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

[peter]

TIGER EVOLUTION - SUMMARY

After reading the Balkenhof, Fickel, Kramer-Schadt and Kitchener article on intraspecific variation in tigers, I was ready to respond. When I tried to find my last post, however, I was amazed at the barrage of information I found. It's clear that some of us are interested in tiger evolution and taxonomy. My guess is the debate will continue for some time.  

As to tiger evolution and taxonomy. In spite of the different views, a few things seem to stand out:

a - All agree there was a Pleistocene population bottleneck about 75 000 - 100 000 years ago. Many think it was related to the Toba eruption. As the results of the eruption are still visible in many mammals, the conclusion is it must have been devastating. 

b - Although interglacial periods were not uncommon in the Pleistocene, the climate definitely changed just before the Holocene. About 10 000 - 20 000 years ago, global warming resulted in rising sea levels, expanding forests, more mammals (and herbivores in particular) and new chances for big carnivores.

c - The evolution of mammals ultimately seems to be related to climate. They expanded when opportunities were offered and suffered or disappeared completely when the ice returned. Seen from a distance (thousands of years), evolution is characterized by waves of expansion and periods of destruction.

d - Tigers might have evolved a few million years ago somewhere in southern China. During periods of global warming, they expanded both to the south and the north. Although remnants of the types which developed in these regions might have survived long periods of destruction, there's no question they are different from modern tigers. Modern tigers again spread from China. 

e - When sea levels rose, Sunda tigers became isolated. Same perhaps, but to a lesser extent, for Malaysian tigers. The local races in mainland Asia most probably were a result of human expansion, meaning tigers were pushed to inaccessible and isolated areas not connected to others areas that had tigers. After the introduction of fire-arms, tigers were actively hunted and often locally destroyed. The answer to the question whether they are (or were) subspecies or not largely depends on the definition of what a subspecies is. All mainland tiger subspecies probably developed in the last 20 000 years.

f - Conservationwise, the debate about (the status of) subspecies doesn't seem to be decisive in that it is known that captive tigers of different subspecies quickly interbreed. Wild animals of different subspecies probably also interbreed and adapt to local circumstances, resulting in local types sooner or later. Judging from what is seen in isolated regions like the Sunderbans, where tigers dramatically changed in less than two centuries, my guess would be sooner. Another, indirect and partly artificial, example would be the experiment with China tigers in South Africa. In a few generations, they adapted to the local conditions by increasing in size. Same, but the other way round, in regions in which tigers are (were) severely hunted. 

Manipur is directly south of the Naga Hills, where tigers were decidedly smaller than those in other parts of India. But half a century earlier, males well exceeding an average Naga Hills male were not uncommon in that area. Manipur, especially the eastern part close to Burma (Myanmar), was wild country, whereas the Naga Hills had many estates (tea). Big cats apparently quickly adapt to local circumstances and it often shows in size. Isolation and stress (as a result of habitat destruction and poaching) often result in miniature tigers in a few generations only. If they don't decrease in length, they, like in eastern Russia, lose weight. The opposite also is true.    

g - Although Kitchener's suggestion regarding the development of local types (cline) is more accurate than the suggestion of others (tiger subspecies are clearly different from others), it has to be said that the skulls of the subspecies I saw suggest a cline would be an oversimplification. If size only would be used as a criterium, he is close, but there often are marked differences between different subspecies. Most of these haven't been quantified. As no quantification in science equals absence, they officially do not exist. But they do and can be seen at a glance. A good topic for a debate, I think. Reply