Rapid dwarfing of red deer on Jersey in the last interglacial.

The dwarfing of large mammals on islands occurred repeatedly in the Pleistocene. Elephants, deer, hippopotami and other species became dwarfed on islands in Indonesia, the Mediterranean, the east Pacific and elsewhere. In most cases, the full-sized ancestral form can be recognized among the adjacent mainland fauna, but evolutionary rates cannot be estimated because the entry of the ancestor onto the island, and appearance of the dwarf form, are poorly dated. Here I give the first example in which the island dwarf is well dated, the full-sized ancestor is found in demonstrably older deposits on the island, and a good estimate can be made for the duration of the isolation leading to dwarfing. In the Last Interglacial, red deer on Jersey, Channel Islands, became reduced to one sixth of their body weight in less than six thousand years.