The nation's smallest fox lives on an island. So does the world's largest tortoise. And until 13,000 years ago, so did the world's smallest humans.
Biologists say it's no coincidence.
The forces of evolution create island creatures that are very different from their continental cousins. Animals that migrate to islands often shrink or grow over time in response to pressures from predators, food supplies and competition for mates.
Known as the island rule, it's a concept that biologists developed in the 1960s. "You have entirely different habitats - sometimes there are no predators," said Virginie Millien, a paleoecologist at McGill University. "The evolutionary forces become accelerated on islands."
The rule - actually more of a theory - may explain the unusual size of some animals. They range from California's 3-pound Channel Island fox to the 500-pound Galapagos tortoise and the 3-foot-tall, hobbitlike humans whose remains, discovered in an Indian cave, were disclosed in a report last month.
Discovery of the extinct species of human, the smallest known, is the first conclusive evidence that prehistoric people were as susceptible to the island rule as other animals, experts say. Scientists from Australia and Indonesia described the discovery last month in the journal Nature.
"There was nothing surprising about finding another dwarfed mammal on an island. But I was shocked they had found a dwarfed human," said Lawrence R. Heaney, curator and head of mammals at the Field Museum in Chicago.
Explaining the rule
Given time and natural selection, the island rule means that animals smaller than squirrels will sometimes grow larger on islands and animals larger than squirrels may shrink. Squirrel-size animals will usually stay the same size.
Biologists say large animals probably shrink over time because food supplies are more limited than in continental habitats. The small animals may grow because they have fewer predators and less competition for food.
"We still aren't exactly sure," said Mary V. Ashley, an evolutionary biologist at the University of Illinois at Chicago.
The process requires generations of isolation, and modern humans are probably too well-traveled for natural selection to create any 3-foot-tall tribes. But the rule may still apply to other animals, experts say.
"It's an ongoing thing. Things are going extinct and colonizing islands all the time," said Eric P. Palkovacs, an expert on evolutionary biology at Yale.
