Not true. There are few species of pinnipeds (seals/sea lions/walruses) in the tropics, and these are mostly very rare. They reach their highest diversity and abundance in the Arctic and Antarctic, precisely because of the high productivity of these waters. (They are found in temperate areas especially in upwelling areas which are likewise of high productivity for fish).
While it’s not true that large animals “starve faster” in famines than small animals, it is true that small animals reach reproductive maturity faster and tend to have more offspring. A 100,000 g baby elephant takes a lot more energy to produce than a 1 g baby mouse. But a mother elephant can gather a lot more food in an hour of foraging than a mother mouse can.
There are multiple advantages to being larger and multiple advantages to being smaller, and every species has selective pressure in both directions. When the pressures are unequal, the species will evolve to a different size, when the pressures are equal the species will remain at the equilibrium size. And we see in island species that frequently large species will evolve to be smaller, and small species will evolve to be larger. So we get dwarf island elephants and giant island rodents, and so on.
The (semi-joking) rule of thumb I’ve heard is that everything bigger than a rabbit gets smaller, and everything smaller than a rabbit gets bigger.
What drove certain island tortoises to get so huge against this trend?
That’s not against the trend. When they arrived on the island they were much smaller. The closest relative on the mainland appears to be the Chaco Tortoise, which is typically less than 10 inches long.
When I’m referring to size, I mean the size of the ancestor from the mainland, not the size the animal becomes on the island.
The rule of thumb I heard was that mammals get smaller and reptiles get bigger.
While there may be a difference in the response of warm-blooded and cold-blooded animals, that’s certainly not true. There are lots of cases of island gigantism in rodents and insectivores.
There are a few cases of island dwarfism in reptiles, but not many.
As already noted, many mammals get larger on islands. Conversely larger reptiles like giant tortoises or Komodo Dragons are smaller generally than their mainland relations.
I think you have inadvertently misstated this, since the giant tortoises and Komodo Dragons are the largest of their kind in the world.
They are today. however before humans killed all the continental forms, there were giant tortoises on all continents, and IIRC every continent had larger forms than those that existed on the adjacent islands. There were giant monitors in Australian and mainland Asia larger than the Komodo dragons, the Australian species being so closely related that there was some debate at one stage whether they were the same species.
IOW, while Aldabran and Galapogos tortoises and Komodo dragons are are the largest of their kind in the world today, they are just island remnants of widespread giant monitors and tortoises that have survived because they managed to reach islands without people. As far as we can tell, they aren’t mainland species that grew large on islands, they are descendants of large mainland forms that have shrunk.
Always the peril with looking at isolated populations and saying “Ah ha, they shrank/grew because they were isolated”, when in fact it may just be a case that all the populations that weren’t isolated died out because they were too large/small to survive. The remnants never faced the same pressure so they survived, but there size never changed, or even changed ion the opposite direction to what is assumed.
Certainly some island species really do seem to have changed size compared to ancestral forms, but in many cases we can’t really be sure because if the ancestral form was the same as the current island form, it would be expected to be extinct for various reasons.
It is a bit like if the Wrangel Island mammoths had survived until Europeans discovered them. We’d be saying “Look elephants get bigger on islands”, when in fact elephants that size were ubiquitous once, but were exterminated everywhere but on isolated islands. That’s not an example of island gigantism, it’s just an example of how large species often survive on uninhabited islands.
In Madagascar, I understand the giant lemur was one of the first to go extinct (among the animals discovered by man of course.)
I further understand that the Komodo dragon is a not that insular but is somewhat cosmopolitan, like the salt water crocodile. I may be wrong this this though.
It depends on the current status of the taxonomic debate concerning whether Megalaniaand the Komodo dragon are the same species. If it’s accepted that they are the same species then yeah, it was widespread though Indonesia and Australia. At the very least the Komodo dragon is just the local representative of a group of very closely related “Giant Goannas” that originated in Australia and spread throughout Indomalaya. And it is very clearly a dwarf representative of that line, not the island giant it was assumed to be for 200 years.
Just to be clear, I was only reporting the “rule of thumb” as I had heard it, not vouching for its veracity. I remember hearing this when the initial discovery of* H. floresiensis* was announced. It was the explanation given for that species’ small size, the small size of the elephants they hunted and the large size of the monitor lizards they also hunted. Could have been the scientist over-simplifying things or the science reporter getting it wrong ( as is often the case).