Complex determination of queen body size in the queen size dimorphic ant Leptothorax rugatulus (Formicidae: Hymenoptera).

In order to understand the evolution of natural variability, and polymorphisms in particular, it is essential to study proximate causes. Our study is the first work on ants to determine formally the heritability of quantitative traits in a quantitative genetic framework. We investigated the causes of queen size dimorphism of the ant Leptothorax rugatulus and derive from the results a possible scenario for its evolutionary maintenance. Mother size was highly predictive of daughter size in field colonies. This finding could be repeated under constant laboratory conditions. Data suggested that maternal effects via egg size are not the cause for the transmission of body size. In colonies with coexisting large and small mother queens, daughter size did not correlate with mother size, and in an additional experiment we found a negative effect of queen number on daughter size. The integration of these various results suggests a high transmissibility of body size from generation to generation. However, social (queen) influences also affect daughter size, especially in the case of mixed colonies. This complex determination fits well with an adaptive adjustment of queen size to alternative reproductive strategies.