Island of the Giant Hedgehogs

Hoplitomeryx matthei

A well-documented phenomenon in evolution is that of insular dwarfism. What happens is that a population of large, usually herbivorous, animals become trapped by rising sea levels, finding themselves on an island where previously they had been able to roam free across a much wider region. In response, over the course of many, many generations, their descendants become smaller.

There are two main reasons why this happens. Firstly, there isn't so much food to eat on an island, so smaller animals that eat less will be at an advantage, and able to have more offspring that survive to have offspring of their own. But, while the problem of a limited food supply is more obvious on a smallish island, even on a large continent, the supply is never likely to be inexhaustible. In which case, why do large animals exist in the first place?

That, of course, is where the second reason comes in: predators. Being large is a protection against being eaten; lions and tigers, for instance, rarely eat elephants, rhinos, or hippos. Large predators, being at the precarious pinnacle of the food chain, and present in smaller numbers to start with, find it even harder to survive on small islands than herbivores, and they often simply die out. That removes the need for herbivores to avoid them: there's no real need to be too large for a lion to eat if the biggest thing you'll ever face is a fox, anyway.