EFFECTS OF ENVIRONMENT ON PHENOTYPIC VARIATION AND SEXUAL DIMORPHISM IN DIPODOMYS SIMULANS (RODENTIA, HETEROMYIDAE)

The magnitude of sexual dimorphism in size among populations of some s mall mammals Living in different habitats may be a reflection of the d egree of habitat partitioning between sexes or nutritional quality of available food. We tested these hypotheses by quantifying between-sex differences in size in populations of the Dulzura kangaroo rat (Dipodo mys simulans) from several macrohabitat types at regional and local ge ographic scales. On a regional scale, our results indicated a signific ant shift in morphology of males and females along a north-south latit udinal gradient. Large kangaroo rats inhabited warm and arid southernm ost latitudes, whereas small animals inhabited relatively cool and moi st regions to the north. This morphologic gradient represented an incr ease in size with less seasonal variation in precipitation and tempera ture. This pattern was clearly counter to Bergmann's rule, but consist ent with the hypothesis that temporal variation in availability of foo d may be an important factor leading to geographic variation in size. External and cranial measures of size dimorphism were not correlated s ignificantly with any major geographic factor and only cranial measure s were associated significantly with variation in climate, At a local level, kangaroo rats living in different vegetation communities within the same latitudinal zone also exhibited significant sexual dimorphis m in size; however, macrohabitat heterogeneity appeared to affect male s and females equally. Both sexes were larger in Sierran Montane Conif er Forest and Californian Chaparral macrohabitats. Smaller animals occ upied Californian Grassland and Coastal-scrub, and Vizcaino vegetation communities. This pattern of variation was observed in all three-way comparisons and suggests the possibility of habitat mediated phenotypi c responses. Finally, only one of eight comparisons involving kangaroo rats Living in adjacent plant communities showed a significant two-wa y interaction between dimorphism and macrohabitat type. Our results, t herefore, did not provide strong evidence to substantiate the hypothes es that the magnitude of dimorphism among populations of D. simulans L iving in different macrohabitats is a reflection of the degree of part itioning between sexes or nutritional quality of available food.