Intra-specific Body Size Variation of Ground Beetles (Сoleoptera: Сarabidae) in Latitudinal Gradient

Abstract

Background. Large-scale patterns of body size variation have occupied biologists for over a century, yet the causes of some of these patterns remain elusive. Bergmann’s rule is a classical eco-geographic rule that relates body size of homoiothermal animals with environmental temperature (or latitude). Contemporary data indicate that latitudinal clines in body size, predicted by Bergmann’s rule also exist in ectothermic organisms and in some groups appear to be fairly common. Despite plenty of data from literature on latitudinal intraspecific body size variation in insects are scarce and in Ground Beetles are absent at all. The aim of this research was to: (i) model the effect of the region of habitation into the body size variation in widespread carabid species; (ii) describe body size variation of mentioned species in latitudinal gradient.

Materials and methods. The design of our study was organized as follows: (1) we study the intraspecific variation of body size in carabids, bearing in mind that closely related species could act very differently, each individual species following or countering Bergmann's Rule in its own way; (2) in all studied species the same six traits were measured; (3) we used linear models to isolate concretely latitude effects (but not another environmental factors) to traits variation; (4) beetles were sampled in the same space over the vast regions covering their ranges. Six carabid species from different regions of Russia were analyzed. Linear models were applied to quantify contribution of region’s latitude where beetles had been sampled into that variation. We applied voice-counting method to reveal trends of traits size variation in latitude gradient and relative warp analysis (a principal component analysis of the weight matrix) when analyzing effect of latitude on body shape shifts in studied species of carabids.

Results. Different traits of certain species varied in differing ways under the influence of the same environmental factor. Contribution of latitude into the traits variation was not similar in studied species. The compiled data over the all six traits variation in latitude gradient in each species showed that   C. granulatus followed Bergmann's rule, P. cupreus – converse Bergmann's rule, in C. cancellatus, C. hortensis and P. melanarius the number of cases which followed direct Bergmann's clines, converse ones and non-significant shifts were equal, P. niger - didn't show any clines at all. Nevertheless the separate analysis of the only elytra length variation showed that in three Carabus species and P. niger elytra length decreased towards the high latitudes, P. melanarius demonstrated saw-tooth elytra length variation in latitude gradient and Poec. cupreus – the counter-gradient one.

Conclusion.

Explanation that takes into account the natural history, climatic correlations and sexual size dimorphism is needed to assess the observed contrasting geographic patterns and differences between species, morphometric traits and sexes, since size clines (e.g. Bergmannian clines) may obey to multiple selection pressures that are not only dependent on temperature constraints but also on other climatic and biotic factors that could influence body size.

Key words: carabids, environmental factors, voice-counting method, converse Bergman clines, saw-tooth variation, counter-gradient variation, linear models.