Integrating nutritional mutualists into the evolution of defense

Historically, mutualisms have been categorized by their benefits: nutrition, defense, or transportation. However, many nutritional mutualists have secondary effects on defense. In this review, we propose that nutritional mutualists are major, overlooked drivers of defense evolution and identify four distinct mechanisms by which they can affect the evolution of defense traits. Direct tests of all four mechanisms are scarce. We argue that this is because most work has focused on mutualist effects on trait expression rather than on the parameters that govern evolution: genetic variances, genetic correlations, and natural selection. We highlight new questions that this perspective brings into focus and outline experiments to test them. Finally, we propose that overlap in mutualism and defense resource budgets can unite mutualism and defense theory.