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Island dwarfism in mammals |
 When populations of animals become isolated on an island, large species often evolve to become smaller, and small ones evolve to become larger. This is known as Van Valen’s island rule.Other unusual trends in island populations, such as the parallel evolution of parts of their skeleton, the possession of proportionately large teeth, and more controversially, the apparent reduction in the size of their brain, continue to puzzle scientists. In large part this is because most examples of dwarfed large mammals from islands are now extinct and are known only from fossils. Although these fossils can be incredibly abundant, they are usually only represented by isolated bones or teeth and partial skulls, making it impossible to perform detailed analyses of changes in bodily proportion such as brain size. It can also be difficult to determine what the mainland ancestor of some of these dwarf mammals was, as some dwarf mammals are believed to have been isolated for several million years. One exception however is the island of Madagascar, where fossils of recently extinct pygmy hippos, giant lemurs and other so-called megafauna are exceptionally preserved. Also, the arrival of the hippopotamus on the island is believed to be relatively recent. My work on these dwarf hippos has yielded new insights into the phenomenon of island dwarfing more generally. For example, it may help interpret the dwarf human Homo floresiensis discovered on the Indonesian island of Flores in 2004, and its possible evolutionarily divergent relationship to modern man. |