Effects of altitude and canopy cover on the nest size and colony size of the red wood ants Formica lugubris and Formica paralugubris

Variations in life-history characteristics across geographic gradients may have implications for the impact of environmental change on animals. Linking one of the most important life-history characteristics and a geographic gradient, Bergmann’s rule describes body size increase with increasing latitude. Due to comparable thermal patterns between latitude and altitude, a similar process is expected to apply across altitude. For social insects, the colony could be biologically analogous to the body of a unitary organism. This study investigates the relationship between altitude and colony size in social insects. The model species used were wood ants Formica lugubris and F. paralugubris. These species have a flexible nesting strategy known as polydomy. I therefore considered both nest size and colony size. Initially, I developed an accurate mark-release-recapture method to estimate nest size, and found that mound volume can be a useful nest size index. A detailed case-study focused on canopy cover effects and showed that nests were larger in shadier areas. Informed by the results, I finally assessed the relationship between altitude, canopy cover, polydomy, nest size and colony size. The results reveal that colony size follows Bergmann’s rule along altitude when canopy cover is controlled for: microclimatic factors can be more significant than geographic factors in determining colony size. A systematic review in the Appendix shows that F. lugubris populations in different locations differ in mean nest size, but shows no evidence of a trade-off between nest size and multi-nest organisation. This thesis not only provides the first intra-specific evidence of Bergmann’s rule acting at the colony level across altitude, but also indicates the prominent role of microclimate on a key life-history characteristic. The work therefore sheds light on the evolution of an eco-geographic cline and the effects which climate change may have on the cline.