The scent of inbreeding: a male sex pheromone betrays inbred males

Inbreeding depression results from mating among genetically related individuals and impairs
reproductive success. The decrease in male mating success is usually attributed to an impact on
multiple fitness-related traits that reduce the general condition of inbred males. However, the reduced
mating success of inbred males could also be a consequence of strong selection on females to avoid
mating with an inbred male. This is especially relevant if females gain direct benefits through
appropriate mate choice (parental care, territory defences, nuptial gifts) or if they suffer direct costs
should they mate with an inbred male (decreased offspring viability and fertility). Here we find that the
production of the male sex pheromone is reduced significantly by inbreeding in the butterfly Bicyclus
anynana. Other traits indicative of the general condition, including flight performance, are also
negatively affected in male butterflies by inbreeding. Yet we unambiguously show that only the
production of male pheromones affects mating success. Thus, this pheromone signal informs females
about the inbreeding status of their mating partners. We also identify the specific chemical component,
hexadecanal, likely responsible for the decrease in male mating success. Our results advocate giving
increased attention to olfactory communication as a major causal factor of mate-choice decisions and
sexual selection.