Insular dwarfism is basically an adaptation seen in mammals where average body sizes decrease in response to the reduced range and resources available on a small island. We see examples of this in extinct species, such as the dwarf elephants seen in the islands of Indonesia, and more famously, Homo floresiensis. We also see this in contemporary species, even in humans.

But the opposite is true for reptiles and avians; their average sizes tend to increase in such environments. This is why Komodo dragons and monitor lizards are so big, and why moas and elephant birds could exist quite happily in their island habitats.

Now, some would wonder why mammals shrink while birds and reptiles tend towards larger body sizes, but there are two factors that explain this away: predation, and the fine balance between population size and available resources. Mammals, having rather fast metabolisms, tend towards shrinkage to ensure they have enough individuals to maintain a breeding population, while not overwhelming the carrying capacity of their environment. Birds and reptiles, on the other hand, less threatened by predation, are free to grow to a larger average body size. An example of this would be the dodo, which evolved from some pigeon-like bird that made it to the island of Mauritius --- no longer under the threat of predation, it lost its energy-intensive powers of flight and grew much, much larger than its ancestors.

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