Dwarf elephants and mammoths

No account of proboscidean evolution would be complete or accurate without mention of the remarkable reversal in body size that occurred during the Pleistocene. Several species of elephants (including mammoths) isolated on islands during the fluctuating sea levels of the Pleistocene underwent rapid dwarfing. This phenomenon was not necessarily confined to the Pleistocene, but may have occurred much earlier in the Southeast Asian islands, although the evidence is fragmentary. There are some clear examples from these islands for Pliocene dwarf proboscideans. The best-studied examples of dwarf proboscideans, however, come from islands off the Californian coast and those in the Mediterranean during Pleistocene times (fig 1.6).

Many of the Southeast Asian islands (Java, Sumatra, Borneo) have been repeatedly connected to or isolated from the mainland, depending on past sea levels. Other islands, such as the Philippines, Sulawesi (Celebes), Flores, Sumba, and Timor, have apparently remained isolated, but the surrounding sea was not a barrier to proboscideans swimming to these islands. The remains of a dwarf proboscidean from Sulawesi dating to the late Pliocene were described by Dirk Hooijer as Archidiskodon celebensis. This species was later recog-

Figure 1.6

A model of the dwarf elephant Elephas falconeri found during the Pleistocene in certain islands of the Mediterranean. (Photo courtesy of A. M. Lister.)

Figure 1.6

A model of the dwarf elephant Elephas falconeri found during the Pleistocene in certain islands of the Mediterranean. (Photo courtesy of A. M. Lister.)

nized as belonging to Elephas and hence was named E. celebensis. Vincent Mag-lio considered this to be derived from a dwarfing of E. planifrons, but others think it also was likely to have descended directly from the more primitive Stegotetrabelodon. The advanced characters of E. celebensis, such as the short, high skull or high-crowned molars, are consistent with either an E. planifrons origin or a parallel evolutionary trend from a more primitive ancestor. Interestingly, a dwarf Stegodon is also known from the mainland Javan fauna of Kedung Brubus during the middle Pleistocene, while another Stegodon of large size or mainland proportions is known from the island of Flores. Dwarfing of proboscideans in Southeast Asia thus demands explanations that go beyond habitat insularity and probably are related to factors such as time of isolation, habitable area, vegetation, predation, and so on.

A more straightforward "island effect" explanation is clear for other instances of dwarfing. Several islands in the Mediterranean have shown such dwarf proboscideans. The best known among these is Elephas falconeri from the islands of Malta and Sicily, dating to at least 500,000 years ago when the two islands were joined. Believed to be derived from E. namadicus (or E. anti-quus as the European branch of E. namadicus is sometimes named), E. falconeri stood only 1 m tall as an adult and is the smallest known elephant. In comparison to body size, the molars of E. falconeri were proportionately larger than in E. antiquus. A second arrival of E. antiquus here during the later Pleistocene gave rise to a somewhat larger dwarf named E. mnaidriensis. A dwarf mammoth is known from the island of Sardinia, while some dwarfs from Cyprus were descendants of E. antiquus. Other dwarfs of uncertain affinities have been discovered in Crete, Cyclades, and Dodecanese.

The Channel Islands off the coast of California have the best example of dwarf proboscideans in the New World. During the late Pleistocene, the Columbian mammoth M. columbi seems to have colonized these islands by swimming across the 30-km oceanic separation from the mainland rather than using a land bridge. The resulting dwarf mammoths, M. exilis, found on these islands, particularly on Santa Rosa, typically stood 1.2-1.8 m (4-6 feet) tall as adults. Allometric scaling suggests body weights between 200 kg and 2 tons compared to 5-10 tons for the Columbian mammoth. These survived until the very late Pleistocene, about 12,000 years ago.

An amazing discovery was reported from the remote Wrangel Islands of the Siberian Arctic in 1993 by S. L. Vartanyan, V. E. Garutt, and A. V. Sher. The woolly mammoth had not become extinct by the end of the Pleistocene, 10,000 years ago, as earlier believed, but survived as a reduced form until as recently as 4,000 years ago. Normal-size woolly mammoths also lived here 12,000 years ago when Wrangel was still connected to Siberia. Rising sea levels at the end of the Pleistocene cut off Wrangel from the mainland. By about 7,000 years ago, the mammoth had reduced in size to about 1.8 m (6 feet). Over a 5,000-year period, or about 100 mammoth life spans, the height of the species was reduced by over 40% and there was an even greater reduction in body mass, an extremely rapid evolutionary transition, if not as extreme as that of the Mediterranean and the Southeast Asian species.

Several explanations are possible for this dwarfing of elephantids on islands. Often, an island has not been colonized by a large predator or is too small to hold a viable predator population. Once it is free from predation pressure, large body size is not of much advantage to a herbivore. An island habitat has limited food resources and also increases competition. A smaller body size and a need for fewer resources would thus be favored. Interestingly, the island rule is reversed for small mammals such as rodents, for which gigan-tism is favored under insular conditions.

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