Co-option of worker organs as indicators of superorganism complexity: Short title: Co-option of worker organs in ant superorganisms.

Ant colonies are superorganisms containing a wingless, nonreproductive, worker caste. The suppression (loss or reduction) of worker organs involved in mating and reproduction is a feature of many ant superorganisms. It is largely assumed that these worker organs, where present, are functionless and in the process of being eliminated through neutral degeneration or selection. Here, we argue that an underappreciated dimension of social evolution involves the co-option of worker organs to serve superorganism-level functions. We draw on two examples: the wings and ovaries. In some ant species, transiently appearing wing rudiments have been co-opted to regulate worker differentiation into soldiers. Likewise, in many ant species, the ovaries in adult workers have been co-opted to produce trophic eggs. We propose that the degree of organ co-option may itself be an indicator of superorganismal complexity. We discuss the broader implications of this idea and suggest novel avenues for future work.