Pheidole in the far north: How ants with tropical ancestors overcame the perils of winter

The ant genus Pheidole is hyper-diverse with more than 1,100 described species. Yet, fewer than 10 species occur at cold latitudes above 42° N. We studied the annual cycle and function major workers in Pheidole pilifera in an area with high seasonality and harsh winters (Massachusetts, USA). Unlike their neotropical congeners that produce brood continuously, P. pilifera exhibit a quasi-heterodynamous brood cycle, overwintering with a cohort of feeding-stage larvae that later pupate during summer. Colonies excavate seasonally dynamic nests, often exceeding 250 cm in depth, where they store seeds and actively feed larvae during an 8-month cold season. These long-lived larvae play a key role in colony survival from November to June. Minor and major workers occur in a predictable 8:1 ratio within colonies, independent of colony size. Our experiments revealed that large-headed major workers 1) specialize on carrying large gravel and soil particles during nest excavation, 2) open more seeds and larger seeds than minor workers, 3) gain weight disproportionately prior to overwintering, and 4) likely act as seasonal repletes, losing more body fat than minors. Our study demonstrates how specialization of morphological castes, seed storage, nest architecture, and demography can be modified to overcome the challenges associated with a compressed warm season. Other winter-adapted ant genera, like Myrmica, combat freezing temperatures with a different set of developmental and physiological strategies, often requiring larval dormancy and cold exposure to later induce pupation. Pheidole pilifera have solved the ‘problem’ of overwintering in another way entirely.