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© 1996 International Society for Behavioral Ecology
research-article |
Productivity in a social wasp: per capita output increases with swarm size
aDepartments of Entomology and Zoology, University of Wisconsin, Madison, WI 53706 bDepartment of Statistics, University of Wisconsin, Madison, WI 58706
ABSTRACT
We measured the productivity of newly-founded colonies of Polybia ocddentalis, a Neotropical swarm-founding social wasp, over their first 25 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 significantly on these models, but the proportion of queens was a significant factor explaining brood production in one of two sampling years. Earlier work on P. ocddentalis suggests that the mechanism behind the pattern is that workers transferring materials to one another experience increasing queuing delays as group size decreases. The largest colony in each of the two years produced unusually low outputs of brood. One interpretation is that the curve of group-size related brood productivity 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: these colonies were constrained to low brood production by a low colony-level oviposition rate. A third possibility is diat these were mature colonies, and mature colonies may allocate a smaller fraction of resources to brood rearing than do younger colonies. Our result contradicts earlier findings for a variety of social and subsocial Hymenoptera that 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. ocddentalis can be generalized to other swarm-founders, then these wasps have evolved a mode of colony organization fundamentally different from that of other wasps. Thus, our result places new significance on the role of group dynamics as a factor affecting group size in different taxa.
Key words: colony size, Polistinae, productivity, social wasps, swarms, Vespidae.
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