Body size is an important determinant for many biological functions and characters closely associated with fitness (e.g., Peters, 1983; Calder, 1984). Many of the genes that determine traits like body size probably are invariant in a population (Fisher, 1930) and have low heritability due to past selection. Indeed, body size in mammals has been shown to have low heritability (Falconer, 1989). Body size also tends to be quite responsive to changes in certain environmental factors that in turn serve as the ultimate sources of selection (Falconer, 1989). Studies of animals that have changed body size rapidly in the fossil record could be useful to an understanding of evolution in response to variations in paleoecology.
White-tailed deer (Odocoileus virginianus) have varied greatly in body size through space (Rees, 1969; Koch, 1986) and time (Purdue, 1986, 1989, 1991). In the American Midwest, the shifting size through time was associated with changes in climate and vegetation (Purdue, 1989, 1991). Small deer prevailed during the warm and dry middle Holocene period that produced prairie expansion (~8.5–4 ky BP). In the late Holocene, climatic conditions became more mesic, and deer became large.
Late Holocene environments on the coastal plain in the Southeast contrasted sharply with those in the Midwest (Delcourt and Delcourt, 1985, 1987; Webb, 1988). Southern pine forests began to replace oaks and hickories after 8 ky BP, with the trend continuing through the late Holocene (Delcourt and Delcourt, 1985; Webb, 1988). Other environmental changes were also evident in the late Holocene, in particular the development from 4 ky BP to the present of the active floodplain of the Savannah River in the upper coastal plain (Brooks and Sassaman, 1990).