Social selection occurs when traits of interaction partners influence an individual’s fitness and can alter total selection strength. However, we have little idea of what factors influence social selection’s strength. Further, social selection only contributes to overall selection when there is phenotypic assortment, but simultaneous estimates of social selection and phenotypic assortment are rare. Here we estimated social selection on body size in a wild population of New Zealand giraffe weevils (Lasiorhynchus barbicornis). We measured phenotypic assortment by body size and tested whether social selection varied with sex-ratio, density, and interacted with the body size of the focal individual. Social selection was limited and unaffected by sex ratio or the size of the focal individual. However, at high densities social selection was negative for both sexes, consistent with size-based competitive interactions for access to mates. Phenotypic assortment was always close to zero, indicating negative social selection at high densities will not impede the evolution of larger body sizes. Despite its predicted importance, social selection may only influence evolutionary change in specific contexts, leaving direct selection to drive evolutionary change.

The methods are described in full in the associated paper and in “Directional selection on body size but no apparent survival cost to being large in wild New Zealand giraffe weevils” Rebecca J. Le Grice et al. 2019, Evolution, doi:10.1111/evo.13698 In brief aggregations of weevils were located on the trunks of karaka trees (Corynocarpus laevigatus), each weevil in the aggregation was measured and individually marked, and then the mating success of each weevil in a one hour period was recorded. This was repeated over several days, giving repeated measures of the composition of the aggregation of weevils on the tree and their mating success. We then used these records to determine the mean body size of same sex rivals in for each weevil in each day, as well as the density and sex ratio of the aggregation on each day. Using these values and the measures of individual body size, we were able to estimate direct and social selection, phenotypic assortment, and how these things varied with density and other factors.

Please see the README file for details on data use.