Ecology and evolution of offspring size in Pogonomyrmex colony founding

Mortality scales with individual size in many organisms. In social insect colonies, mortality peaks early in colony development, when colonies are young and small. The workers that compose these new colonies are extremely small individuals, called nanitics. I assume a size-based mortality schedule for social insect colonies and investigate the extent to which production of nanitics and other aspects of early colony development could be adaptations to the aforementioned mortality schedule. I ask specifically (1) whether temperature, food availability, and the social environment limit colonies in the production of their first brood of workers, (2) whether the size and number of workers in a colony’s first brood affect the colony’s growth rate, (3) whether adult worker size affects the production of a batch of new workers, and (4) whether the number of workers in the colony’s first brood affect a colony’s probability of victory in direct competition with another new colony. The absence of adult workers in a new colony limits the size and number of workers that the colony can produce. Colony growth in the first 8 weeks beyond the first clutch is limited by the size of the first clutch; colonies with larger first clutches grow more quickly. Adult nanitics may produce new workers as quickly as do adult non-nanitics. First clutch size does not affect the probability of colony survival in direct competition with other new colonies in the lab. Together these results suggest that early colony development has been shaped by selection for fast colony growth. Production of nanitics (1) may enable production of relatively large first clutches, which may accelerate colony growth and (2) seems to confer little if any cost in brood care. That colonies normally stop producing nanitics after colony founding could simply reflect an increase in the availability of resources for worker production, such that both the size and number of workers produced in a clutch increase with the same overall allocation pattern among worker size and clutch size as in new colonies. It is also possible that selection favors increased investment in worker size in later stages of colony life.Biology and Biochemistry, Department o