The Island Rule
Foster’s rule (also known as the “island rule”) is a hypothesis that mammals change their size on islands depending on resources available in their environment.
In 1964, a biologist named J. Bristol Foster documented the tendency for mammal populations on islands to have body sizes that are different from their mainland counterparts. Specifically, large mammals seem to shrink (“dwarfism”), while small mammals appear to enlarge (“gigantism”). This initial observation led to the formulation of ‘the island rule’ hypothesis, which states that, on an isolated, insular environment where large predators are limited or absent, small mammals would become bigger to exploit a wider range of resources, move greater distances, enhance metabolic efficiency, and better fend off competitors. On the other hand, larger mammals would reduce in size to limit their food requirements and increase their reproductive rate, to better cope with environmental changes and stressful conditions.
While scientists today agree on some general conclusions of the island rule, many studies have demonstrated exceptions where island animal populations do not obey this rule. In the case of Flores, because of the continued isolation by sea, animal taxa with sea-crossing abilities (such as elephants) are over-represented in the island while mammalian carnivores and other ungulates are absent. An island the size of Flores would simply not be big enough to carry a population of tigers, even if they would have been able to reach the island by crossing the sea straits separating Flores from the Asian continent. In no time these large carnivores would have depleted their prey species. That is why the niche of top predator is often filled by reptiles on islands, in the case of Flores Komodo dragons and crocodiles. Because reptiles are cold-blooded, they require less food and equilibrium between predator and prey populations can establish. The Flores fauna also show a strong phylogenetic continuity, meaning that the immigration of new species into the environment over time has been minimal.
Some of the Pleistocene vertebrate faunal communities do exhibit features that follow the island rule. For instance, Stegodon florensis appears to have downsized on Flores between about 700, 000 and 100, 000 years ago. Among the faunal remains recovered from Liang Bua, there is evidence of a dwarfed Stegodon subspecies (Stegodon florensis insularis) with a body size 30% smaller than their Middle Pleistocene ancestors on the island. There are also remains of several endemic species of giant rats (e.g., Papagomys armandvillei, theodorverhoeveni, Spelaeomys florensis). These body size changes in some mammals can be accompanied by adaptation in limbs through shifts in locomotion. In the absence of predators, animals may show a tendency towards a reduction in the relative length of limbs and also the fusion of elements. Such changes would result in shorter and more robust limbs that would have provided greater stability (at the cost of reduced speed) and lower energy expenditure during locomotion.
It is possible that Homo floresiensis was subjected to similar evolutionary forces to those responsible for the island characteristics of certain animal populations on Flores. Based on skeletal features, Homo floresiensis has been hypothesised to be a descendant of large-bodied hominin ancestors (e.g. Homo erectus) and subsequently evolved through dwarfing as a result of long-term isolation on the island. While the relatively short legs, robust limb bones, and long feet of Homo floresiensis have been suggested to represent primitive traits associated with an ancestry related to earlier Australopithecus hominins, other researchers have argued that these skeletal features can in fact be explained by island adaptation. Specifically, these changes would suggest a shift in locomotion that provided greater stability, which might be favoured in a resource-restricted and predator-free environment.
© University of Wollongong