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Island Area and Body Size of Insular Mammals: Evidence from the Tri-Colored Squirrel (Callosciurus prevosti) of Southeast Asia

Lawrence R. Heaney
Evolution
Vol. 32, No. 1 (Mar., 1978), pp. 29-44
DOI: 10.2307/2407408
Stable URL: http://www.jstor.org/stable/2407408
Page Count: 16
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Island Area and Body Size of Insular Mammals: Evidence from the Tri-Colored Squirrel (Callosciurus prevosti) of Southeast Asia
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Abstract

Callosciurus prevosti occurs throughout Malaysia and much of Indonesia; it is generally restricted to the Sunda Shelf, a continental shelf underlying the Greater Sunda Islands, the Malay Peninsula, and adjacent archipelagoes. The islands on which it has been found range in size from 4-7.5 × 105 km2. Earlier studies have found that the Sunda Shelf was exposed as dry land during the Pleistocene; this allowed terrestrial mammals including C. prevosti, to reach the islands by overland dispersal and suggests geographic isolation for about 5,000 yr. The body size of tri-colored squirrels is related to the size of the island on which they live; the smallest squirrels occur on the smallest islands and increase in body size on islands up to about 104 km2. On islands larger than this, body size decreases significantly as island area increases. The trends in body size changes of island-dwelling mammals are reviewed. The tendencies of small mammals to increase and of large mammals to decrease in size as island area decreases are interpreted as a single phenomenon. A model is presented which predicts that body size plotted against island area for a mammal occurring on a series of islands of varying sizes will form a curve, if the following conditions are met: all islands connected to a continental land mass at approximately the same time in the past, equivalent climate, no gene flow, and no special selective pressures. The position of the curve is determined by its apex, i.e., the greatest body size of the species. The curve may be positioned in such a way that only one side is apparent; this is to be expected for most species. Predation, food limitation, interspecific competition, and selection for physiological efficiency, are thought to be major factors in determining the body size of these squirrels and of other mammals. Each of these factors is thought to be related to island area and to be of varying importance on different sizes of islands.

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