Sexual Size Dimorphism Does Not Change Systematically in Latitude/Longitude Gradient, but its Standard Deviation Declines Significantly

The clarifying ecogeographical rules (Bergmann's, Rensch' etc.) contributes greatly into evolutionary theory and elucidation of mechanisms in animals adaptation to environmental factors, including climate impact. We have verified the theory about the greater steepness of body size changes in males than in females in geographical gradients (Blanckenhorn et al., 2004; 2006). The advantage of our study was that we sampled and measured beetles ourselves (not complying the data from published papers) using the single methods.

We took the data set which had been used in our earlier paper on intraspecific beetles body size variation in latitude gradient (Sukhodolskaya et al.) and supplemented it with the data from other regions from Russia and abroad. In total 9 samples of ground beetle Pterostichus oblongopunctatus Fabricius, 1787 were studied from regions differing in location at 4 degrees in latitude and 57 degrees in longitude. Sexual size dimorphism (SSD) was estimated in 6 morphometric traits. In total 3847 individuals were measured. We used RMA II additionally to estimate females and males sensitivity to environmental factors.

SSD in different traits changed sometimes significantly in investigated gradients. But its mean values for all 6 traits together had the same values in all regions. In the regions where beetles size was the smallest, males were more sensitive to environment. And vice versa: in the regions with the larger individuals SSD was female-biased and females were more sensitive to environment. Standard deviation in SSD values declines significantly towards the north and towards the east.