Amur Tigers

[Balam]

@GuateGojira  you are completely right about the prey densities argument. Many people online have this misconception that Bergmann's rule is often referred to as a "rule" in biology because it describes a consistent pattern observed in the distribution of body sizes among related species across different environments. This is anything but the case, Bergmann's rule is not an absolute or universal law, but rather a hypothesis or generalization based on empirical observations.

There are numerous factors that can influence body size within species and populations, including evolutionary history, ecological niche, genetic variability, and adaptation to specific environmental conditions. In some cases, smaller individuals might be more successful in colder environments due to factors such as heat conservation or resource availability. The latter has a lot to do with prey densities, which is why you will often find that carnivores in warmer, prey-rich, or less competitive environments will have a greater body mass than their counterparts in colder climates. We even see this between populations that live in similar environments but have different degrees of access to resources. For example, Amur leopards are among the smallest leopard populations in the world, with males seldomly surpassing 50 kg in weight. This is in contrast to their Persian counterparts in Central Asia which average in excess of 65 kg and can weight up to 90 kg. Leopards from the Amur River Valley not only have to compete with dominant predators like brown bears, tigers, and wolves, but the prey base there is generally poorer than in the Hyrcanian forests of Central Asia due to overhunting and niche partitioning, especially with the larger tiger. The only competitors of leopards in Central Asia are the much smaller Syrian brown bears and wolves, hence why despite the environments being similar, you see such a drastic difference in size: the prey availability and competitive pressure favor smaller body sizes on the Amur population.

Additionally, some cases where populations within a species happen to occur in colder areas are an example where correlation doesn't mean causation. In pumas, for example, the largest individuals are found closer to the poles where it tends to be colder. Specimens from the Rocky Mountains, Pacific Northwest Forests, or Patagonian are not bigger than those from the Amazon or Central America because they live in cooler environments (like some individuals in the AVA forums you mentioned that have very little understanding of biology like to falsely claim), but because the highest prey densities where these cats live happen to occur in those colder regions. The common denominator here is the prey biomass available rather than the climate. If a puma from Costa Rica has to survive on raccoons and monkeys, while one from Patagonia survives on guanacos, and one from the Rocky Mountains survives on elk, it is obvious which populations have better access to abundant food and which ones are going to produce bigger individuals as a result.

I also believe that the obsession between the sizes of Amur and Bengal tigers lacks so much nuance to the point that it becomes childish. Amur tigers weigh less because they have fewer resources than Bengal tigers, it's not rocket science. If tomorrow we reversed the situation and increased the ungulate populations in Siberia and Manchuria, increased natural corridors, and allowed tigers to breed through a selection of the best genes, while hunting the tigers and their prey in India until their numbers are greatly reduced and inbreeding becomes more common, we'd see the Amur population growing in body mass and the Indian one decreasing.

It's clear that the mainland tiger subspecies has a standard body size that can manifest to its best potential when all the right variables are in place, and it can also decrease in weight or even skeletal frame size if it needs to survive with lesser resource, this is what we call size plasticity and it happens in all carnivorans. It doesn't matter if it happens in India, Russia, or Malaysia, these are all tigers at the end of the day. Reply