The relationship between nest queen frequency, worker number and month of sampling, in the ant Leptothorax tuberum where both queen frequency and worker number reach a maximum during December and January and a minimum during the summer, supports the thesis of seasonal polydomy, where nests coalesce in the winter and fragment in the summer. Coalescence was found to occur randomly, between nests, in the laboratory whereas in the field coalescence seems to occur genealogically. Coalescence in the laboratory appeared to be facilitated the closer to the winter solstice the nests were sampled in the field and more so when one or both nests tested did not have a queen. From these observations we provisionally conclude that a proximate cause of coalescence is day length. We also report a possible role for thermoregulation as the ultimate cause of seasonal polydomy and cooperative behavior in this species with seasonal temperature variation as an additional proximate cause of this condition. Furthermore the seasonal coalescing and fragmenting of nests may explain a phenomenon, previously recorded in this species, at present uniquely in ants, of a form of intra specific parasitism known as egg dumping.