Island visitors are often struck by how large some animals get compared to their mainland ancestors: flightless (extinct) dodos (a pigeon!) on Mauritius, giant hissing cockroaches on Madagascar, Komodo dragons on Indonesian islands, giant tortoises on the Galapagos, to name a few. And Puerto Rico has its “giant” land iguanas, which lay around on sand or shore, unintimidated by human presence.
But not every species that manages to find an island home grows into a giant, and thus Island Rule #1: small-bodied species tend to get larger on islands while large-bodied species tend to get smaller (depending on the existing competition for food there and who got to the island first).
In particular, large-bodied herbivores (e.g., elephants, hippos, deer) develop into dwarfs of their mainland ancestors, and relatively quickly — within just 5,000 years in some cases. This makes sense because there simply wouldn’t be enough food on an island to support a population of large-bodied, warm-blooded, leaf eaters.
Case in point — the Key Deer in the Florida keys are about 1/2 the size of their White-tailed ancestors. Food is limited on the key islands, and deer don’t seem to be great swimmers.
Gigantism occurs most often on islands where there are no mammalian predators, and is more frequent among smaller-bodied mammals (rodents, in particular), reptiles, and some bird species. However, when an increased body size impacts foraging habits, like flight in birds, or hanging from ceilings in geckos, then food acquisition puts constraints on the increased body size.
Warning to those visiting tropical islands: watch out for the giant rats!