ISLAND AREA AND BODY SIZE OF INSULAR MAMMALS: EVIDENCE FROM THE TRI-COLORED SQUIRREL (CALLOSCIURUS PREVOSTI) OF SOUTHEAST ASIA.

Abstract

The presence of Pleistocene elephants on Crete only 1 m in shoulder height, of giant rodents on Caribbean Islands, and of other equally remarkable dwarf and giant mammals, has prompted a variety of responses from their startled discoverers. Most of the mechanisms suggested for their origins are simplistic-usually a single factor, ranging from extreme food deprivation to isolation of antediluvian species. To my knowledge, no one has described adequately either the pattern of variation of mammals on islands, or synthesized the many suggested causes (Van Valen, 1973). This paper is an initial attempt to redress this deficiency. It is not intended as a thorough review; rather, it presents significant new data, briefly summarizes trends in body size of mammals, and presents a causal model of body size evolution in insular mammals. It is possible that the processes and patterns discussed here apply to other animals but I have directed my studies to maimmals alone. Each topic discussed below is worthy of a lengthy review, and it is my hope that this paper will stimulate additional research and discussion. During a study of the systematics of Callosciurus prevosti, the tri-colored squirrel of Malaysia, Indonesia, and peninsular Thailand, it became evident that body size of these squirrels was related to the size of the island on which the squirrels lived. This observation stimulated a more detailed study of size variation in these squirrels and led to the present paper. Body size of C. prevosti is discussed at the beginning of the paper, because variation in this species provided insight into the evolution of body size of insular mammals and formed the basis of my hypotheses and models. The biology of C. prevosti is poorly known. Externally tri-colored.squirrels resemble the familiar North American tree squirrels but are more brightly colored. Most populations have a black dorsum and tail, a broad white lateral stripe, and a red venter; many have a prominent shoulder patch which may be black, gray, red, white, or a mixture of these. In Selangor, Malaysia, tri-colored squirrels nested in trees and fed on coconut and rubber seeds, and oil palm fruit (Harrison and Traub, 1950; Harrison, 1962). Davis (1962) stated that in Sabah, North Borneo, they were found in primary and old logged forest where they inhabited chiefly the smaller trees of the middle story. Stomach contents consisted of pulpy fruit and about 5% insect matter, chiefly ants and termites. Three pregnant females taken on 20 June, 24 June, and 2 August had two, two, and three embryos, respectively. At Bukit Lanjong, West Malaysia, they have been seen feeding on the fruit of fig trees (Ficus) in the upper story (MacKinnon and MacKinnon, 1974:36). Nothing definite is known of the predators or competitors of these squirrels. ' Parts of this study were carried out while the author was resident at the Division of Mammals, Smithsonian Institution, Washington, D.C. 20560, and the Bell Museum of Natural History, University of Minnesota, Minneapolis, Minnesota 55455.