Phenotypic assortment by morphology in social partners of the forked fungus beetle Bolitotherus cornutus

Social interactions with conspecifics can dramatically affect an
individual’s fitness. The positive or negative consequences of interacting
with social partners typically depend on the value of traits that they
express. These pathways of social selection connect the traits and genes
expressed in some individuals to the fitness realized by others, thereby
altering the total phenotypic selection on and evolutionary response of
traits across the multivariate phenotype. The downstream effects of social
selection are mediated by the patterns of phenotypic assortment between
focal individuals and their social partners (the interactant covariance,
Cij , or the multivariate form, CI). Depending on the sign and magnitude
of the interactant covariance, the direction of social selection can be
reinforced, reversed, or erased. We report estimates of Cij from
a variety of studies of forked fungus beetles to address the largely
unexplored questions of consistency and plasticity of phenotypic
assortment in natural populations. We found that phenotypic assortment of
male beetles based on body size or horn length was highly variable among
subpopulations, but that those differences also were broadly consistent
from year to year. At the same time, the strength and direction of
Cij changed quickly in response to experimental changes in
resource distribution and social properties of populations. Generally,
interactant covariances were more negative in contexts in which the number
of social interactions was greater in both field and experimental
situations. These results suggest that patterns of phenotypic assortment
could be important contributors to variability in multilevel selection
through their mediation of social selection gradients.