Predictors of nest growth: diminishing returns for subordinates in the paper wasp Polistes dominula

Abstract

Abstract: In cooperative breeders, subordinates that have alternative reproductive options are expected to stay and help dominant breeders only as long as they contribute to group productivity, if their fitness is linked with colony success. Female Polistes dominula paper wasps live as cooperative breeders in small groups of typically fewer than 10 females. Subordinates tend to have high-quality outside options, and so could choose alternative breeding tactics if their work efforts increased productivity negligibly. In the founding stage before workers emerge, we tested the effect of various predictors on nest growth, as a proxy for group productivity, and explored the shape of the relationship between group size and nest growth. We found group size to be the only significant predictor of nest growth: variation among body sizes within the group showed no effect, suggesting a lack of size-dependent task specialization in this species. Average body size and average genetic relatedness between group members similarly showed no effects on nest growth. Group size had a non-linear effect so that per-capita benefits to nest growth decreased in larger groups, and groups of 10 or more would benefit negligibly from additional group members. Hence, females might be better off pursuing other options than joining a large group. This finding helps to explain why P. dominula groups are usually relatively small in our study population. Further studies may illuminate the mechanisms behind the smaller per-capita nest growth that we found in larger groups.

Significance statement: Identifying which factors influence the productivity of animal groups is key to understanding why different species breed cooperatively in groups of varying sizes. In the paper wasp Polistes dominula, we investigated the growth rate of nests as a measure of group productivity. We found that average body size, the variation in body sizes within the group, and average genetic relatedness between group members did not affect nest growth, while group size had a strong, positive effect: nests grew faster with more group members, but the per-capita benefit decreased in larger groups. The addition of extra group members in groups of 10 or more had negligible effects on nest growth. Hence, wasps may be better off pursuing other options than joining large groups. This finding helps to explain why groups normally consist of fewer than 10 wasps in this population.

Keywords: Altruism; Cooperation; Group living; Michener’s paradox; Social evolution; Social insects.

Fig. 1

Nest growth measured as the average number of cells added per day plotted against group size (number of group members). a Average observed nest growth with standard errors (except for group size = 12 because N = 1). b Predicted nest growth based on the main model (model: nest growth = group size + group size^2 + median first cell count + median first observation date); shaded area represents 95% confidence bands

Fig. 2

Predicted increase (a) and predicted percentwise increase (b) in nest growth based on the main model (model: nest growth = group size + group size^2 + median first cell count + median first observation date) plotted against group size; e.g. the point at group size = 2 represents the increase if one extra wasp was added to make a group of 3. The grey line indicates the point at which adding extra group members no longer translates to an increase in nest growth

Fig. 3

Brood value (egg = 1, small larva = 1.5, medium larva = 2, large larva = 3, pupa = 4) at the time when the first worker(s) matured, as a measure of nest productivity, plotted against nest growth (number of cells added per day) in a subset of 17 nests. The full line is a regression line indicating a significant correlation (LM, p < 0.0001)