Why Humans (and Other Large Animals) Shrink When They Live on Islands

A model of H. floresiensis.

Somewhere between 50 and 100 millennia ago, a humanlike species thrived on the Indonesian island of Flores. Like modern humans, members of this species — named Homo floresiensis, after their home — built stone tools, hunted prey, and may even have used fire. Unlike modern humans, they had small brains, large teeth, receding foreheads, and relatively large feet.

They also stood at approximately three feet six inches tall.

Why did Homo floresiensis — also known as “hobbits” — have such an unusually short stature? Some scientists believe it may be the result of a controversial theory known as island dwarfing, in which living on an island causes certain animals to shrink in size.

Hominins aren’t the only animals thought to experience island dwarfing. Isolated on the tropical Flores, Homo floresiensis likely hunted animals such as small pygmy elephants for food. If you had roamed the Mediterranean island of Cyprus around 10,000 B.C., you may also have come across Hippopotamus minor, a species of dwarfed hippopotamuses that stood about two and a half feet tall and measured around four feet long. Or, if you had visited the island of Crete, you might have seen Mammuthus creticus, the Cretan dwarf mammoth, a species that stood a little over three feet tall.

“Classic observations indicate that large animals tend to decrease in size when evolving in insularity, whereas small animals tend to increase,” said Aida Gomez-Robles, an anthropologist at George Washington University. Because island ecosystems typically have fewer predators than mainland ecosystems, she explained, larger animals may start to shrink, making it so they require fewer resources to survive. (Homo floresiensis’ smaller stature may have been advantageous for this reason.) By contrast, smaller animals get larger, making them more metabolically efficient, or better able to use fat as an energy source.

Many famous examples of the island rule come from the very same island where H. floresiensis once flourished: Along with the small hominins and elephants, Flores was home to oversize rats measuring 16 to 17 inches long, as well as the Komodo dragon, a species of lizard that can grow up to ten feet and weigh more than 300 pounds.

Despite these examples, though, the island rule is hotly disputed. A 2007 paper from the Proceedings of the Royal Society, for instance, claimed that “there is no evidence for the existence of the island rule,” and that the patterns observed by past scientists were more nuanced than any universal rule would allow for. Instead, the authors argued, the size of island-dwelling species is determined by the particular components of each island’s ecosystem and the characteristics of the species in question.

The researchers traced the origins of the island rule back to a 1964 paper by J. Bristol Foster, who first quantified the differences in body size of island and mainland mammals of a closely related species. Foster showed that rodents often grow larger on islands, while carnivores and certain larger mammals often grow smaller. After publication, though, other scientists took his results to be evidence for a larger pattern, arguing that his work illustrated that the pattern held more generally across all island mammals. (The term “the island rule” was coined in 1973 by evolutionary biologist Leigh Van Valen, who used it in his paper “Pattern and the Balance of Nature.”)

“The overall evidence is not as strong as it might appear when you first package it in a neat narrative,” said Matt Tocheri, a researcher at the Smithsonian Institution’s Human Origins Program. The same evolutionary processes that power the island rule, he continued, can also happen on the mainland whenever certain ecosystems become isolated from others.

Even the origin of Homo floresiensis, one of the island rule’s most compelling pieces of evidence, is still not certain. Robles noted that while many scientists favor the hypothesis that Homo floresiensis evolved from the larger Homo erectus, shrinking once the species was isolated on an island, other theories propose that Homo floresiensis evolved from other hominins that were already small, such as Homo habilis, or from even more small and primitive Australopithecus-like hominins. After all, our human ancestors and relatives were much smaller than modern humans. It’s possible their short stature was typical of the time, rather than a product of their ecosystem.

Tocheri agreed, noting the lack of fossil evidence that supports the island dwarfing of Homo floresiensis. To confirm that this particular species shrunk on islands, we would need not only specimens of Homo floresiensis, but specimens of the hominins they descended from. That would require both a rich discovery of fossils and a careful analysis of how such fossils relate to one another in the geological time scale. And as Tocheri emphasized, we still don’t have a strong understanding of what early hominins in southeast Asia looked like — it may be that even those on the mainland evolved to be much smaller than some scientists have assumed.

In other words, while there’s plenty of evidence that suggests some species change size on islands, it’s not yet enough to constitute a “rule.” This is especially true when it comes to the question of how island life affected our species’ distant relatives. “Knowing is generations away,” Tocheri said, adding that there’s still too much scientists don’t know about H. floresiensis to understand how they fit into the island-rule hypothesis. “We all want to have that narrative, but we also have to be honest that we may never have that narrative in our own lifetimes.”

Why Humans Shrink When They Live on Islands