Colony size as a buffer against seasonality: Bergmann's rule in social insects

In eusocial species, the size of the superorganism is the summed sizes of its component individuals. Bergmann's rule, the cline of decreasing size with decreasing latitude, applies to colony size in ants. Using data from the literature and our own collections, we show that colony sizes of tropical ant species are about one-tenth the average size of temperate species. This pattern holds when species or genera are sample units. Further, this trend is shown in 17 of 19 genera and five of six subfamilies. Bergmann's rule may arise if seasonal famine favors larger organisms, given their increased energy reserves. We constructed three colony sizes of the ant Solenopsis invicta. We deprived these colonies of food, or food and water. Queens, when surrounded by 102workers or 104workers, survived longer than solitary queens. When deprived only of food, days of queen survival had an allometry of M0.21(where M is mass), not significantly different from the predicted M0.25for unitary organisms. We propose that shorter growing seasons in the temperate latitudes cull small-colony species through overwintering starvation, which contributes to Bergmann's rule in social insects.