Here’s one of the fundamental rules of biology: islands are awesome. Islands, by virtue of their extreme isolation, provide a plethora of unique living laboratories in which to examine evolution at work. There is a reason that Darwin’s trip to the Galapagos was a formative experience for his theory of evolution. Just for your edification, here’s a very overview of some of the biological oddities you can find on islands:
Madagascar is the only place you’ll find the nearly 100 species of lemur, a ‘primitive’ branch of the primate tree found which exists nowhere else in the world today. Australia is the only place where marsupial mammals (mammals with pouches like kangaroos or koalas) reign over placental mammals, which dominate mammal populations on all the other continents. The world’s largest lizard, the Komodo dragon, is found only on four islands (Komodo, Rinca, Flores, and Gil Motang) in Indonesia.
The uniqueness of islands can be attributed to their isolation: islands provide interest, self-contained habitats with limited contact to the outside world. In some cases, this isolation allows lines that have gone extinct on the mainland to flourish (lemurs), in others it allows for a shuffling of ecological roles — the lack of large mammalian predators on islands in Indonesia probably allowed the Komodo dragon to attain its impressive size and stature as top predator.
The impressive size of Komodo dragons probably has something to do with insular gigantism.
Now that I’ve finished plugging islands as biological hot-spots, let’s take a look at the inverse of the growth spurt shown by the Komodo dragon — i.e. island dwarfism (or insular dwarfism, if you want to be proper). Sometimes, in the unique environments provided by islands, a lineage which is usually quite large may find itself shrinking over evolutionary time. This process is most visible among mammals, but it can theoretically affect any group (including dinosaurs).
One example of an extant product of insular dwarfism is the pygmy three-toed sloth, a resident of the miniscule (approx. 1.3 mi2) Isla Escudo de Veraguas off the Caribbean coast of Panama. These animals weigh 40% less than their mainland cousins, the regular three-toed sloth.
Another is the island foxes — six unique subspecies of gray fox found only on the Channel Islands off of California. These foxes are markedly smaller than their already-not-that-big mainland cousins, coming in at only 2-6 pounds. A good size equivalent is a housecat — and a lean one, at that.
One of the island foxes, related to mainland gray foxes.
Much of our evidence of insular dwarfism actually comes from the fossil record, wherein miniaturized versions of megafauna have been discovered in islands. In comparatively recent history, consider the remains of dwarf elephants found in the larger islands of the Mediterranean. There were actually several dwarf species on different islands, indicating that the dwarfing processes occurred more than once, and independently. The theory is that the islands were colonized by elephants from the mainland several times whenever sea levels dropped enough to permit travel, and each visit could give rise to a new dwarf species.
Model of a pygmy elephant.
Also in the fossil record, but a bit more contentious, is the apparently dwarfed human species Homo floresiensis. For a summary of the story behind this interesting human ancestor, dubbed ‘hobbits’, check out this answer I wrote a few weeks ago.
Going back much further, to the Cretaceous, we come to the large, offshore island of Hateg — which was in an area corresponding to the region around modern-day Hateg, Romania. Paleontologists have determined that there were a handful of indigenous dinosaurs species on the island, most of which were dwarfed versions of the mainland megafauna. Interestingly, Hateg also provided an example of the inverse of insular dwarfism (insular gigantism) in the form of the pterosaur Hatzegopteryx, which is one of the largest pterosaurs on record.
Why does this happen? Why the fun-sized animals? There are several theories — and likely the exact factors at play in each instance of insular dwarfism are different — but the primary theory has to do with resource management. Islands, by virtue of their smaller size and isolated nature, tend to have more limited resources than the mainland. Large species which find themselves trapped on an island will need to evolve to become smaller and use less resources, or die out because the island cannot support them.
Really, you could write an entire book about islands. In fact, David Quammen,one of my favorite writers, already has. I urge anybody who found this article interesting to check out his book The Song of the Dodo — it’s a treasury of interesting island biogeography and pithy prose.
There are really many more examples of insular dwarfism than I could cover here, so I’ll leave it up to you to visit some of these citations and get a more comprehensive list.
– The Song of the Dodo by David Quammen
This refers to the fact that when large animals (and lagomorphs) become isolated on islands, they usually evolve over time to be smaller than their mainland relatives. Other, smaller species also tend to become much larger: these size changes are referred to as Van Valen’s island rule. Dwarfism can be observed in Madagascar pygmy hippos, Wrangel Island pygmy mammoths and even in a homonid, Homo floresiensis, that lived on the island of Flores and was much smaller than modern man. Many of the dwarf animals we know about are now extinct. There are many, complex possible reasons for island dwarfism, but one appears to be that small animals do better when there are limited resources and space.
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