Abstract. As stated by the island rule, small mammals evolve toward gigantism on islands. In addition they are known to evolve faster than their mainland counterparts. Body size in island mammals may also be influenced by geographical climatic gradients or climatic change through time. We tested the relative effects of climate change and isolation on the size of the Japanese rodent Apodemus speciosus and calculated evolutionary rates of body size change since the last glacial maximum (LGM). Currently A. speciosus populations conform both to Bergmann's rule, with an increase in body size with latitude, and to the island rule, with larger body sizes on small islands. We also found that fossil representatives of A. speciosus are larger than their extant relatives. Our estimated evolutionary rates since the LGM show that body size evolution on the smaller islands has been less than half as rapid as on Honshu, the mainland-type large island of Japan. We conclude that island populations exhibit larger body sizes today not because they have evolved toward gigantism, but because their evolution toward a smaller size, due to climate warming since the LGM, has been decelerated by the island effect. These combined results suggest that evolution in Quaternary island small mammals may not have been as fast as expected by the island effect because of the counteracting effect of climate change during this period.Key words. Apodemus, climate change, evolutionary rates, incisor, island rule, Japan, size.Received December 15, 2003. Accepted January 27, 2004 Populations that evolve on islands often provide remarkable examples of size evolution. Among island mammals, it is frequently found that small species evolve toward a larger size and large species toward a smaller size (Foster 1964;Van Valen 1973;Lomolino 1985), and this tendency has come to be known as the island rule (Van Valen 1973). The dwarfing of large mammals on island is remarkably well illustrated by Pleistocene elephants (Sondaar 1977;Roth 1992), hippopotami (Simmons 1988), deer (Lister 1989, and sloths (Anderson and Handley 2002). Similarly, the trend toward gigantism of island rodents has been observed in numerous extant species (Hall 1938;Delany 1970;Heaney 1978;Vaughan and Schwartz 1980;Musser 1981;Lawlor 1982;Melton 1982; Davis 1983;Musser and Newcomb 1983;Angerbjö rn 1986;Collins and George 1990;Musser and Heaney 1992;Smith 1992;Libois et al. 1993;Vigne et al. 1993;Adler and Levins 1994;Adler 1996; Berry 1996;Ramalhinho et al. 1996;Olmos 1997) as well as among some fossil species (Crusafont-Pairo and Petter 1964;Freudenthal 1976Freudenthal , 1985Brandy 1978;Mein and Adrover 1982;Daams and Freudenthal 1985;Agusti 1986;Hutterer et al. 1988;Vigne 1992;Biknevicius et al. 1993;Mezzabotta et al. 1996;Michaux et al. 1996;Millien and Jaeger 2001). However, not all island rodent populations show strong body size differences from mainland populations (Foster 1964; Case 1978;Lomolino 1985). The strength of the island effect on size evolution is usually presumed to be rou...