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ABSTRACT
The spatial and temporal size variation of Meriones tristrami (Rodentia: Gerbillidae) over the geographic range of Israel and northern Sinai and during the late Middle and Upper Quaternary of Israel was correlated with environmental factors. The allopatric fossil populations were found to fluctuate in accordance with Bergmann's rule (i.e. the maximum body size was obtained during colder phases). Recent populations did not show north—south body size gradient or high correlation with climatic factors. A sudden shift in body size of M. tristrami is observed when sympatry with its congener M. sacramenti occurs in the Coastal Plain, and another (and smaller) shift is noticeable when sympatry with a third congener (M. crassus) takes place in the Negev region. The allopatric convergence and sympatric divergence suggest an ecological interaction (character displacement) among the congener species. A significant north—south increase in the relative volume of bulla tympanica of M. tristrami is shown, with the mastoid portion playing the main role in geographic variation. The bulla volume is highly correlated with climatic factors and shows no divergent shifts within the sympatric region. It is suggested that in fossil populations the bulla tympanica can be used for evaluating palaeoclimatic fluctuations.