Superorganismal Anisogamy: a Comparative Test of an Extended Theory

Philip Ashley Downing , Jussi Lehtonen, Louis Bell-Roberts, Heikki Helanterä

Multicellular organisms and superorganisms (e.g., ant colonies) are both products of major evolutionary transitions in individuality, and they share many analogous traits. Theory developed to explain the evolution of one such trait, anisogamy, has recently been adapted to explain its superorganismal analogue: large egg-like queens and small sperm-like males. To test this theory with comparative data, we first extended it to incorporate variation in how colonies with multiple queens arise. We then used data from 732 ant species to investigate the effects of colony size, worker caste number, and queen number on queen size (thorax volume) and queen-male dimorphism. Queen size and queen-male size dimorphism both increased with colony size and number of worker castes, consistent with predictions. Contrary to predictions, queen size and queen-male size dimorphism were not associated with queen number. To further understand the commonalities and idiosyncrasies of evolution at different hierarchical levels, future work should consider the adaptive and non-adaptive causes for correlated evolution between queen and male, and egg and sperm sizes.