Abstract
Previous work on the Glanville fritillary butterfly (Melitaea cinxia) shows substantial inbreeding depression in both of our two study regions, Finland and southern France. The influence of inbreeding depression on population dynamics should depend on the strength of inbreeding avoidance. We conducted mate choice experiments to ascertain whether and to what extent butterflies avoid mating with their sibs. Experiments with similar design were done in the laboratory with Finnish butterflies and in the field with French butterflies. Each female was given a choice of mates with equal opportunity to mate with a sib or with a non-sib. In neither experiment was there a trend towards avoidance of sib mating. 95% confidence intervals for the proportion of non-sib matings were 12–62% in the laboratory experiment and 28–69% in the field experiment. Any preference for non-sibs must be slight, and can provisionally be ignored in modelling the dynamics of M. cinxia populations.