Scented nectar and the challenge of measuring honest signals in pollination

1. Nectar scents are thought to function as honest signals of reward used by pollinators, but this hypothesis has rarely been tested. 2. Using Penstemon digitalis, we examined honest signalling of the nectar volatile (S)-(+)-linalool and pollinator responses to linalool in both field and laboratory settings. Because our previous work showed that linalool emission was associated with higher female fitness and that nectar is scented with linalool, we hypothesized that linalool was an honest signal of nectar reward. To assess honesty, we measured linalool-nectar associations including nectar volume, sugar amount, concentration, and production rate for inflorescences and flowers in several populations. We also assessed whether Bombus impatiens, the main pollinator of P. digitalis at our sites, can use linalool as a foraging signal. We supplemented real or artificial flowers in the field and lab with varying linalool-nectar combinations to measure pollinator behavioural responses. 3. We foun...