Island rules – part I

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.

Head to tail, these land iguanas may stretch more than 3 feet, and they can move surprisingly fast when they want to.

Head to tail, these land iguanas may stretch more than 3 feet, and they can move surprisingly fast when they want to.

Several years ago, I remember a particular iguana that had specialized in stealing food from beach picnickers.

Several years ago, I remember a particular iguana that had specialized in stealing food from beach picnickers and terrorized my daughter into dropping her cookies.  However, this guy paid no attention to us, but was on a mission to somewhere.

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.

Full-grown Key Deer are about the size of a 6 month old White-tailed fawn.  Photograph taken on Big Pine key, near Marathon, FL.

Full-grown Key Deer are about the size of a 6 month old White-tailed fawn. Photograph taken on Big Pine key, near Marathon, FL.

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!

Fictional, of course.

Fictional, of course.

7 thoughts on “Island rules – part I

  1. Great photos and fascinating info. I am pretty confident that a three-foot long iguana like that could intimidate me into giving up my cookies. It was especially intriguing to think about the changes brought about by being on an island (and I figure that man is not affected in the same way, because he is not as constrained by the available food sources).

    • Interestingly, humans definitely are subject to food constraints in the same way that animals are, and there is recent evidence of dwarf-like humans living on islands in Indonesia, as recently as 3000 years ago. You may have read that some scientists decided the skeletons on the island of Flores represented a completely different humanoid species, although others think these people were Homo sapiens but simply responding to the same limitations that other large animals found on islands.

    • They usually get there by floating on vegetation, swimming, or flying (even ballooning on spider silk). Some animals have made it to various islands on ships.

  2. Those full grown iguanas were a little unnerving the first time I came across one of them in Mexico. They did seem harmless enough. I can’t say the same for a creature I thought was a cat but was told by the resort workers that it was a Coati and should probably stay away from it.

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