reports were dismissed as easily as reports on large Indian tigers, it is an undisputed fact skulls exceeding 15 inches found their way to Rowland Ward every now and then. Pocock mentioned one in his article on tigers that was published in the JBNHS (1929). 

It isn't easy to find your way in records, but I think there's no question Annam and Laos produced male tigers well exceeded 9 feet straight and 400 pounds on a regular basis. In books in which exceptional animals featured, I also read tigers in other parts of French Indochina were quite small. I do not doubt some tigers exceeded 11 feet 'over curves' in Annam. At least one of them was 260 kg. (Bazé). This animal only hunted very large herbivores. A German hunter shot a 298 cm. male estimated at about 240 kg. in the west of what was then South-Vietnam in the early sixties of the last century. Some of the photographs I saw pointed towards tigers that definitely compared to an average male Indian tiger. But less than 100 miles away, a male tiger only very seldom exceeded 9 feet and 350 pounds. Tigers often swimming to the numerous islands in the Mekong were described as 'water tigers'. Many of them were stocky, but short.             

17 - What was the function of the Isthmus of Kra?

The more I read, the more I'm convinced this part of Asia could have been a kind of bridge between mainland Asia tigers and those in Malaysia. In the days of unrestricted gene flow, tigers in Malaysia, although smaller than those in India, exceeded 9 feet straight on a regular basis. There's plenty of indisputable evidence. Today, as a result of restricted access and the downfall of tigers in general, large tigers are unheard of. In the extreme south of Malaysia (Johore and Terengganu), tigers seem to compare to Sumatran tigers. If they reach the size of an average Sumatran tiger at all. One wonders why tigers living in isolated regions suddenly lose size overnight (less than a century). An issue neglected by many, so it seems. Reply