PRODUCTIVITY IN A SOCIAL WASP - PER-CAPITA OUTPUT INCREASES WITH SWARM SIZE

We measured the productivity of newly-founded colonies of Polybia occi dentalis, a Neotropical swarm-founding social wasp, over their first 2 5 days. By both of the measures we used, number of nest cells built by the swarm and dry weight of brood produced, colony-level productivity was a significant positive quadratic function of the number of adults in the swarm, indicating that per capita output increased with swarm size. Subdividing adults into queens and workers did not improve signi ficantly on these models, but the proportion of queens was a significa nt factor explaining brood production in one of two sampling years. Ea rlier work on P. occidentalis suggests that the mechanism behind the p attern is that workers transferring materials to one another experienc e increasing queuing delays as group size decreases. The largest colon y in each of the two years produced unusually low outputs of brood. On e interpretation is that the curve of group-size related brood product ivity peaks at intermediate group size and that these colonies are on the downward part of the curve. That these same two colonies also had the lowest proportions of queens suggests a second interpretation: the se colonies were constrained to low brood production by a low colony-l evel oviposition rate. A third possibility is that these were mature c olonies, and mature colonies may allocate a smaller fraction of resour ces to brood rearing than do younger colonies. Our result contradicts earlier findings for a variety of social and subsocial Hymenoptera tha t per capita productivity declines as group size increases. We suspect that Michener's result for swarm-founding wasps is an artifact of his having to lump colonies of different species and different stages of development to obtain adequate sample sizes to plot. If our result for P. occidentalis can be generalized to other swarm-founders, then thes e wasps have evolved a mode of colony organization fundamentally diffe rent from that of other wasps. Thus, our result places new significanc e on the role of group dynamics as a factor affecting group size in di fferent taxa.